FIGURE CAPTION – This artist’s impression shows how jets from supermassive black holes could form galaxies, thereby explaining why the mass of black holes is larger in galaxies that contain more stars.

“The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today,” says lead author David Elbaz. “Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.”

To reach such an extraordinary conclusion, the team of astronomers conducted extensive observations of a peculiar object, the nearby quasar HE0450-2958 (see ESO PR 23/05 for a previous study of this object), which is the only one for which a host galaxy has not yet been detected [1]. HE0450-2958 is located some 5 billion light-years away.

Until now, it was speculated that the quasar’s host galaxy was hidden behind large amounts of dust, and so the astronomers used a mid-infrared instrument on ESO’s Very Large Telescope for the observations [2]. At such wavelengths, dust clouds shine very brightly, and are readily detected. “Observing at these wavelengths would allow us to trace dust that might hide the host galaxy,” says Knud Jahnke, who led the observations performed at the VLT. “However, we did not find any. Instead we discovered that an apparently unrelated galaxy in the quasar’s immediate neighbourhood is producing stars at a frantic rate.”

These observations have provided a surprising new take on the system. While no trace of stars is revealed around the black hole, its companion galaxy is extremely rich in bright and very young stars. It is forming stars at a rate equivalent to about 350 Suns per year, one hundred times more than rates for typical galaxies in the local Universe.

Earlier observations had shown that the companion galaxy is, in fact, under fire: the quasar is spewing a jet of highly energetic particles towards its companion, accompanied by a stream of fast-moving gas. The injection of matter and energy into the galaxy indicates that the quasar itself might be inducing the formation of stars and thereby creating its own host galaxy; in such a scenario, galaxies would have evolved from clouds of gas hit by the energetic jets emerging from quasars.

“The two objects are bound to merge in the future: the quasar is moving at a speed of only a few tens of thousands of km/h with respect to the companion galaxy and their separation is only about 22 000 light-years,” says Elbaz. “Although the quasar is still ‘naked’, it will eventually be ‘dressed’ when it merges with its star-rich companion. It will then finally reside inside a host galaxy like all other quasars.”

Hence, the team have identified black hole jets as a possible driver of galaxy formation, which may also represent the long-sought missing link to understanding why the mass of black holes is larger in galaxies that contain more stars [3].

“A natural extension of our work is to search for similar objects in other systems,” says Jahnke.

Future instruments, such as the Atacama Large Millimeter/submillimeter Array, the European Extremely Large Telescope and the NASA/ESA/CSA James Webb Space Telescope will be able to search for such objects at even larger distances from us, probing the connection between black holes and the formation of galaxies in the more distant Universe.

Notes

[1] Supermassive black holes are found in the cores of most large galaxies; unlike the inactive and starving one sitting at the centre of the Milky Way, a fraction of them are said to be active, as they eat up enormous amounts of material. These frantic actions produce a copious release of energy across the whole electromagnetic spectrum; particularly spectacular is the case of quasars, where the active core is so overwhelmingly bright that it outshines the luminosity of the host galaxy.

[2] This part of the study is based on observations performed at mid-infrared wavelengths, with the powerful VLT spectrometer and imager for the mid-infrared (VISIR) instrument at the VLT, combined with additional data including: spectra acquired using VLT-FORS, optical and infrared images from the NASA/ESA Hubble Space Telescope, and radio observations from the Australia Telescope National Facility.

[3] Most galaxies in the local Universe contain a supermassive black hole with a mass about 1/700th the mass of the stellar bulge. The origin of this black hole mass versus stellar mass relation is one of the most debated subjects in modern astrophysics.

More Information

This research was presented in papers published in the journal Astronomy & Astrophysics: “Quasar induced galaxy formation: a new paradigm?” by Elbaz et al., and in the Astrophysical Journal “The QSO HE0450-2958: Scantily dressed or heavily robed? A normal quasar as part of an unusual ULIRG” by Jahnke et al.

The team is composed of David Elbaz (Service d’Astrophysique, CEA Saclay, France), Knud Jahnke (Max Planck Institute for Astronomy, Heidelberg, Germany), Eric Pantin (Service d’Astrophysique, CEA Saclay, France), Damien Le Borgne (Paris University 6 and CNRS, Institut d'Astrophysique de Paris, France) and Géraldine Letawe (Institut d'Astrophysique et de Géophysique, Université de Liège, Belgium).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

• Research papers: http://www.aanda.org/10.1051/0004-6361/200912848/pdf
  and http://arxiv.org/abs/0906.0365
• Web page of David Elbaz: http://david.elbaz3.free.fr
• Web page of Knud Jahnke: http://www.mpia.de/coevolution

Contacts

David Elbaz
CEA, Saclay, France
Phone: +33 (0)1 69 08 54 39
E-mail: delbaz (at) cea.fr

Knud Jahnke
Max Planck Institute for Astronomy, Heidelberg
Phone: +49 6221 528 398
E-mail: jahnke (at) mpia-hd.mpg.de

When targeting spiral galaxy bulges, astronomers often seek edge-on galaxies, as their bulges are more easily distinguishable from the disc. This exceptionally detailed edge-on view of NGC 4710 taken by the Advanced Camera for Surveys (ACS) aboard Hubble reveals the galaxy's bulge in the brightly coloured centre. The luminous, elongated white plane that runs through the bulge is the galaxy disc. The disc and bulge are surrounded by eerie-looking dust lanes.

When staring directly at the centre of the galaxy, one can detect a faint, ethereal "X"-shaped structure. Such a feature, which astronomers call a "boxy" or "peanut-shaped" bulge, is due to the vertical motions of the stars in the galaxy's bar and is only evident when the galaxy is seen edge-on. This curiously shaped puff is often observed in spiral galaxies with small bulges and open arms, but is less common in spirals with arms tightly wrapped around a more prominent bulge, such as NGC 4710.

NGC 4710 is a member of the giant Virgo Cluster of galaxies and lies in the northern constellation of Coma Berenices (the Hair of Queen Berenice). It is not one of the brightest members of the cluster, but can easily be seen as a dim elongated smudge on a dark night with a medium-sized amateur telescope. In the 1780s, William Herschel discovered the galaxy and noted it simply as a "faint nebula". It lies about 60 million light-years from the Earth and is an example of a lenticular or S0-type galaxy – a type that seems to have some characteristics of both spiral and elliptical galaxies.

Astronomers are scrutinising these systems to determine how many globular clusters they host. Globular clusters are thought to represent an indication of the processes that can build bulges. Two quite different processes are believed to be at play regarding the formation of bulges in spiral galaxies: either they formed rather rapidly in the early Universe, before the spiral disc and arms formed; or they built up from material accumulating from the disc during a slow and long evolution. In this case of NGC 4710, researchers have spotted very few globular clusters associated with the bulge, indicating that its assembly mainly involved relatively slow processes.

Notes for editors:

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Image credit: NASA & ESA

These observations were obtained by a team led by Paul Goudfrooij from the Space Telescope Science Institute in Baltimore, Maryland, USA.

Contacts:

Colleen Sharkey
Hubble/ESA, Garching, Germany
Tel: +49 89 3200 6306
Cell: +49 151 153 73591
E-mail: csharkey@eso.org

PASADENA, CA (Nov. 9, 2009) – Astronomers, conducting the broadest survey to date of galaxies from about 800 million years after the Big Bang, have found 22 early galaxies and confirmed the age of one by its characteristic hydrogen signature at 787 million years post Big Bang. The finding is the first age-confirmation of a so-called dropout galaxy at that distant time and pinpoints when an era called the reionization epoch likely began. The research will be published in a December issue of the Astrophysical Journal.

FIGURE CAPTION – This is a composite of false color images of the galaxies found at the early epoch around 800 million years after the Big Bang. The upper left panel presents the galaxy confirmed in the 787 million year old universe. These galaxies are in the Subaru Deep Field. (Credit: These images are created by M. Ouchi et al., which are the reproduction of Figure 3 in the Astrophysical Journal December 2009 issue.)

With recent technological advancements, such as the Wide-Field Camera 3 on the Hubble Space Telescope, there has been an explosion of research of the reionization period, the farthest back in time that astronomers can observe. The Big Bang, 13.7 billion years ago, created a hot, murky universe. Some 400,000 years later, temperatures cooled, electrons and protons joined to form neutral hydrogen, and the murk cleared. Some time before 1 billion years after the Big Bang, neutral hydrogen began to form stars in the first galaxies, which radiated energy and changed the hydrogen back to being ionized. Although not the thick plasma soup of the earlier period just after the Big Bang, this star formation started the reionization epoch. Astronomers know that this era ended about 1 billion years after the Big Bang, but when it began has eluded them and intrigued researchers like lead author Masami Ouchi of the Carnegie Observatories.

The U.S. and Japanese team led by Ouchi used a technique for finding these extremely distant galaxies. “We look for ‘dropout’ galaxies,” explained Ouchi. “We use progressively redder filters that reveal increasing wavelengths of light and watch which galaxies disappear from or ‘dropout’ of images made using those filters. Older, more distant galaxies ‘dropout’ of progressively redder filters and the specific wavelengths can tell us the galaxies’ distance and age. What makes this study different is that we surveyed an area that is over 100 times larger than previous ones and, as a result, had a larger sample of early galaxies (22) than past surveys. Plus, we were able to confirm one galaxy’s age,” he continued. “Since all the galaxies were found using the same dropout technique, they are likely to be the same age.”

Ouchi’s team was able to conduct such a large survey because they used a custom-made, super-red filter and other unique technological advancements in red sensitivity on the wide-field camera of the 8.3-meter Subaru Telescope. They made their observations from 2006 to 2009 in the Subaru Deep Field and Great Observatories Origins Deep Survey North field. They then compared their observations with data gathered in other studies.

Astronomers have wondered whether the universe underwent reionization instantaneously or gradually over time, but more importantly, they have tried to isolate when the universe began reionization. Galaxy density and brightness measurements are key to calculating star-formation rates, which tell a lot about what happened when. The astronomers looked at star-formation rates and the rate at which hydrogen was ionized.

Using data from their study and others, they determined that the star-formation rates were dramatically lower from 800 million years to about one billion years after the Big Bang, than thereafter. Accordingly, they calculated that the rate of ionization would be very slow during this early time, because of this low star-formation rate.

“We were really surprised that the rate of ionization seems so low, which would constitute a contradiction with the claim of NASA’s WMAP satellite. It concluded that reionization started no later than 600 million years after the Big Bang,” remarked Ouchi. “We think this riddle might be explained by more efficient ionizing photon production rates in early galaxies. The formation of massive stars may have been much more vigorous then than in today’s galaxies. Fewer, massive stars produce more ionizing photons than many smaller stars,” he explained.

http://www.ciw.edu/prouchiz7falsecolornotype_jpg 

Plot of ionization history http://www.ciw.edu/prouchiz7galaxyionizationhist_jpg

Cosmic star-formation history http://www.ciw.edu/prouchiz7galaxyageplots_jpg

------------------

The work was funded by the Carnegie Institution. The research is based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the Hubble Space Telescope, operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555; the Spitzer Telescope, managed by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

“Matter is not distributed uniformly in the Universe,” says Masayuki Tanaka from ESO, who led the new study. “In our cosmic vicinity, stars form in galaxies and galaxies usually form groups and clusters of galaxies. The most widely accepted cosmological theories predict that matter also clumps on a larger scale in the so-called ‘cosmic web’, in which galaxies, embedded in filaments stretching between voids, create a gigantic wispy structure.”

These filaments are millions of light years long and constitute the skeleton of the Universe: galaxies gather around them, and immense galaxy clusters form at their intersections, lurking like giant spiders waiting for more matter to digest. Scientists are struggling to determine how they swirl into existence. Although massive filamentary structures have been often observed at relatively small distances from us, solid proof of their existence in the more distant Universe has been lacking until now.

The team led by Tanaka discovered a large structure around a distant cluster of galaxies in images they obtained earlier. They have now used two major ground-based telescopes to study this structure in greater detail, measuring the distances from Earth of over 150 galaxies, and, hence, obtaining a three-dimensional view of the structure. The spectroscopic observations were performed using the VIMOS instrument on ESO’s Very Large Telescope and FOCAS on the Subaru Telescope, operated by the National Astronomical Observatory of Japan.

Thanks to these and other observations, the astronomers were able to make a real demographic study of this structure, and have identified several groups of galaxies surrounding the main galaxy cluster. They could distinguish tens of such clumps, each typically ten times as massive as our own Milky Way galaxy — and some as much as a thousand times more massive — while they estimate that the mass of the cluster amounts to at least ten thousand times the mass of the Milky Way. Some of the clumps are feeling the fatal gravitational pull of the cluster, and will eventually fall into it.

“This is the first time that we have observed such a rich and prominent structure in the distant Universe,” says Tanaka. “We can now move from demography to sociology and study how the properties of galaxies depend on their environment, at a time when the Universe was only two thirds of its present age.”

The filament is located about 6.7 billion light-years away from us and extends over at least 60 million light-years. The newly uncovered structure does probably extend further, beyond the field probed by the team, and hence future observations have already been planned to obtain a definite measure of its size.

More Information

This research was presented in a paper published as a letter in the Astronomy & Astrophysics Journal: The spectroscopically confirmed huge cosmic structure at z = 0.55, by Tanaka et al. 

The team is composed of Masayuki Tanaka (ESO), Alexis Finoguenov (Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany and University of Maryland, Baltimore, USA), Tadayuki Kodama (National Astronomical Observatory of Japan, Tokyo, Japan), Yusei Koyama (Department of Astronomy, University of Tokyo, Japan), Ben Maughan (H.H. Wills Physics Laboratory, University of Bristol, UK) and Fumiaki Nakata (Subaru Telescope, National Astronomical Observatory of Japan).

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

A team of six CfA astronomers led by Junfeng Wang, together with one of their colleagues, used the Chandra X-ray Observatory to study the galaxy NGC 4151, at about 40 million light-years away one of the closest active galactic nuclei. Its radio jet is small, only about 700 light-years long, making this galaxy a good example of the more conventional jet sources. Moreover, the Hubble Space Telescope has provided detailed optical images of the inner regions. The scientists' new observations are the first very deep X-ray images of this nucleus.

The astronomers were able to compare the detailed morphology of the X-ray emitting gas with that of the ionized light seen in the optical, comparing for example the knots of activity along the jets. They find that the overall physical conditions of these knots are the same independent of the distance of a knot from the black hole. The scientists conclude as a result that one of the most commonly advanced theories about the emission, one that relies on magnetic fields, is not supported, at least in this class of galaxy. Instead, the new results tend to indicate that an outflowing wind is slamming into clouds of gas in the local environment, and that these interactions are generating the X-rays. The results help to explain how and why the jets in these more modest sources compare to those in the more extreme examples, and thereby also lend credibility to our general understanding of these amazing cosmic beacons.

Not surprisingly, interacting galaxies have a dramatic effect on each other. Studies have revealed that as galaxies approach one another massive amounts of gas are pulled from each galaxy towards the centre of the other, until ultimately, the two merge into one massive galaxy. The object in the image, NGC 2623, is in the late stages of the merging process with the centres of the original galaxy pair now merged into one nucleus. However, stretching out from the centre are two tidal tails of young stars showing that a merger has taken place. During such a collision, the dramatic exchange of mass and gases initiates star formation, seen here in both the tails.

The prominent lower tail is richly populated with bright star clusters — 100 of them have been found in these observations. The large star clusters that the team have observed in the merged galaxy are brighter than the brightest clusters we see in our own vicinity. These star clusters may have formed as part of a loop of stretched material associated with the northern tail, or they may have formed from debris falling back onto the nucleus. In addition to this active star-forming region, both galactic arms harbour very young stars in the early stages of their evolutionary journey.

Some mergers (including NGC 2623) can result in an active galactic nucleus, where one of the supermassive black holes found at the centres of the two original galaxies is stirred into action. Matter is pulled toward the black hole, forming an accretion disc. The energy released by the frenzied motion heats up the disc, causing it to emit across a wide swath of the electromagnetic spectrum.

NGC 2623 is so bright in the infrared that it belongs to the group of very luminous infrared galaxies (LIRG) and has been extensively studied as the part of the Great Observatories All-sky LIRG Survey (GOALS) project that combines data from some of the most advanced space-based telescopes, including Hubble. Additional data from infrared and X-ray telescopes can further characterise objects like active galactic nuclei and nuclear star formation by revealing what is unseen at visible wavelengths.

The GOALS project includes data from NASA/ESA's Hubble Space Telescope, NASA's Spitzer Space Telescope, NASA's Chandra X-ray Observatory and NASA's Galaxy Evolution Explorer (GALEX). The joint efforts of these powerful observing facilities have provided a clearer picture of our local Universe.

This data used for this colour composite were taken in 2007 by the Advanced Camera for Surveys (ACS) aboard Hubble. The observations were led by astronomer Aaron S. Evans. A team of over 30 astronomers, including Evans, recently published an important overview paper, detailing the first results of the GOALS project. Observations from ESA's X-ray Multi-Mirror Mission (XMM-Newton) telescope contributed to the astronomers' understanding of NGC 2623.

Notes for editors:

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Image credit: NASA, ESA and A. Evans (Stony Brook University, New York & National Radio Astronomy Observatory, Charlottesville, USA)

Links:

GOALS
NGC 2623 paper
GOALS overview paper

Contacts:

Colleen Sharkey
Hubble/ESA, Garching, Germany
Tel: +49 89 3200 6306
Cell: +49 151 153 73591
E-mail: csharkey@eso.org

Aaron S. Evans
University of Virginia, Charlottesville, USA
National Radio Astronomy Observatory, Charlottesville, USA
Tel: +1-434-924-4896
E-mail: aevans@virginia.edu

[GH / HOR]

Related links:

[1] The H.E.S.S. Homepage at the MPI for Nuclear Physics

Original work:

F. Acero, F. Aharonian et al.
Detection of Gamma Rays From a Starburst Galaxy
Science Express, September 24, 2009

Ram pressure is the drag force that results when something moves through a fluid — much like the wind you feel in your face when bicycling, even on a still day — and occurs in this context as galaxies orbiting about the centre of the cluster move through the intra-cluster medium, which then sweeps out gas from within the galaxies.

The spiral galaxy NGC 4522 is located some 60 million light-years away from Earth and it is a spectacular example of a spiral galaxy currently being stripped of its gas content. The galaxy is part of the Virgo galaxy cluster and its rapid motion within the cluster results in strong winds across the galaxy as the gas within is left behind. Scientists estimate that the galaxy is moving at more than 10 million kilometres per hour. A number of newly formed star clusters that developed in the stripped gas can be seen in the Hubble image.

Even though this is a still image, Hubble's view of NGC 4522 practically swirls off the page with apparent movement. It highlights the dramatic state of the galaxy, with an especially vivid view of the ghostly gas being forced out of it. Bright blue pockets of new star formation can be seen to the right and left of centre. The image is sufficiently deep to show distant background galaxies.

The image of NGC 4402 also highlights some telltale signs of ram pressure stripping such as the curved, or convex, appearance of the disc of gas and dust, a result of the forces exerted by the heated gas. Light being emitted by the disc backlights the swirling dust that is being swept out by the gas. Studying ram pressure stripping helps astronomers better understand the mechanisms that drive the evolution of galaxies, and how the rate of star formation is suppressed in very dense regions of the Universe like clusters.

Both images were taken by the Advanced Camera for Surveys on Hubble before it suffered from a power failure in 2007. Astronauts on Servicing Mission 4 in May 2009 were able to restore ACS during their 13-day mission.

Notes for editors:

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Image credit: NASA & ESA

Contacts:

Colleen Sharkey
Hubble/ESA, Garching, Germany
Tel: +49 89 3200 6306
Cell: +49 151 153 73591
E-mail: csharkey@eso.org

FIGURE CAPTION – Seen edge-on, observations of NGC 4945 suggest that this hive of stars is a spiral galaxy much like our own Milky Way, with swirling, luminous arms and a bar-shaped centre. Sites of active star formation, known as H II regions, are seen prominently in the image, appearing bright pink. These resemblances aside, NGC 4945 has a brighter centre that likely harbours a supermassive black hole, which is devouring reams of matter and blasting energy out into space. NGC 4945 is about 13 million light-years away in the constellation of Centaurus (the Centaur) and is beautifully revealed in this image taken with data in five bands (B, V, R, H-alpha and S II) with the 2.2-metre MPG/ESO telescope at La Silla. The field of view is 30 x 30 arcminutes. North is up, East is to the left.

As NGC 4945 is only about 13 million light-years away in the constellation of Centaurus (the Centaur), a modest telescope is sufficient for skygazers to spot this remarkable galaxy. NGC 4945’s designation comes from its entry number in the New General Catalogue compiled by the Danish–Irish astronomer John Louis Emil Dreyer in the 1880s. James Dunlop, a Scottish astronomer, is credited with originally discovering NGC 4945 in 1826 from Australia.

Today’s new portrait of NGC 4945 comes courtesy of the Wide Field Imager (WFI) instrument at the 2.2-metre MPG/ESO telescope at the La Silla Observatory in Chile. NGC 4945 appears cigar-shaped from our perspective on Earth, but the galaxy is actually a disc many times wider than it is thick, with bands of stars and glowing gas spiralling around its centre. With the use of special optical filters to isolate the colour of light emitted by heated gases such as hydrogen, the image displays sharp contrasts in NGC 4945 that indicate areas of star formation.

Other observations have revealed that NGC 4945 has an active galactic nucleus, meaning its central bulge emits far more energy than calmer galaxies like the Milky Way. Scientists classify NGC 4945 as a Seyfert galaxy after the American astronomer Carl K. Seyfert, who wrote a study in 1943 describing the odd light signatures emanating from some galactic cores. Since then, astronomers have come to suspect that supermassive black holes cause the turmoil in the centre of Seyfert galaxies. Black holes gravitationally draw gas and dust into them, accelerating and heating this attracted matter until it emits high-energy radiation, including X-rays and ultraviolet light. Most large, spiral galaxies, including the Milky Way, host a black hole in their centres, though many of these dark monsters no longer actively “feed” at this stage in galactic development.

More Information

ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Contact

Henri Boffin

ESO

Phone: +49 89 3200 6222

E-mail: hboffin (at) eso.org

ESO Press Officer in Chile: Valeria Foncea - +56 2 463 3123 - vfoncea@eso.org

This belief, based on years of research, has been tipped on its side with new data from NASA's Galaxy Evolution Explorer. The ultraviolet telescope has found proof that small stars come in even bigger bundles than previously believed; for example, in some places in the cosmos, about 2,000 low-mass stars may form for each massive star. The little stars were there all along but masked by massive, brighter stars.

"What this paper is showing is that some of the standard assumptions that we've had - that the brightest stars tell you about the whole population of stars - this doesn't seem to work, at least not in a constant way," said Gerhardt R. Meurer, principal investigator on the study and a research scientist at Johns Hopkins University, Baltimore, Md.

Astronomers have long known that many stars are too dim to be seen in the glare of their brighter, more massive counterparts. Though the smaller, lighter stars outnumber the big ones, they are harder to see. Going back to a grocery story analogy, the melons grab your eyes, even though the total weight of the blueberries may be more.

Beginning in the 1950s, astronomers came up with a method for counting all the stars in a region, even the ones they couldn't detect. They devised a sort of stellar budget, an equation called the "stellar initial mass function," to estimate the total number of stars in an area of the sky based on the light from only the brightest and most massive. For every large star formed, a set number of smaller ones were thought to have been created regardless of where the stars sat in the universe.

"We tried to understand properties of galaxies and their mass by looking at the light we can see," Meurer said.

But this common assumption has been leading astronomers astray, said Meurer, especially in galaxies that are intrinsically small and faint.

To understand the problem, imagine trying to estimate the population on Earth by observing light emitted at night. Looking from above toward North America or Europe, the regions where more people live light up like signposts. Los Angeles, for example, is easily visible to a scientist working on the International Space Station. However, if this method were applied to regions where people have limited electricity, populations would be starkly underestimated, for example in some sections of Africa.

The same can be said of galaxies, whose speckles of light in the dark of space can be misleading. Meurer and his team used ultraviolet images from the Galaxy Evolution Explorer and carefully filtered red-light images from telescopes at the Cerro Tololo Inter-American Observatory in Chile to show that many galaxies do not form a lot of massive stars, yet still have plenty of lower-mass counterparts. The ultraviolet images are sensitive to somewhat small stars three times or more massive than the sun, while the filtered optical images are only sensitive to the largest stars with 20 or more times the mass of the sun.

The effects are particularly important in parts of the universe where stars are spread out over a larger volume -- the rural Africa of the cosmos. There could be about four times as many stars in these regions than previously estimated.

"Especially in these galaxies that seem small and piddling, there can be a lot more mass in lower mass stars than we had previously expected from what we could see from the brightest, youngest stars," Meurer said. "But we can now reduce these errors using satellites like the Galaxy Evolution Explorer."

This research was published in the April 10, 2009, issue of Astrophysical Journal.

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

FIGURE 1 –This image of the Himiko object is a composite and in false color. The thick horizontal bar at the lower right corner presents a size of 10 thousand light year. (Credit: This image is created by M. Ouchi et al., which is the reproduction of Figure 2 in the article of The Astrophysical Journal May 2009 - 10 v696 issue.)

The researchers are puzzled by the object. Even with superb data from the world’s best telescopes, they are not sure what it is. Because it is one of the most distant objects ever found, its faintness does not allow the researchers to understand its physical origins. It could be ionized gas powered by a super-massive black hole; a primordial galaxy with large gas accretion; a collision of two large young galaxies; super wind from intensive star formation; or a single giant galaxy with a large mass of about 40 billion Suns. Because this mysterious and remarkable object was discovered early in the history of the universe in a Japanese Subaru field, the researchers named the object after the legendary mysterious queen in ancient Japan.

“The farther out we look into space, the farther we go back in time, “ explained lead author Masami Ouchi, a fellow at the Observatories of the Carnegie Institution who led an international team of astronomers from the U.S., Japan, and the United Kingdom. “I am very surprised by this discovery. I have never imagined that such a large object could exist at this early stage of the universe’s history. According to the concordance model of Big Bang cosmology, small objects form first and then merge to produce larger systems. This blob had a size of typical present-day galaxies when the age of the universe was about 800 million years old, only 6% of the age of today’s universe!”

Extended blobs discovered thus far have mostly been seen at a distance when the universe was 2 to 3 billion years old. No extended blobs have previously been found when the universe was younger. Himiko is located at a transition point in the evolution of the universe called the reionization epoch—it’s as far back as we can see to date. And at 55 thousand light years, Himiko is a big blob for that time.

This reionizing chapter in the universe was at the cosmic dawn, the epoch between about 200 million and one billion years after the Big Bang. During this period, neutral hydrogen began to form quasars, stars, and the first galaxies. Astronomers probe this era by searching for characteristic hydrogen signatures from the scattering of photons created by ionized gas clouds.

The team initially identified Himiko among 207 distant galaxy candidates seen at optical wavelengths using the Subaru telescope from the Subaru/XMM-Newton Deep Survey Field located in the constellation of Cetus. They then made spectroscopic observations to measure the distance with the Keck/DEIMOS and Carnegie’s Magellan/IMACS instrumentation. Himiko was an extraordinarily bright and large candidate for a distant galaxy. “We hesitated to spend our precious telescope time by taking spectra of this weird candidate. We never believed that this bright and large source was a real distant object. We thought it was a foreground interloper contaminating our galaxy sample,” continued Ouchi. “But we tried anyway. Then, the spectra exhibited a characteristic hydrogen signature clearly indicating a remarkably large distance—12.9 billion light years!”

“Using infrared data from NASA’s Spitzer Space Telescope and the United Kingdom Infrared Telescope, radio data from the VLA, and X-ray imaging from the XMM-Newton satellite, we were able to estimate the star-formation rate and stellar mass of this galaxy and to investigate whether it contains an active nucleus powered by a super-massive black hole,” remarked James Dunlop a team member at Edinburgh. “We found that the stellar mass of Himiko is an order of magnitude larger than other objects known at a similar epoch, but we cannot as yet tell if the center houses an active and growing black hole.”

“One of the puzzling things about Himiko is that it is so exceptional,” said Carnegie’s Alan Dressler, a member of the team. “If this was the discovery of a class of objects that are ancestors of today’s galaxies, there should be many more smaller ones already found—a continuous distribution. Because this object is, to this point, one-of-a-kind, it makes it very hard to fit it into the prevailing model of how normal galaxies were assembled. On the other hand, that’s what makes it interesting!”

The research is published in the May 10, 2009, issue of The Astrophysical Journal. The work was funded by the NASA through an award issued by JPL/Caltech, the Department of Energy, and the Carnegie Institution. The research is based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA); the Spitzer Telescope, managed by JPL for NASA; the Magellan telescopes operated by a consortium consisting of the Carnegie Institution, Harvard University, MIT, the University of Michigan, and the University of Arizona; and the United Kingdom Infrared Telescope, which is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK.

FIGURE CAPTION – This brilliant image, courtesy of NASA/ESA's Hubble Space Telescope, is a fitting 19th anniversary tribute to the workhorse space observatory. This interacting group contains several galaxies, along with a "cosmic fountain" of stars, gas and dust that stretches over 100 000 light-years. Resembling a pair of owl's eyes, the two nuclei of the colliding galaxies can be seen in the process of merging at the upper left. The bizarre blue bridge of material extending out from the northern component looks as if it connects to a third galaxy but in reality the galaxy is in the background and not connected at all. The blue "fountain" is the most striking feature of this galaxy troupe and it contains complexes of super star clusters that may have as many as dozens of individual young star clusters in them. (Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA))

Over the past 19 years Hubble has taken dozens of exotic pictures of galaxies going "bump in the night" as they collide with each other and have a variety of close encounters of the galactic kind. Just when you thought these interactions couldn’t look any stranger, this image of a trio of galaxies, called Arp 194, looks as if of the galaxies has sprung a leak. The bright blue streamer is really a stretched spiral arm full of newborn blue stars. This typically happens when two galaxies interact and gravitationally tug at each other gravitationally.

Resembling a pair of owl's eyes, the two nuclei of the colliding galaxies can be seen in the process of merging at the upper left. The bizarre blue bridge of material extending out from the northern component looks as if it connects to a third galaxy but in reality this galaxy is in the background and not connected at all. Hubble's sharp view allows astronomers to try and sort out visually which are the foreground and background objects when galaxies, superficially, appear to overlap.

The blue "fountain" is the most striking feature of this galaxy troupe and it contains complexes of super star clusters that may have as many as dozens of individual young star clusters in them. It formed as a result of the interactions among the galaxies in the northern component of Arp 194. The gravitational forces involved in a galaxy interaction can enhance the star formation rate and give rise to brilliant bursts of star formation in merging systems.

Hubble's resolution shows clearly that the stream of material lies in front of the southern component of Arp 194, as shown by the dust that is silhouetted around the star cluster complexes.

The details of the interactions among the multiple galaxies that make up Arp 194 are complex. The system was most likely disrupted by a previous collision or close encounter. The shapes of all the galaxies involved have been distorted by their gravitational interactions with one another.

Arp 194, located in the constellation of Cepheus, resides approximately 600 million light-years away from Earth. Arp 194 is one of thousands of interacting and merging galaxies known in our nearby Universe. These observations were taken in January 2009 with the Wide Field Planetary Camera 2. Blue, green and red filters were composited together to form this rather picturesque image of a galaxy interaction.

This picture was issued to celebrate the 19th anniversary of the launch of the Hubble Space Telescope aboard the space shuttle Discovery in 1990. Hubble has made more than 880 000 observations and snapped over 570 000 images of 29 000 celestial objects over the past 19 years.

Notes for editors:

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Contacts:

Colleen Sharkey
Hubble/ESA, Garching, Germany
Tel: +49-89-3200-6306
Cell: +49-151-153-73591
E-mail: csharkey@eso.org

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
Tel: +1-410-338-4514
E-mail: villard@stsci.edu

The most distant galaxies ever photographed are as far as 10 billion to 13 billion light-years away. A light-year is the distance that light travels in a vacuum in a year -- about 5.88 trillion miles (9.46 trillion kilometers). Galaxies range in diameter from a few thousand to a half-million light-years. Small galaxies have fewer than a billion stars. Large galaxies have more than a trillion.

The Milky Way has a diameter of about 100,000 light-years. The solar system lies about 25,000 light-years from the center of the galaxy. There are about 100 billion stars in the Milky Way.

Only three galaxies outside the Milky Way are visible with the unaided eye. People in the Northern Hemisphere can see the Andromeda Galaxy, which is about 2 million light-years away. People in the Southern Hemisphere can see the Large Magellanic Cloud, which is about 160,000 light-years from Earth, and the Small Magellanic Cloud, which is about 180,000 light-years away.

Groups of Galaxies

Galaxies are distributed unevenly in space. Some have no close neighbor. Others occur in pairs, with each orbiting the other. But most of them are found in groups called clusters. A cluster may contain from a few dozen to several thousand galaxies. It may have a diameter as large as 10 million light-years.

Clusters of galaxies, in turn, are grouped in larger structures called superclusters. On even larger scales, galaxies are arranged in huge networks. The networks consist of interconnected strings or filaments of galaxies surrounding relatively empty regions known as voids. One of the largest structures ever mapped is a network of galaxies known as the Great Wall. This structure is more than 500 million light-years long and 200 million light-years wide.

Shapes of Galaxies

Astronomers classify most galaxies by shape as either spiral galaxies or elliptical galaxies. A spiral galaxy is shaped like a disk with a bulge in the center. The disk resembles a pinwheel, with bright spiral arms that coil out from the central bulge. The Milky Way is a spiral galaxy. Like pinwheels, all spiral galaxies rotate -- but slowly. The Milky Way, for example, makes a complete revolution once every 250 million years or so.

New stars are constantly forming out of gas and dust in spiral galaxies. Smaller groups of stars called globular clusters often surround spiral galaxies. A typical globular cluster has about 1 million stars.

Elliptical galaxies range in shape from almost perfect spheres to flattened globes. The light from an elliptical galaxy is brightest in the center and gradually becomes fainter toward its outer regions. As far as astronomers can determine, elliptical galaxies rotate much more slowly than spiral galaxies or not at all. The stars within them appear to move in random orbits. Elliptical galaxies have much less dust and gas than spiral galaxies have, and few new stars appear to be forming in them.

 Galaxies of a third kind, irregular galaxies, lack a simple shape. Some consist mostly of blue stars and puffy clouds of gas, but little dust. The Magellanic Clouds are irregular galaxies of this type. Others are made up mostly of bright young stars along with gas and dust.

Galaxies move relative to one another, and occasionally two galaxies come so close to each other that the gravitational force of each changes the shape of the other. Galaxies can even collide. If two rapidly moving galaxies collide, they may pass right through each other with little or no effect. However, when slow-moving galaxies collide, they can merge into a single galaxy that is bigger than either of the original galaxies. Such mergers can produce spiral filaments of stars that can extend more than 100,000 light-years into space.

Emissions from Galaxies

All galaxies emit (give off) energy as waves of visible light and other kinds of electromagnetic radiation. In order of decreasing wavelength (distance between successive wave crests), electromagnetic radiation consists of radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All these forms of radiation together make up the electromagnetic spectrum.

The energy emitted by galaxies comes from various sources. Much of it is due to the heat of the stars and of clouds of dust and gas called nebulae. A variety of violent events also provide a great deal of the energy. These events include two kinds of stellar explosions: (1) nova explosions, in which one of the two members of a binary star system hurls dust and gas into space; (2) much more violent supernova explosions, in which a star collapses, then throws off most of its matter. One supernova may leave behind a compact, invisible object called a black hole, which has such powerful gravitational force that not even light can escape it. Another supernova may leave behind a neutron star, which consists mostly of tightly packed neutrons, particles that ordinarily occur only in the nuclei of atoms. But some supernovae leave nothing behind.

The intensity of the radiation emitted by a star at various wavelengths depends on the star's surface temperature. For example, the sun, which has a surface temperature of about 5500 ¡C (10,000 ¡F) emits most of its radiation in the visible part of the electromagnetic spectrum. Radiation of this type, whose intensity depends on temperature as it does for the sun and other normal stars, is called thermal radiation.

A small percentage of galaxies called active galaxies emit tremendous amounts of energy. This energy results from violent events occurring in objects at their center. The distribution of the wavelengths of the emissions does not resemble that of normal stars, and so the emissions are known as nonthermal radiation. The most powerful such object is a quasar, which emits a huge amount of radio, infrared, ultraviolet, X-ray, and gamma-ray energy. Some quasars emit 1,000 times as much energy as the entire Milky Way, yet look like stars in photographs. Quasar is short for quasi-stellar radio source. The name comes from the fact that the first quasars identified emit mostly radio energy and look much like stars. A radio galaxy is related to, but appears larger than, a quasar.

A Seyfert galaxy is a spiral galaxy that emits large amounts of infrared rays as well as large amounts of radio waves, X rays, or both radio waves and X rays. Seyfert galaxies get their name from American astronomer Carl K. Seyfert, who in 1943 became the first person to discover one.

Some active galaxies emit jets and blobs of highly energetic, electrically charged particles. These particles include positively charged protons and positrons and negatively charged electrons. Electrons and protons are forms of ordinary matter, but positrons are antimatter particles. They are the antimatter opposites of electrons -- that is, they have the same mass (amount of matter) as electrons, but they carry the opposite charge. See Antimatter.

The cause of the intense activity in active galaxies is thought to arise from a colossal black hole at the galactic center. The black hole can be as much as a billion times as massive as the sun. Because the black hole is so massive and compact, its gravitational force is powerful enough to tear apart nearby stars. The resulting dust and gas fall toward the black hole, adding their mass to a disk of matter called an accretion disk that orbits the black hole. At the same time, matter from the inner edge of the disk falls into the black hole. As the matter falls, it loses energy, thereby producing the radiation and jets that shoot out of the galaxy.

The Milky Way is not an active galaxy, but it does have a powerful source of radiation called Sagittarius A* at its center. The cause of this radiation may be a black hole a million times as massive as the sun.

Origin of Galaxies

Scientists have proposed two main kinds of theories of the origin of galaxies: (1) bottom-up theories and (2) top-down theories. The starting point for both kinds of theories is the big bang, the explosion with which the universe began 10 billion to 20 billion years ago. Shortly after the big bang, masses of gas began to gather together or collapse. Gravity then slowly compressed these masses into galaxies.

The two kinds of theories differ concerning how the galaxies evolved. Bottom-up theories state that much smaller objects such as globular clusters formed first. These objects then merged to form galaxies. According to top-down theories, large objects such as galaxies and clusters of galaxies formed first. The smaller groups of stars then formed within them. But all big bang theories of galaxy formation agree that no new galaxies -- or very few -- have formed since the earliest times.

Astronomers have found evidence of what conditions were like before the galaxies formed. In 1965, American physicists Arno Penzias and Robert Wilson detected faint radio waves throughout the sky. According to the big bang theory, the waves are radiation left over from the initial explosion. The strength of the radio waves appeared to be very nearly the same in every direction. But in 1992, a satellite called the Cosmic Background Explorer (COBE) detected tiny differences in the strength of radio waves coming from different directions. The differences in strength arise from tiny increases in the density of matter in the universe shortly after the big bang. The small regions of increased density had a stronger gravitational force than the surrounding matter. Clumps of matter therefore formed in these regions; and the clumps eventually collapsed into galaxies.

Most astronomical observations made to date support big bang theories. According to these theories, the universe is still expanding. Two kinds of observations strongly support the idea of an expanding universe. These observations indicate that all galaxies are moving away from one another and that the galaxies farthest from the Milky Way are moving away most rapidly. This relationship between speed and distance is known as the Hubble law of recession (moving backward), or Hubble's law. The law was named after American astronomer Edwin P. Hubble, who reported it in 1929.

Astronomers estimate the speed at which a galaxy is moving away by measuring the galaxy's redshift. The redshift is an apparent lengthening of electromagnetic waves emitted by an object moving away from the observer. A redshift can be measured when light from a galaxy is broken up and spread out into a band of colors called a spectrum. The spectrum of a galaxy contains bright and dark lines that are determined by the galaxy's temperature, density, and chemical composition. These lines are shifted toward the red end of the spectrum if the galaxy is moving away. The greater the amount of redshift, the more rapid the movement.

Scientists estimate the distance to galaxies by measuring the galaxies' overall brightness or the brightness of certain kinds of objects within them. These objects include variable stars as well as supernovae.

Evolution of Spiral Galaxies

Astronomers do not understand clearly how galactic spirals evolved and why they still exist. The mystery arises when one considers how a spiral galaxy rotates. The galaxy spins much like the cream on the surface of a cup of coffee. The inner part of the galaxy rotates somewhat like a solid wheel, and the arms trail behind. Suppose a spiral arm rotated around the center of its galaxy in about 250 million years -- as in the Milky Way. After a few rotations, taking perhaps 2 billion years, the arms would "wind up," producing a fairly continuous mass of stars. But almost all spiral galaxies are much older than 2 billion years.

According to one proposed solution to the mystery, differences in gravitational force throughout the galaxy push and pull at the stars, dust, and gas. This activity produces waves of compression. A familiar example of waves of compression are ordinary sound waves. Because the galaxy is rotating, the waves seem to travel in a spiral path, leading to the appearance of spiral arms of dense dust and gas. Stars then form in the arms.

LEARN MORE »

Galaxy Classification – Hubble Tuning Fork

In the composite image above, galaxies are organized by shape, according to the Hubble-Tuning Fork. In this structure, elliptical galaxies sit on the left side of the poster, creating the tuning fork's handle. They are designated by the letter "E", and given a number from zero to seven. An "E0" galaxy looks round, while an E7 galaxy is very long and thin. Spiral galaxies are located to the right side of the poster creating the fork's two prongs. The top prong is made up of regular spiral galaxies, and identified by the letter "S." Barred spiral galaxies make up the bottom prong, and are branded "SB." Meanwhile, letters – "a", "b", and "c" – indicate how tightly the spiral arms are wound. An "Sa" galaxy's arms are wound very tightly, while an "Sc" galaxy's spiral arms are very loosely wound.

Irregular galaxies are organized on bottom-left side of the poster because they were not represented in Hubble's original Tuning Fork.

The galaxies in this poster are three-color composites where blue depicts the galaxies at a light wavelength of 3.6 microns, while 8.0 microns is green, and 24 microns is red. Blue colors reveal light from an older population of stars. Tints of green represent organic molecules called polycyclic aromatic hydrocarbons, while red lumps show clouds of warm dust and gas heated by radiation from newborn stars. These galactic portraits were collected as part of the Spitzer Infrared Nearby Legacy project (SINGS).

For more, see: Hubble Classification of Galaxies - Hubble Tuning Fork Diagram »

The Standard Big Bang Model is more or less rooted in the idea that we are not located in some privileged position in space. All observers at the present epoch will see the universe on average the same way that we see it around us. In Steady State Cosmology, not only does the universe look much the same at any location in space, but according to the Perfect Cosmological Principle, it looks much the same at any point in time. There is no evolution of matter, and a dilution of the universe as the expansion proceeds. With this, the Steady State model was created in 1948 by H. Bondi, T. Gold and F. Hoyle. There was no beginning to the universe. It has existed throughout all eternity much as we see it today. It is expanding, but as each galaxy recedes, a new one gradually forms over the eons to maintain the density of galaxies in intergalactic space.

Related EoC Articles

• Universe
• Universe: Chinese Genesis
• Universe: Cold Big Bang
• Universe: Creation
• Universe: Cyclical
• Universe: Einstein-DeSitter
• Universe: Expansion
• Universe: Friedman
• Universe: Higher Dimensions
• Universe: Hot Big Bang
• Universe: Infinity
• Universe: Many-Worlds
• Universe: Maori
• Universe: Mixmaster
• Universe: Multiverse
• Universe: Mythology
• Universe: Oscillating
• Universe: Platonic
• Universe: Pre-relativistic
• Universe: Scandinavian
• Universe: Static
• Universe: Steady State
• Universe: Watery Abyss
• Universe: Zuni Indian

External Links

• Cosmological Principle - Wikipedia.
• Foundations of Big Bang Cosmology - Universe 101, Big Bang Theory, Wilkinson Microwave Anisotropy Probe, NASA.
• Steady State Theory - Wikipedia.

Preview Image

Depiction of a steady-state universe. (Source: NASA.)

In the Gamow, Alpher and Fermi theory of the expanding universe ca 1948, the universe started out in a hot, dense state. Conditions were changing rapidly as the universe expanded so that temperatures and densities were falling. There was no equilibrium condition that could be sustainable for too long. Temperatures were in the billions of degrees. There was a mixture of neutrons, protons and electrons that existed by about 13 minutes after the expansion had started. Gamow called this matter 'ylem' after a Webster's Dictionary entry that identified this as an archaic term meaning "the first substance from which the elements were supposed to be made."

A variation on Big Bang cosmology is Brans-Dicke cosmology which is based on a modification to Einstein's general relativity in which a 'scalar field' is added to the usual spacetime metric representing the gravitational field. This 'scalar-tensor' theory of gravity is not supported by careful investigations of general relativity because it requires that gravitational forces change slowly over billions of years. The cosmological implications of Brans-Dicke theory would have their strongest impact on the expansion rate of the universe which would depend not just on time, but on a second physical parameter that depends on the strength of this new scalar field. There would still exist an initial singularity of the kind predicted by Big Bang cosmology.

"We assume that the universe expands from a highly condensed state. It is possible that in the intense gravitational field of this condensed state, matter is created [but] in our view of our present state of ignorance, there seems to be little point in speculating about the process involved. In any case, the creation process lies outside the present [Brans-Dicke] theory.."

Although the initial temperature of the Big Bang is believed to be somewhere near the Planck Temperature of 10³² K, in 1969 Hagedorn proposed that by a temperature of a few trillion degrees, the Big Bang saturates and ceases to grow hotter at earlier times. What happens is that the energy that would have gone into thermal motion goes into creating more and more exotic particles without limit. This quenches the primeval temperature increase as we look back to earlier epochs.

In all of the Big Bang cosmological models spawned by the Friedman solutions to Einstein's equation for gravity, a peculiar state called a "Singularity" is absolutely demanded. This condition occurs at the time of the Big Bang and represents a state in which the curvature of spacetime becomes infinite. This also means that temperature and density also become infinite. Steven Hawking and Roger Penrose developed a powerful set of theorems in the early 1970's that proved that this condition is absolutely unavoidable in homogeneous universes such as our own. All world lines terminate on the initial spacetime Singularity. Every scrap of matter in the universe can be traced back in time to its emergence from this state of lethal curvature, and in the future if our universe is destined to re-collapse, a similar condition will be unavoidable.

Related EoC Articles

• Universe
• Universe: Chinese Genesis
• Universe: Cold Big Bang
• Universe: Creation
• Universe: Cyclical
• Universe: Einstein-DeSitter
• Universe: Expansion
• Universe: Friedman
• Universe: Higher Dimensions
• Universe: Hot Big Bang
• Universe: Infinity
• Universe: Many-Worlds
• Universe: Maori
• Universe: Mixmaster
• Universe: Multiverse
• Universe: Mythology
• Universe: Oscillating
• Universe: Platonic
• Universe: Pre-relativistic
• Universe: Scandinavian
• Universe: Static
• Universe: Steady State
• Universe: Watery Abyss
• Universe: Zuni Indian

External Links

• Cosmic Background Explorer (COBE) - Developed by NASA's Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe to the limits set by our astrophysical environment. NASA.
• Wilkinson Microwave Anisotropy Probe (WMAP) - A NASA Explorer mission that launched June, 2001 to make fundamental measurements of cosmology – the study of the properties of our universe as a whole. WMAP has been stunningly successful, producing our new Standard Model of Cosmology. WMAP continues to collect high quality scientific data. NASA.

Preview Image

Artists' concept of the initial "big bang." (Source: NASA.)

Supermassive black holes have generally been recognized as the most destructive force in nature. But in recent years, they have undergone a dramatic shift in paradigm. These objects may have been critical to the formation of structure in the early universe, spawning bursts of star formation and nucleating proto-galactic condensations. Possibly half of all the radiation produced after the Big Bang may be attributed to them, whose number is now known to exceed 300 million. The most accessible among them is situated at the Center of Our Galaxy. Here, we will examine the evidence that has brought us to this point, and we will understand why many expect to actually image the event horizon of the Galaxy's central black hole within this decade.

The supermassive black hole story begins in 1963, at the Mount Palomar observatory, where Schmidt (1963) was pondering over the nature of a star-like object with truly anomalous characteristics.

Meanwhile, at the University of Texas, Kerr (1963) was making a breakthrough discovery of a solution to Einstein's field equations of general relativity. Kerr's work would eventually produce a description of space and time surrounding a spinning black hole, now thought to power the compact condensations of matter responsible for producing the mystery on Schmidt's desk that year.

The development and use of radio telescopes in the 1940s had led to the gradual realization that several regions of the sky are very bright emitters of centimeter-wavelength radiation.

In the early 1960's, the British astronomer Cyril Hazard's idea of using lunar occultation to determine with which, if any, of the known visible astronomical objects the emitter of centimeter-wavelength radiation was associated, lead to the successful identification of 3C 273 as a star-like object in Virgo. Its redshifted lines, however, indicated that this was not a star at all, but rather an object lying at great cosmological distances. A recent image of this historic source was made with the Chandra X-ray telescope, and is shown in Figure 1.

3C 273's total optical output varies significantly in only 10 months or so, implying that its size cannot exceed a few light-years—basically the distance between the Sun and its nearest stellar neighbor. So it was clear right from the beginning that the quasar phenomenon must be associated with highly compact objects. Even more impressively, their X-ray output has now been seen to vary in a matter of only hours, corresponding to a source size smaller than Neptune's orbit. Each quasar typically releases far more energy than an entire galaxy, yet the central engine that drives this powerful activity occupies a region smaller than our solar system.

The idea that such small volumes could be producing the power of a hundred billion Suns led to their early identification as radiative manifestations of supermassive black holes (see, e.g., Salpeter 1964, Zel'dovich and Novikov 1967, and Lynden-Bell 1969). But are they naked—deep, dark pits of matter floating aimlessly across the primeval cosmic soup—or are they attached to more recognizable structures in the early universe?

In recent years, the task of source identification has been made easier using the Hubble Space Telescope (see Figure 2). The most widely accepted view now is that quasars are found in galaxies with active, supermassive black holes at their centers. Because of their enormous distance from Earth, the "host" galaxies appear very small and faint, and are very hard to see against the much brighter quasar light at their center.

Quasars actually reside in the nuclei of many different types of galaxy, from the normal to those highly disturbed by collisions or mergers. A supermassive black hole at the nucleus of one of these distant galaxies "turns on" when it begins to accrete stars and gas from its nearby environment; the rate at which matter is converted into energy can be as high as 10 solar masses per year. So the character and power of a quasar depend in part on how much matter is available for consumption. Disturbances induced by gravitational interactions with neighboring galaxies can trigger the infall of material toward the center of the quasar host galaxy (see Figure 3). However, many quasars reside in apparently undisturbed galaxies, and this may be an indication that mechanisms other than a disruptive collision may also be able to effectively fuel the supermassive black hole residing at the core.

Some supermassive black holes may not be visible as quasars at all, but rather just sputter enough to become the fainter galactic nuclei in our galactic neighborhood. Our Milky Way galaxy and our neighbor, the Andromeda galaxy, harbor supermassive black holes with very little nearby plasma to absorb. The question concerning how the undisturbed galaxies spawn a quasar is still not fully answered. Perhaps the Next Generation Space Telescope, now under development and expected to fly soon after 2010, will be able to probe even deeper than the Hubble Space Telescope has done, and expose the additional clues we need to resolve this puzzle.

The Quasar/Supermassive Black Hole Census

By now, some 15,000 distant quasars have been found, though the actual number of supermassive black holes discovered thus far is much greater. Because of their intrinsic brightness, the most distant quasars are seen at a time when the universe was a mere fraction of its present age, roughly one billion years after the Big Bang. The current distance record is held by an object found with the Sloan Digital Sky Survey (SDSS), with a redshift of about 6.3, corresponding to a time roughly 800 million years after the Big Bang.

The SDSS has shown that the number of quasars rose dramatically from a billion years after the Big Bang to a peak around 2.5 billion years later, falling off sharply at later times toward the present. Quasars turn on when fresh matter is brought into their vicinity, and then fade into a barely perceptible glimmer not long thereafter.

However, not all the supermassive black holes in our midst have necessarily grown through the quasar phase. Quasars typically have masses of around one billion solar masses. Yet the black hole at the center of our galaxy is barely 3.4 million times the mass of our Sun. In other words, not all the supermassive black holes in our vicinity are dormant quasars.

A recent discovery suggests how some of these "smaller" black holes may have gotten their start. Chandra has identified what appears to be a mid-sized black hole 600 light-years from the center of M82 (Matsumoto et al. 2001). With a mass of about 500 Suns, this object could conceivably sink to the center of M82, and then grow to become a supermassive black hole in its own right, without having passed through the rapid accretion phase of a quasar.

So the class of known quasars may be a good tracer of supermassive black holes, but it clearly does not encompass all of them. Taking advantage of two patches of sky relatively devoid of nearby objects, Chandra produced two of the deepest images ever made of the distant cosmos at X-ray energies, one in the southern hemisphere and the other in the north—the latter, called the Chandra Deep Field North, is shown in Figure 4. Based on the number of suspected supermassive black holes in these images, one infers an overall population of about 300 million throughout the cosmos.

And yet, these X-ray detections speak only of those particular supermassive black holes whose orientation facilitates the transmission of their high-energy radiation toward Earth. Their actual number must be higher than this; indeed, there is now growing evidence that many—perhaps the majority—of the supermassive black holes in the universe are obscured from view. The faint X-ray background pervading the intergalactic medium has been a puzzle for many years. Unlike the cosmic microwave background radiation left over from the Big Bang, the photons in the X-ray haze are too energetic to have been produced at early times. Instead, this radiation field suggests a more recent provenance associated with a population of sources whose overall radiative output may actually dominate over everything else in the cosmos. Stars and ordinary galaxies simply do not radiate profusely at such high energy, and therefore cannot fit the suggested profile.

A simple census shows that in order to produce such an X-ray glow with quasars alone, for every known source there ought to be ten more obscured ones. This would also mean that the growth of most supermassive black holes by accretion is hidden from the view of optical, UV, and near infrared telescopes. Fabian et al. (2000) have reported the discovery of an object they call a Type-2 quasar. Invisible to optical light telescopes, the nucleus of this otherwise normal looking galaxy betrayed its supermassive guest with a glimmer of X-rays. The implication is that many more quasars, and their supermassive black-hole power sources, may be hidden in otherwise innocuous-looking galaxies.

And so, the all-pervasive X-ray haze, in combination with the discovery of gas-obscured quasars, now point to supermassive black holes as the agents behind perhaps "half" of all the universe's radiation produced after the Big Bang. Ordinary stars no longer monopolize the power as they had for decades prior to the advent of space-based astronomy.

Black Holes in the Nuclei of Normal Galaxies

Much closer to Earth, within hundreds of thousands of light-years as opposed to the 11 billion-light-year distance to the farthest quasars, supermassive black holes accrete at a lower rate than their quasar brethren and are therefore much fainter. An archetype of this group, Centaurus A, graces the southern constellation of Centaurus at a distance of 11 million light-years (see Figure 5). At the center of the dark bands of dust, HST recently uncovered a disk of glowing, high-speed gas, swirling about a concentration of matter with the mass of two million Suns. This enormous mass within the central cavity cannot be due to normal stars, since these objects would shine brightly, producing an intense optical spike toward the middle, unlike the rather tempered look of the infrared image shown here.

Centaurus A is apparently funneling highly energetic particles into beams perpendicular to the dark strands of dust. It may therefore have much in common with the X-ray jet-producing black hole in 3C 273 (see Figure 1), and another well-known active galactic nucleus, Gygnus A, shown in Figure 6.

The luminous extensions in this figure project out from the nucleus of Cygnus A, an incredible distance three times the size of the Milky Way. Yet located 600 million light-years from Earth, they cast an aspect only one-tenth the diameter of the full moon. Radio and X-ray observations show that objects such as this accelerate plasma to relativistic speeds on scales of 10 to 100 Schwarzschild radii (see Figure 7), and that these jets are not rare.

An important conclusion to draw from the morphology of jets like those in Cygnus A is that the process responsible for their formation must be stable for at least as long as it takes the streaming particles to journey from the center of the galaxy to the extremities of the giant radio lobes. Evidently, these pencil-thin jets of relativistic plasma have retained their current configuration for over one million years. The most conservative view regarding the nature of these objects is that a spinning black hole is ultimately responsible for this activity. The axis of its spin functions as a gyroscope, whose direction determines the orientation of the jets. Although the definitive mechanism for how the ejection takes place is yet to be determined, almost certainly the twisting motion of magnetized plasma near the black hole's event horizon is causing the expulsion. The Kerr spacetime, which describes the dragging of inertial frames about the black hole's spin axis, provides a natural setting for establishing the preferred direction for this ejective process.

Weighing Supermassive Black Holes

Black hole masses are measured with a variety of techniques, though all have to do with the dynamics of matter within their gravitational influence. Knowledge of the radiating plasma's distance from the central object, and the force required to sustain its motion at that distance, is sufficient for us to extract the central mass.

One of the more compelling applications of this technique has been made to the spiral galaxy NGC 4258. Using global radio interferometry, Miyoshi et al. (1995) observed a disk of dense molecular material orbiting within the galaxy's nucleus at speeds of up to 650 miles per second. This disk produces sufficient radiation to excite condensations of water molecules, leading to strong maser emission at radio wavelengths. The disk within which these water molecules are trapped is small compared to the galaxy itself, but it happens to be oriented fortuitously so that beams of microwaves are directed along our line-of-sight.

The maser clouds appear to trace a very thin disk, with a motion that follows Kepler's laws to within one part in 100, reaching a velocity (inferred from the Doppler shift of the lines) of about 650 miles per second at a distance of 0.5 light-years from the center. The implied central mass is 35 to 40 million solar masses, concentrated within 0.5 light years of the center in NGC 4258. This points to a matter density of at least 100 million Suns per cubic light-year. If this mass were simply a highly concentrated star cluster, the stars would be separated by an average distance only somewhat greater than the diameter of the solar system, and with such proximity, they would not be able to survive the inevitable catastrophic collisions and mergers with each other. Because of the precision with which we can measure this concentration of dark mass, we regard the object in the nucleus of NGC 4258 as one of the two most compelling supermassive black holes now known, the other being the object sitting at the center of our own galaxy, about which we will have more to say shortly.

The fortuitous arrangement of factors that permits the use of this particular technique does not occur often, however, so other methods must be used. Often, clouds of gas orbiting the nucleus are irradiated by the central engine, and they in turn produce a spectrum with emission lines indicative of the plasma's ionized state. The method used to determine the distance of these ionized clouds from the black hole is known as reverberation. By monitoring the light emitted by the supermassive black hole and, independently, the radiation from its halo of irradiated clouds of gas, one can determine when a variation in the radiative output has occurred. When the quasar varies its brightness, so does the surrounding matter—but only after a certain time delay. The lag is clearly due to the time it took the irradiating light from the center to reach the clouds, and using the speed of light, this delay provides a measure of the distance between the nucleus and the orbiting plasma. Again, this procedure provides the speed of matter and its distance from the center, which together yield a determination of the gravitating mass.

Having said this, the best mass determination one can make is still based on kinematic studies of stars orbiting the central object. The center of our Galaxy is close enough for this method to work spectacularly. Known as Sagittarius A*, the black hole at the center of the Milky Way may not be the most massive, nor the most energetic, but it is by far the closest, only 28,000 light-years away. Figure 8 shows an infrared image of Galactic center produced recently with the 8.2-meter VLT YEPUN telescope at the European Southern Observatory in Paranal, Chile. The image we see here is sharp because of the use of adaptive optics, in which a mirror in the telescope moves constantly to correct for the effects of turbulence in the Earth's atmosphere. Sagittarius A* is so close to us compared to other supermassive black holes, that on an image such as this, we can identify individual stars orbiting a mere seven to 10 light-days from the source of gravity. In the nucleus of Andromeda, the nearest major galaxy to the Milky Way, the best we could do right now is about two light-years.

In the Galactic center, stars orbit Sagittarius A* at speeds of up to five million kilometers per hour. This motion is rapid, in fact, that we can easily detect their proper motion on photographic plates taken only several years apart. Some of them complete an orbit about the center in only 15 years (Schoedel, Ott, Genzel, et al. 2002). In the middle of the photograph in Figure 8, it appears that one of the fainter stars—designated as S2—lies right on top of the position where the black hole is inferred to be. S2 is an otherwise "normal" star, though some 15 times more massive and seven times larger than the Sun. This star, S2, has now been tracked for over ten years and the loci defining its path trace a perfect ellipse with one focus at the position of the supermassive black hole. This photograph, taken near the middle of 2002, just happens to have caught S2 at the point of closest approach (known as the perenigricon), making it look like it was sitting right on top of the nucleus.

At perenigricon, S2 was a mere 17 light-hours away from the black hole—roughly three times the distance between the Sun and Pluto, while traveling with a speed in excess of 5,000 kilometers per second, by far the most extreme measurements ever made for such an orbit and velocity. We infer from these data that the mass of Sagittarius A* is 3.4 million solar masses, compressed into a region no bigger than 17 light-hours. For this reason, Sagittarius A*, and the central object in NGC 4258, are considered to be the most precisely "weighed" supermassive black holes yet discovered.

The Formation of Supermassive Black Holes

An increasingly important question being asked in the context of supermassive black holes is which came first, the central black hole, or the surrounding galaxy? Quasars seem to have peaked 10 billion years ago, early in the universe's existence. The light from galaxies, on the other hand, originated much later—after the cosmos had aged another 2 to 4 billion years. Unfortunately, both measurements are subject to uncertainty, and no one can be sure we are measuring "all" of the light from quasars and galaxies, so this argument is not quite compelling. But we do see quasars as far out as we can look, and the most distant among them tend to be the most energetic objects in the universe, so at least "some" supermassive black holes must have existed near the very beginning. At the same time, images such as Figure 3 provide evidence of mergers of smaller structures into bigger aggregates, but without a quasar. Perhaps not every collision feeds a black hole or, what is more likely, at least some galaxies must have formed first. Several scenarios for the formation of supermassive black holes are currently being examined. In every case, growth occurs when matter condenses following either the collapse of massive gas clouds, or the catabolism of smaller black holes in collisions and mergers.

All of the structure in the universe traces its beginnings to a brief era shortly after the Big Bang. Very few "fossils" remain from this period; one of the most important is the cosmic microwave background radiation. The rapid expansion that ensued lowered the matter density and temperature, and about one month after the Big Bang, the rate at which photons were created and annihilated could no longer keep up with the thinning plasma. The radiation and matter began to fall out of equilibrium with each other, forever imprinting the conditions of that era onto the radiation that reaches us to this day from all directions in space.

We now know that the temperature anisotropies are smaller than one part in a thousand, a limit below which density perturbations associated with ordinary matter would not have had sufficient time to evolve freely into the nonlinear structures we see today. Only a gravitationally dominant dark-matter component could then account for the strong condensation of mass into galaxies and supermassive black holes.

The thinking behind this is that whereas the cosmic microwave background radiation interacted with ordinary matter, it would retain no imprint at all of the dark matter constituents in the universe. The nonluminous material could therefore be condensed unevenly (sometimes said to be "clumped") all the way back to the Big Bang and we simply wouldn't know it.

The first billion years of evolution following the Big Bang must have been quite dramatic in terms of which constituents in the universe would eventually gain primacy and lasting influence on the structure we see today. The issue of how the fluctuations in density, mirrored by the uneven cosmic microwave background radiation, eventually condensed into supermassive black holes and galaxies is currently a topic of ongoing work. This question deals with the fundamental contents of the universe, and possibly what produced the Big Bang and what came before it. The evidence now seems to be pointing to a coeval history for these two dominant classes of objects—supermassive black holes and galaxies—though as we have already noted, at least some of the former must have existed quite early. One possibility proposed by Balberg and Shapiro (2002) is that the first supermassive objects formed from the condensation of dark matter alone; only later would these seed black holes have imposed their influence on the latter. But this dark matter has to be somewhat peculiar, in the sense that its constituents must be able to exchange heat with each other. As long as this happens, a fraction of its elements evaporate away from the condensation, carrying with them the bulk of the energy, and the rest collapse and create an event horizon. The net result is that the inner core of such a clump forms a black hole, leaving the outer region and the extended halo in equilibrium about the central object. Over time, ordinary matter gathers around it, eventually forming stars, and planets.

Ordinary matter could not have achieved this early condensation because it simply wasn't sufficiently clumped initially. Perhaps this material formed the first stars, followed by more stars, eventually assembling a cluster of colliding objects. Over time, the inner core of such an assembly would have collapsed due to the evaporation of some of its members and the ensuing loss of energy into the extended halo, just as the dark matter did (see, e.g., Haehnelt and Rees 1993). A seed black hole might have formed in the cluster's core. Estimates show that once formed, such an object could have doubled its mass every 40 million years, so over the age of the universe, even a modestly appointed black hole could have grown into a billion-solar-mass object. The problem is that this could not have happened in only 700 million years, when the first supermassive black holes appeared.

Yet another method leading to black hole growth results from ongoing collisions between galaxies, which eventually lead to the merger of the black holes themselves. An example of such a process occurring right now is shown in Figure 9.

Almost every large, normal galaxy harbors a supermassive black hole at its center. Some evidence for this has been provided by a recently completed survey of 100 nearby galaxies using the VLA, followed by closer scrutiny with the Very Long Baseline Interferometry array. At least 30 percent of this sample showed tiny, compact central radio sources bearing the unique signature of the quasar phenomenon. Of the hundreds of millions of supermassive black holes seen to pervade the cosmos, none of them appear to be isolated. And even more compelling is the work of Kormendy and Richstone (1995), who set about the task of systematically measuring as many black hole masses as is currently feasible. Direct measurements of supermassive black holes have been made in over 38 galaxies, based on the large rotation and random velocities of stars and gas near their centers.

These objects are all relatively nearby because these direct methods don't work unless we can see the individual stars in motion about the central source of gravity. Curiously, none of the supermassive black holes have been found in galaxies that lack a central bulge (Figure 10). Galaxies with a central bulge may have undergone one or more mergers in their past. Thus, a collision like that seen in Figures 3 and 11 may have been required to create a central supermassive object. 

Black Hole Animation

"Animation of Star Ripped Apart by Giant Black Hole"
This animation shows a yellow star that travels too close to a giant black hole in the center of the galaxy RX J1242-11. As it nears, the star is stretched by tidal forces from the black hole and is quickly torn apart. Most of the yellow gaseous debris from the star escapes the black hole in parabolic orbits. However, a small amount of material is captured by the black hole and then forms a rotating disk of gas. X-rays are emitted as the gas in the disk is heated (as shown by the blue color) and is gradually swallowed by the black hole, eventually emptying the disk.

QuickTime: High Res (25.3 MB), Low Res (5.1 MB).
MPEG: High Res (8.9 MB), Low Res (6.7 MB).
All open in new browser window. Run Time: 0:43.
(Source: NASA-Chandra Observatory.)

References

• Balberg, S., and S. L. Shapiro (2002). Gravothermal Collapse of Self-Interacting Dark Matter Halos and the Origin of Massive Black Holes, "Physical Review Letters" 88, 101301.1.
• Bower, G. A., D. O. Richstone, G. D. Bothun, and T. M. Heckman (1993). A Search for Dead Quasars Among Nearby Luminous Galaxies. I. The Stellar Kinematics in the Nuclei NGC 2613, NGC 4699, NGC 5746, and NGC 7331, "Astrophysical Journal" 402, 76
• Fabian, A. C., et al. (2000). Testing the Connection Between theX-ray and Submillimeter Source Populations Using Chandra,"Monthly Notices of the Royal Astronomical Society" 315, L8.
• Haehnelt, M. G., and M. J. Rees (1993). The Formation of Nuclei inNewly Formed Galaxies and the Evolution of the Quasar Population, "Monthly Noticesof the Royal Astronomical Society" "263, 168.
• Kerr, R. P. (1963). Gravitational Field of a Spinning Mass as an Example ofAlgebraically Special Metrics, "Physical Review Letters" 11, 237.
• Kormendy, J., and D. Richstone (1995). Inward Bound—The Search forSupermassive Black Holes in Galactic Nuclei, "Annual Reviews of Astronomy and Astrophysics" 33, 581.
• Lynden-Bell, D. (1969). Galactic Nuclei as Collapsed Old Quasars,"Nature" 223, 690.
• Matsumoto, H., et al. (2001). Discovery of a Luminous, Variable, Off-Center Source inthe Nucleus of M82 with the Chandra High-Resolution Camera, "Astrophysical JournalLetters" 547, L25.
• Miyoshi, M., et al. (1995). Evidence for a Black Hole from High RotationVelocities in a Sub-Parsec Region of NGC 4258, "Nature" 373, p. 127.
• Salpeter, E. E. (1964). Accretion of Interstellar Matter by Massive Objects,"Astrophysical Journal" 140, 796.
• Schmidt, M. (1963). 3C 273: A Star-like Object with Large Redshift, "Nature" 197, 1040.
• Schoedel, R., T. Ott, R. Genzel, et al. (2002). A Star in a 15.2-yearOrbit Around the Supermassive Black Hole at the Centre of the Milky Way,"Nature" 419, 694.
• Zel'dovich, Ya. B., and I. D. Novikov (1967). The Hypothesis of Cores Retarded During Expansion and the Hot Cosmological Model,"Soviet Astronomy" 10, 602.

Further Reading

• "The Black Hole at the Center of Our Galaxy" by Fulvio Melia, Princeton University Press, 2003.
• "The Edge of Infinity: Supermassive Black Holes in the Universey" by Fulvio Melia, Cambridge University Press, 2003.
• "The Galactic Supermassive Black Hole" by Fulvio Melia, Princeton University Press, 2007.

External Links

• Chandra X-ray Center & Telescope - Operated for NASA by the Smithsonian Astrophysical Observatory/Harvard-Smithsonian Center for Astrophysics.
• Visit Fulvio Melia's Website for more information and other links.

Preview Image

"Black Hole Gobbles a Star" - The illustration above depicts a supermassive black hole ripping apart a star and consuming a portion of it, a long-predicted astronomical event confirmed by NASA's Chandra and the European Space Agency's XMM-Newton X-ray Observatories. Astronomers believe a doomed star came too close to a giant black hole after being thrown off course by a close encounter with another star. As it neared the enormous gravity of the black hole, the star was stretched by tidal forces until it was torn apart. This discovery provides crucial information about how these black holes grow and affect surrounding stars and gas.  (Illustration-Source: NASA/CXC/M.Weiss.)

Galaxies range in size from the smallest dwarf galaxies only a few hundred light-years across with just a few million stars, through normal galaxies like our own Milky Way Galaxy, with a few hundred billion stars, to giant ellipticals spanning over hundreds of thousands of light-years and containing several trillion stars.

Various categorization schemes have been devised to bring order to the galactic zoo by pigeonholing galaxies according to one or more properties, including shape, spectrum, and luminosity.

Most galaxy formation is thought to have taken place in the early universe, beginning less than half a billion years after the Big Bang.

Galaxies may be solitary, or in small groups like our Local Group, or in larger clusters of galaxies. In rich clusters of galaxies, the brightest systems tend to be ellipticals and lenticulars, with spirals making up only 5 to 10% of the population. However, the proportion of spirals in these clusters was probably higher in the past, galaxy cannibalism and galaxy mergers having turned them into the more amorphous types. In low-density environments, spirals account for about 80% of the bright galaxy count.

See also:

• Galaxies - Detail
• Galaxies - Detail, Expanded

Click the link below for more details.   
 

Spiral galaxies are a subset of galaxies, tracing the late stages of the Hubble classification. Spiral Galaxies are defined by their thin disks, thick central bulges, the presence of spiral arms, and blue colors.

Characteristics of Spiral Galaxies:

• Spiral galaxies constitute ¾ of the total population of galaxies in the field. This fraction changes both with redshift and the local environment density. (see Morphology-Density Relation)
• Typical lengths: ~ 1-50 kpc.
• Masses: 10⁹ – 10¹² solar masses.
• Star formation rates: ~1-5 solar masses/year.
• M_V ~ -16 to -23.
• Peak rotational velocities: ~ 150-300 km/s.
• They exhibit both old and young stellar populations.
• They are composed of both dynamically cold and hot stars. The former follow elliptical or near-circular orbits in the disk with little random motions, while the latter follow more chaotic orbits with high dispersions in the bulge.
• Most spiral arms trail the galaxy's direction of rotation.
• More luminous galaxies have higher rotational velocities; this is the basis of the Tully-Fischer relation.

Basic Components of Spiral Galaxies:

Disk: The disk contains metal-rich stars and ample interstellar medium (ISM); disk components are generally dynamically cold. Disks are also sites of spiral arms and their triggered star formation regions. Consequently, spiral arms are especially prominent when viewed in blue and ultraviolet light, which characteristic of the arms' younger, luminous stellar populations.

Bulge: The central bulge is composed of metal-poor to extremely metal-rich, dynamically hot stars.

Bar: These flat, linear structures of stars and ISM are present in about 50% of spiral galaxies. They tend to be as thick as the disk, and have length/width ratios on the order of 5:1. Additionally, bar endpoints can be low-shear environments where triggered star formation may also occur.

Nucleus: Nuclei are the innermost, most dense regions of spiral galaxies; they are thought to contain supermassive black holes or intense starbursting regions at their very centers.

Halo: The low density environment surrounding the galactic disk, halos contain metal-poor stars, globular clusters, and hot, low density ISM.

Dark Matter Halo: The dark halo extends far beyond the visible extent of the galaxy, providing most of the galaxy's mass and controlling its dynamics. We have yet to understand the fundamental composition of this material.

Trends in Spiral Galaxies

While there are really no arms to speak of in the S0 systems, we notice that spiral arms tend to wrap themselves tighter as the Hubble classification proceeds from Sa to Sm. This is no surprise; Hubble based his classification scheme on this phenomenon. Furthermore, systems with an abundance of gas are more likely to develop the flocculent structures (as in NGC 2841) resulting from previous generations of supernovae pushing bubbles of star-forming material outward into the ISM for the next cycle of star birth. Otherwise, the trend compliments a generally increasing supply of gas and dust from Sa to Sm.

Moreover, galaxies tend to get bluer and fainter as they progress from early type to late type spiral. This has much to do with mass and stellar composition; early types are the most massive, showing absorption lines indicative of cooler K and M stars, while the late types are smaller, and display H and K lines of ionized Calcium, among other spectral and photometric indicators of young, blue stars, especially in the active spiral regions.

Rotation Curves and Dark Matter

Thanks to the Virial Theorem, if we can measure an object’s rotational velocity while knowing its radius, we have a measure of the mass internal to that particular radius. In the case of spiral galaxies, we find that this velocity is constant (past a certain distance) as the radius of the galaxy increases, so the mass internal to that radius must also increase. This inferred mass exceeds what we directly observe by as much as an order of magnitude in some galaxies. This unobserved, yet necessary, mass may be provided by dark matter.

In practice, astronomers obtain a rotation curve by either observing the motions of HI (neutral hydrogen) or CO (carbon monoxide) gas at their characteristic radio emission lines out to a galaxy's edge. We can model a so-called spider diagram, whose velocity contours are close to those measured in HI or CO and thereby obtain a galaxy's velocity as a function of radius as well as its inclination. This provides a measure of the total mass of the system. 

References

• "Galaxies - 2: Spirals" by Gronwall & Caryl. Penn State Astronomy and Astrophysics, State College, PA. 2008.
• "Galaxies in the Universe" by Sparke, Linda, S., Gallagher & John S. Cambridge University Press, New York, NY, 379 pp. 2005.
• "Normal Galaxies - The Large-Scale Structure of the Universe,"  James Brau, Knight Professor of Natural Science, Ph.D., Department of Physics, University of Oregon.
• "The Hubble Tuning Fork — Classification of Galaxies" from "Starry Bulges Yield Secrets to Galaxy Growth," HubbleSite News Release Number: STScI-1999-34, October 6, 1999.

Related EoC Articles

• Hubble Classification - Hubble Tuning Fork Diagram
• Spiral Arm Structure

External Links

• Edwin Powell Hubble (1889-1953) – Hubble Space Telescope, NASA.
• "Galactic Astronomy" by Binney, James, and Merrifield, Michael, 1998. Princeton University Press, Princeton, NJ, 796 pp.
• Hubblesite. 2008

Preview Image

"Barred Spiral Galaxy NGC 1672" -  Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 1672, pictured above, was captured in spectacular detail in this recently released image taken by the orbiting Hubble Space Telescope. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 60 million years to reach us from NGC 1672, which spans about 75,000 light years across. NGC 1672, which appears toward the constellation of the Swordfish (Dorado), is being studied to find out how a spiral bar contributes to star formation in a galaxy's central regions. View full-size image.  (Source: NASA/ESA/Hubble Heritage (STScI/AURA). Image Credit: L. Jenkins (GSFC/U. Leicester).)

The structure of arms in spiral galaxies originates with their differential rotation. Spiral galaxies may have a grand design structure, with long, continuous, symmetric arms, or flocculent structure, with short spiral arm pieces, or multiple arms, which exhibit grand design structure in the inner regions but branch to many arms in the outer regions.

Grand Design Arms

Grand design arms are caused by spiral density waves, which are sinusoidal perturbations that ripple through the disk. They may move from the outer regions inward, or the inward regions outward. Companions and internal bars both trigger waves. The wave pattern rotates as a solid body, while the arms rotate diffierentially, at a higher angular rate in the inner disk than in the outer disk. At corotation, stars and gas rotate at the same angular speed as the pattern. Waves are reinforced at corotation, and may bounce between a point outside the Inner Lindblad resonance (where the difference between the disk's angular speed and the pattern's angular speed is equal to the epicyclic period divided by the number of arms) and corotation. In this case standing waves are established, so the overall structure is governed by modes. The waves are destroyed at the Outer Lindblad resonance, which is close to R25 (where the surface brightness is 25 magnitudes per arcsecond squared). In grand design galaxies, both old and young stars are organized by the waves, so the arms are evident in both blue and infrared filters. Multiple arms may result from a superposition of 2-arm and 3-arm patterns.

Flocculent Structure

Flocculent structure may also result from overlapping modes. Alternatively, flocculent structure may be the result of sheared star formation sites (with stochastic self-propagating star formation) that trace out short spiral arcs in a differentially rotating disk. In this case, the spiral structure only manifests itself at blue wavelengths, tracing out the high mass (short-lived) stars.

External Links:

The NASA/Infrared Processing and Analysis Center - Extragalactic Database ("NED") is built around a master list of extragalactic objects for which cross-identifications of names have been established, accurate positions and redshifts entered to the extent possible, and some basic data collected.  The images in the external links below are from the NED collection.

Bibliographic references relevant to individual objects have been compiled, and abstracts of extragalactic interest are kept on line. Detailed and referenced photometry, position, and redshift data, have been taken from large compilations and from the literature. NED also includes images from 2MASS, from the literature, and from the Digitized Sky Survey. NED's data and references are being continually updated, with revised versions being put on-line every 2-3 months.

The NASA/IPAC Extragalactic Database is operated by the Jet Propulsion Laboratory, and the California Institute of Technology, under contract with the National Aeronautics and Space Administration.

• NED's features has more information on NED's many available features.
   
• Guide to NED Image Results - NASA/IPAC Extragalactic Database.
   
• SIMBAD– Data and references for Galactic objects may be retrieved from SIMBAD (Set of Identifications, Measurements, and Bibliography for Astronomical Data), maintained by Centre de Données Astronomiques de Strasbourg, France.
   
• Planetary Data System– Similarly, solar system and planetary data (e.g. for Mars or for Halley's Comet) may be retrieved from NASA's Planetary Data System (PDS) at JPL.
   
• CDS (Centre données astronomiques de Strasbourg)  – Many of the individual catalogs loaded into NED are available from CDS at the Strasbourg Astronomical Observatory, Strasbourg, France.
  

Grand Design Galaxy Examples:

• M51 – NASA/IPAC Extragalactic Database
• M81 – NASA/IPAC Extragalactic Database

Flocculent Spiral Galaxy Examples:

• NGC5055 – NASA/IPAC Extragalactic Database
• NGC7793 – NASA/IPAC Extragalactic Database

Multiple Arm Galaxy Examples:

• NGC5457 – NASA/IPAC Extragalactic Database
• NGC6946 – NASA/IPAC Extragalactic Database

Preview Image

The "Whirlpool Galaxy," Messier 051, also known as (NGC 5194).  An example of a "Grand Design Galaxy."  A multicolor optical image taken by the Hubble Space Telescope.  (View full-size image.)  From the NASA/IPAC Extragalactic Database collection.  A close-up view of the Whirlpool Galaxy was the February 19, 2006 NASA Astronomy Photo of the Day.

FIGURE CAPTION – The three pictured galaxies -- NGC 7173 (middle left), NCG 7174 (middle right) and NGC 7176 (lower right) -- are part of the Hickson Compact Group 90, named after astronomer Paul Hickson, who first catalogued these small clusters of galaxies in the 1980s. NGC 7173 and NGC 7176 appear to be smooth, normal elliptical galaxies without much gas and dust. In stark contrast, NGC 7174 is a mangled spiral galaxy, barely clinging to independent existence as it is ripped apart by its close neighbours. The strong tidal interaction surging through the galaxies has dragged a significant number of stars away from their home galaxies. These stars are now spread out, forming a tenuous luminous component in the galaxy group. (Credit: NASA, ESA and R. Sharples (University of Durham, U.K.))

About 100 million light-years away, in the constellation of Piscis Austrinus (the Southern Fish), three galaxies are playing a game of gravitational give-and-take that might ultimately lead to their merger into one enormous entity.

A new image from the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope allows astronomers to view the movement of gases from galaxy to galaxy, revealing the intricate interplay among them.  
 
The three pictured galaxies — NGC 7173 (middle left), NCG 7174 (middle right) and NGC 7176 (lower right) — are part of the Hickson Compact Group 90, named after astronomer Paul Hickson, who first catalogued these small clusters of galaxies in the 1980s. NGC 7173 and NGC 7176 appear to be smooth, normal elliptical galaxies without much gas and dust

In stark contrast, NGC 7174 is a mangled spiral galaxy, barely clinging to independent existence as it is ripped apart by its close neighbours. The strong tidal interaction surging through the galaxies has dragged a significant number of stars away from their home galaxies. These stars are now spread out, forming a tenuous luminous component in the galaxy grou

Ultimately, astronomers believe that the stars in NGC 7174 will be redistributed into a giant 'island universe', tens to hundreds of times as massive as our own Milky Way.

Notes for editors:

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

For more information:

Colleen Sharkey, Hubble/ESA, Garching, Germany

E-mail: Csharkey @ eso.org

About 10 percent of Active Galactic Nuclei produce powerful outflows which are launched with relativistic (i.e. close to the speed of light) bulk flow velocities. These outflows contain magnetic fields and relativistic particles which produce emission at radio and sometimes optical and X-ray wavelengths via the synchrotron process. Two oppositely directed outflows or jets seem to be generated along the rotation axis of the accretion disk. The jets push their way out through the [Interstellar Medium] of the host galaxy and propagate out to distances of tens to hundreds of kpc.

Types of Radio Galaxies

Radio Galaxies tend to divide into two main types based on their appearance (morphology) and their radio luminosity as first discussed by Fanaroff and Riley in 1974. The lower luminosity sources (FR type I) tend to have jets which are brightest near the AGN and then expand in width and fade in brightness as they propagate, eventually forming faint, diffuse, plume-like structures. In the more luminous (FR type II) sources, the jets tend to be faint, and are often too faint to be detected, but end in bright compact regions called hot spots. Surrounding the jets are often diffuse cocoons of emission. We believe that the main parameter responsible for this difference is the jet Mach number. In the current paradigm, all radio galaxies are launched with similar properties, but the FRIs start to decelerate possibly due to entrainment of ambient gas on scales of tens of parsecs to a kpc or so. The FRIs then become transonic flows which transition into buoyant plumes. On the other hand, the jets in the FRIIs remain relativistic and supersonic all the way to their end. The supersonic jets end at a strong shock at the location of the working surface between the jet and the ambient medium. The jet material goes through the shock and then travels sideways filling up a large cocoon with shocked jet material.

Preview Image

In this radio image, two jets shoot out of the center of active galaxy Cygnus A. GLAST may solve the mystery of how these jets are produced and what they are made of.  (Source: NASA. Credit: NRAO.)

The Einstein gravitational theory predicts that light and all other wavelengths of electromagnetic radiation are bent in the vicinity of any massive body, and it is of course everyday experience that the bending is neglegible in everyday experience. But, in astronomy where vast distances are involved, the bending can be sufficient to distort and displace the image, and in extreme cases can produce multiple images of a single distant source. This phenomenon is called gravitational lensing, and in extreme cases multiple images separated by a few arc-seconds on the sky are found for distant quasars lensed by a foreground galaxy with the mass of the Milky Way galaxy. Since discovery of the first such object by Dennis Walsh in 1979, 80 such objects have been recognized at optical and radio wavelengths. Formation of multiple images is always accompanied by magnification of the source image and amplification of the brightness.

Of course, the magnification properties depend on the mass of the lensing object. For a galaxy lens-mass of a billion suns, the observed separation of the multiple images is approximately an arcsec. But if the lensing galaxy has a grainy mass structure of a typical galaxy made of stars, the individual stars within the galaxy cause further image splitting within the main image with accompanying brightness amplification, in a process known as microlensing. Thus, if the lensed quasar image could be observed in great detail, it would be found to have brightness irregularities caused by the grainy mass distribution within the lens.

Discovery

Because it was understood at the time of the discovery of the first lens in 1979 and that measurement of a time delay associated with the different light paths contained important information about the expansion rate of the universe, several research groups attempted to measure the cosmological time delay. But, soon Chang and Refsdal realized that the microlensing effect could cause important brightness changes that would mask the intrinsic quasar brightness measurements needed to determine the time delay and the expansion rate of the universe. This prediction was confirmed when the first measurement of the time delay revealed the microlensing much as feared, the the subject of time delay estimation in the first object was in a state of confusion for 10 years after its discovery.

During this time, the Einstein Cross quasar also was found to display microlensing behavior, which seemed weaker than expected but was carefully studied even though a time delay would not be measured until 2005 (Vakulik et al. 2005). By then, the microlensing was also seen in 6 more gravitational lens systems.

An unexpected surprise was the detection of a very rapid microlensing, with weak fluctuations lasting just a day. This was immediately recognized to be the signature of an important population of rogue planets in the lens galaxy, since an important signature of small rapid positive and negative amplifications were recognized. For such rapid brightness fluctuations to occur, the source quasar must have finer structure than had been supposed, and further study is allowing new understanding of the optical emitting structures within quasars.

Preview Image

"Finding Planets: Multiple Methods Help Track Elusive Quarry" - Gravitational Microlensing.  This method derives from one of the insights of Einstein's theory of general relativity: gravity bends space. We normally think of light as traveling in a straight line, but light rays become bent when passing through space that is warped by the presence of a massive object such as a star. This effect has been proven by observations of the Sun's gravitational effect on starlight.  When a planet happens to pass in front of a star along our line of sight, the planet's gravity will behave like a lens. This focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star.  Astronomers can use the gravitational microlensing effect to find objects that emit no light or are otherwise undetectable.  (Soure: Cal-Tech/JPL - Planet Quest.)

Some History

Our appreciation of the universe beyond the Milky Way is entirely an achievement of the twentieth century. The objects which we now know to be galaxies had occasionally drawn the curiosity of visual observers from the days of Charles Messier and forwards, particularly William Parsons (the Earl of Rosse), whose 72-inch (1.8-meter) telescope with its speculum-metal mirror had revealed the intriguing spiral forms of certain dim, cloudy objects (nebulae) seen, by and large, far from the encircling band of the Milky Way. However, further tools were to be needed to unravel the true nature of some of these objects.

By the 1920s, photography had revealed that there must be tens of thousands of these objects, by then known as white nebulae to distinguish them from the clearly different gaseous nebulae such as the famous Orion Nebula, accessible to the telescopes of the time. They showed a variety of spiral, elongated, or oval forms. The most plausible theories to account for these nebulae made them either nearby objects – perhaps planetary systems in formation – or extremely distant, truly "island universes" of which our Milky Way, hitherto the entire known Universe, would be merely one among myriads.

The key observation in resolving this dispute came from Edwin Hubble, using the recently completed 100-inch (2.5-meter) telescope on Mount Wilson, California. Targeting the largest and brightest of the "white nebulae," as the ones most likely to be nearby in space, he repeatedly photographed selected portions of them as deeply as the available photographic plates would allow. Faint starlike points had been recognized in these nebulae, but could one show that these were in fact stars such as we know in the solar neighborhood, and thus at the enormous distances required to make them appear so faint?

Hubble's breakthrough came in identifying stars with a particular kind of cyclic change in brightness, which them "standard candles" whose absolute brightness could be determined – Cepheid variable stars. Henrietta Leavitt at Harvard had shown that this class of pulsating stars has the useful property of a tight correlation between the period required to complete one pulsation in surface temperature and size (and thus brightness) and the amount of energy the star gives off (usually expressed as absolute magnitude, the stellar brightness which we would measure if a star were located at a reference distance of ten parsecs). Cepheid variables gave Hubble the first necessary yardstick in the ladder of extragalactic distances. (One of the major programs for the Hubble Space Telescope is the measurement of galaxy distances beyond the reach of ground-based instruments, by identifying Cepheid variables in more and more distant galaxies. One HST team has recently reported success in measuring Cepheids in galaxies of the Virgo Cluster, about twenty times as distant as the galaxies Hubble the astronomer was observing).

This discovery, in one stroke, opened a whole new vista of the Universe. Within a decade, many of the major strands of galaxy research had begun. Clusters and groups were recognized, classification schemes were proposed, and spectroscopic measurements were begun. Spectra of galaxies proved especially rewarding. Early measures by V.M. Slipher at Lowell Observatory, using very delicate multi-night exposures, had shown that some "spiral nebulae" exhibited unusually large Doppler shifts. It eventually developed that galaxies exhibit, on average, a relation between the redshift of features in their spectra and their estimated distances. This gave a way to estimate the distances of ever fainter and more remote galaxies, and provided the first glimpse of an expanding universe.

Kinds of Galaxies

Scientists and naturalists alike have the urge to sort, classify and organize new phenomena, in the hope of seeing underlying patterns that have physical meaning. Several classifications for galaxies were proposed early in their study; Hubble's classification system has proven remarkably robust, correlating well with physically interesting measurements such as stellar content, gas content, and environment despite being designed only to describe the appearance of the galaxy as seen on photographs with blue-sensitive emulsions. With later extensions by Gerard de Vaucouleurs and Sidney van den Bergh, this remains the most commonly used description of galaxy forms.

Elliptical galaxies were denoted by the letter E and a number describing the galaxy's apparent shape – 0 for a completely round form, 5 for one twice as long as wide, and 7 for the apparently flattest genuine ellipticals. We do not know, solely from an image, the true shape of such a galaxy; the same galaxy might have quite different degrees of flattening if viewed from different directions. Elliptical galaxies are, in general, characterized by old stellar populations and very little of the gas and dust needed to form new stars.

Spirals are divided into ordinary and barred spirals; in barred systems the spiral arms arise from a straight "bar" passing through the center, while ordinary spirals have a more S-shaped inner configuration. Ordinary spiral are denoted S and barred systems SB. Both usually contain a central bulge, often sharing many properties with elliptical galaxies, surrounded by a thin rotating disk containing whatever spiral structure there may be. Spirals are subdivided into a sequence jointly defined by the winding and prominence of the spiral arms, and the relative importance of the central bulge. Sa galaxies have a bright bulge and tightly wound arms, while Sc galaxies have loosely wound arms and a relatively less important bulge. This sequence Sa-Sb-Sc-Sd has counterparts SBa-SBb-SBc-SBd in the barred spirals. As more detail was observed in some galaxies, intermediate substeps (Sab,Sbc,Scd,S0/a) could be added when necessary.

Some galaxies show no particular organization, either because some recent event has left them in a disturbed state or because they simply lack the organizing rotation and wave motions of a spiral. These are simply called irregulars; the ones that do not result from external disturbance form, in many respects, an extension beyond Sd of the spiral sequence.

Hubble recognized that the connections among various types left an apparent hole which he called S0 – objects with a bulge and disk, but little or no star formation, dust, or gas. They would form a bridge between ellipticals and spirals. Later, many genuine S0 galaxies were in act recognized, and understanding their origin promises to tell us much about the history and development of galaxies in general.

Several refinements of the Hubble classification have proven widely useful. Gerard de Vaucouleurs introduced distinctions depending on whether the spiral structure proceeds from the nucleus in an S-shape (s) or from an inner ring (r), or some combination (rs) or (sr). He also recognized intermediate cases SAB between barred and nonbarred galaxies. These new dimensions allowed a finer discrimination of galaxy structure and opened the way for more detailed study of the physical properties of spiral galaxies. Sidney van den Bergh noted that the most luminous spirals have long, well-developed spiral arms, and introduced a luminosity classification driven by the organization and distinctness of the arms; Sc I galaxies are in the mean the brightest Sc galaxies, and Sc V the faintest. Note that the classification is based solely on a galaxy's appearance, with its absolute magnitude a correlating quantity.

Some galaxies are not well described by the Hubble system or its variants, even excluding "train wrecks" resulting from galaxy collisions. There exist, usually in rich clusters, enormous elliptical-like systems that may span millions of light-years with more extended outer regions than a similarly huge elliptical would show. These are given the designation cD, from a scheme developed at Yerkes Observatory by W.W. Morgan. Dwarf galaxies may be irregular, elliptical, or spheroidal, depending on their degree of symmetry and central concentration. Recent work has turned up galaxies of very low surface brightness, which must have had a rather different history from familiar spirals. While many of these look like the ghosts of ordinary spirals, it is not at all clear how they connect to the familiar Hubble types.

Content of Galaxies

We observe stars, gas, and dust in galaxies. Stars come in a wide range of age and mass, and are intricately linked to interstellar matter by processes of stellar birth and death. This means that galaxies have a history, which we can probe either by investigating the makeup of a galaxy in detail, or in a kind of fossil probe unique to astronomy, look at galaxies so distant that the light we observe left them when they were much younger than they are "today."

In tracing the makeup of galaxies, there are numerous clues as to the populations of stars present. Different kinds of stars (giant/dwarf, hot/cool, higher/lower abundances of heavy elements) have different patterns and intensities of features in their spectra. In most galaxies, we can observe only their overall (integrated) spectrum, so that a mathematical solution can give constraints on the overall population, but the solution is not completely well-determined without additional assumptions (such a a smoothly varying star-formation rate, or fixed ratios of stars at various masses). To resolve these ambiguities, observations of very nearby galaxies are crucial, where individual (luminous) stars can be observed and counted.

Some components of a galaxy stand out in specific kinds of observations, so that interstellar matter and certain kinds of stars can be studied in isolation. The 21-cm radio emission of cold atomic hydrogen traces this component of a galaxy cleanly, giving one index of its gas content and tracing internal motions beautifully. The gas most immediately associated with the birth of new stars is colder and denser than neutral hydrogen, being mostly molecular hydrogen and best observed via the trace molecule CO, which emits spectral lines in the 1-3 mm range. The most massive young stars emit copious ultraviolet radiation, which may be absorbed by surrounding gas and re-emitted as spectral lines including H-alpha in the visible region, so that using special filters and image processing allows a view of these star-forming regions alone (so long as they are not hidden from view by intervening dust). The dust itself emits longer-wavelength infrared radiation, so we can trace the location of interstellar dust and locate the regions where it is warmed by starlight. Going to the ultraviolet, only the hottest stars give off enough radiation to see, so this region also allows us to trace regions of active star formation. Finally, looking at a galaxy in X-rays, we see only the highest-energy components – binary stars in which material falling onto a neutron star or black hole gives rise to extreme temperatures, emission from gas at millions of degrees, and sometimes emission from central quasar-like active nuclei which may not give an accurate indicator of temperature, since so-called nonthermal processes may be involved.

There is growing evidence that we may be completely ignorant of one of the most important constituents of galaxies – the dark matter. If gravity behaves over ranges of thousands of light years in the way that it does over smaller scales, the motions of stars and gas in galaxies, and of gas and galaxies in clusters, require that most of the mass in these systems is on some completely invisible forms. The main lines of evidence include:

• Flat rotation curves in spiral galaxies. The orbital speed measured for material in the outer parts of spirals is nearly constant with distance, without the dropoff which would show that we are observing orbits outside the main mass concentration.
   
• Velocities of galaxies in clusters. Similarly, the measured motions of galaxies in clusters are too fast for them to be held together by the gravity of the the visible stars comprising the galaxies. Hot gas between the galaxies, revealed by its X-ray emission, adds about an equal amount of mass to the galaxies' stars, but a discrepancy often reaching a factor 10 remains between visible and gravitating masses.
   
• The extent of the hot gas in clusters of galaxies. At its observed temperatures, the amount of mass needed to hold it in place by gravity is comparable to that deduced to from galaxy motions. In fact, in many cases, the gas is regarded as a more reliable tracer, since a cluster contains only so many galaxies which can act as tracer particles, while the hot gas is a continuous medium which can be observed in as much detail as instrumentation permits.
   

The nature of this unseen matter remains elusive, and has provided a happy hunting ground for observer and theoretician alike. Proposals have included brown dwarf stars, Jupiter-like objects, quantum black holes produced in the early Universe, and a whole zoo of exotic particles which would also be remnants of the early Universe. Assorted astronomical and laboratory searches have yet to tell us what makes up most of the matter in the Universe. We are left with the sobering realization that all of our vaunted technology and apparatus has been telling us about only 10% of the cosmos.

Clusters of Galaxies

Early surveys of galaxies on the sky showed that certain regions have more than their share of galaxies; such concentrations as the Virgo cluster were known long before the nature of galaxies was understood. More complete statistics have shown that the distribution of galaxies in space is far from the uniform "sea" first envisaged, with many (perhaps most) galaxies arrayed in groups, clusters with thousands of members, superclusters, and even larger sheets and fingers stretching as far across the Universe as we can reliably map.

Clusters come in a variety of kinds, just as galaxies do. The richest and densest clusters are round assemblages, while sparser clusters have flattened or irregular shapes. The cluster environment is reflected in its galaxy content – dense environments like cluster cores are populated almost solely by elliptical and S0 galaxies, nearly devoid of gas and star formation. Less extreme environments can host, as well, spiral and irregular galaxies. This so-called morphology-density relation has engendered a classic heredity-environment question – were spiral galaxies never formed in those regions which would one day be rich clusters, or are they somehow destroyed or transformed in such clusters? The jury is still out, though there is strong evidence that in some clusters spirals were once numerous and have been transformed by external factors into elliptical or S0 systems. One such transforming mechanism is via galaxy mergers, which, while not common at the high speeds typical of cluster encounters today, might have been more common early on.

A second transforming mechanism could be provided if clusters contain some kind of external medium – intergalactic gas. Such a medium was indeed discovered by early X-ray astronomy satellites, and is known to be ubiquitous in clusters and even galaxy groups. Random motions in the cluster heat this gas to temperatures of 10,000,000 Kelvin, making it visible only by its own X-ray emission. This gas typically has as much mass as do stars in the visible galaxies, and as galaxies move through it, will provide an external wind. This would in principle be strong enough to sweep gas out of a spiral galaxy, and a gas-free spiral will cease star formation and quickly look like an S0. Detailed observations in local clusters such as Virgo in fact show that spirals nearest the center seem to have lost the outermost parts of their gas distributions.

Detailed studies of the distance and redshifts of nearby galaxies have added another dynamic aspect to our understanding of clusters – they are still growing. At greater and greater distances, the gravity of a cluster takes longer to affect the motions of its surrounding galaxies, so that galaxies at larger distances will eventually turn around against the expansion of the Universe and fall into the cluster. Our own local group has a detectable motion toward the Virgo Cluster (more precisely, the core of the Local Supercluster), and such large-scale motions can be found near many nearby clusters. In this sense, the cosmic epoch of cluster formation is now.

Galaxies and Cosmology

The very recognition of galaxies as objects at vast distances led to the first attempts to use thems as tracers of the structure of the Universe as a whole – observational cosmology. Hubble's promulgation of the evidence for a relation between a galaxy's distance and its redshift led to a picture of an expanding Universe. As observational capabilities have increased, so has the volume of space where astronomers can search for signatures of the geometry of space-time.

The Hubble law for galaxy redshifts implied a uniform expansion – one in which every galaxy sees the same linear relation between distance and redshift when looking at other galaxies. The rate of this expansion is characterized by the Hubble constant – the ratio between redshift and distance for a fictitious average galaxy with no peculiar motion of its own with respect to the expansion. Not only does this value give us the size scale of the Universer, but it gives a measure of its age as well. If one runs the clock backwards on a uniform Hubble expansion, at a constant rate, the age of the expansion is the numerical inverse of the Hubble constant. Even if there has been deceleration of the expansion due to gravity, this age – the Hubble time – gives a scaling value for the age of the Universe.

The exact value of the Hubble constant has been contentious, with strong arguments presented for values from 50 to 100 km/sec per megaparsec. These correspond to Hubble times of, respectively, 20 and 10 billion years. Some of the disagreement between workers on the cosmic distance scale comes from different treatments of local galaxy motions superimposed on the smooth expansion, and some from regarding various measures of distance as primary or secondary. One of the major projects for the Hubble Space Telescope deals with direct measures of galaxies distant enough to expect a clean measurement of the Hubble constant.

The distribution of galaxies into groups, clusters, and superclusters carries information on masses and motions in the early Universe. In brief, if the initial distribution of pre-galactic material was as uniform as the COBE satellite data suggests, then in order to form clusters today surrounded by relatively empty areas, the cluster-galaxies-to-be must have been able to move fast enough to cross (at least) the size of these empty areas in a Hubble time. The necessary gravitational clumping to propel such motion early enough proves to be an important constraint on the early Universe.

Classical cosmology was once described as a search for two numbers – the Hubble constant H_0, and a second value, the deceleration parameter q_0. The deceleration parameter described how fast the Hubble constant changes with cosmic time as the overall gravity of all matter in the Universe slows the expansion. A value of 0 would indicate an empty Universe – mathematically simple and appealing, but not very interesting to us! There are there different cases – an open Universe, in which the expansion will never stop; a closed Universe, in which the expansion will someday stop and reverse; and the critical point between them, where the expansion will constantly slow and approach (but never quite reach) zero. These are separated by the critical value q_0=1/2. Many classical tests for the value of q_0 relied on using galaxies as standard candles, but have been defeated by the fact that galaxies evolve on the same timescales that must be probed to measure q_0. Current efforts in this direction use different probes, such as gravitational lensing or the mean mass density in the local Universe, in efforts to circumvent the unknowns of galaxy evolution. In any case, it is remarkable that q_0 is close to its critical value; otherwise the Universe might not have the right properties for us to exist and discover cosmology.

Galaxy Nomenclature and Catalogs

Galaxies are generally denoted only by catalog numbers; only a handful are well-known or unusual enough to rate distinctive names (such as the Whirlpool, Antennae, Pinwheel, and Cartwheel). A given galaxy may sport numbers from several catalogs. The most cited sources are: Messier number – from a list compiled visually by Charles Messier and several colleagues during the eighteenth century. Many of the brightest and most conspicuous galaxies (as well as gaseous nebulae and star clusters) appear in the Messier lists. NGC/IC (New General Catalog and Index Catalog) – compilations by J.L.E. Dreyer from the 1860s-1880s. These included results of the complete sky sweeps performed by William and Jihn Herschel and discoveries by others, plus the first harvests of celestial photography. These catalogs include (besides the usual round of clusters and nebulae) about 10,000 of the most conspicuous galaxies. Until recently, almost all galaxies which could be studied in detail had NGC or IC numbers. Arp – Halton Arp produced an atlas of peculiar and interacting galaxies, which first drew the attention of many astronomers to the strange and spectacular forms that galaxies outside the normal Hubble classification can take. UGC (Uppsala General Catalog) - galaxies only, covering the sky north of -2 degrees 30'. Peter Nilsson produced this catalog of positions, sizes, orientations, and magnitudes from Palomar Sky Survey photographs.

Several other kinds of name comprise coordinate designations (first digits of the object right ascension and declination, either for epoch 1950 or 2000) and a survey name. Examples are the PKS (radio sources from the Parkes radio telescope in Australia) and IRAS (Infrared Astronomical Satellite) surveys. Thus we may have PKS 1413+003 or IRAS 09104+4109.

Many of the originally published versions of these catalogs are either rather obscure (university observatory transaction series, for example), or out of print (the NGC and IC being notable exceptions). Users of personal computers can get modern versions of these and many more on CD-ROMs (from the Astronomical Data Center at the NASA Goddard Space Flight Center, or the Almageste package from the ASP). Such a level of access, without need of a professional astronomical library, makes many kinds of advanced study and observing programs possible.

Note that wavelength/flux/surface brightness selection enters into what galaxies get selected for a particular catalog or study. Malin 1, despite being large and luminous, was long missed for being too large. Several researchers have pointed out that galaxy catalogs are dominated by those kinds of galaxies which are easiest to see against the natural glow of the night sky, so we may still be ignorant of important parts of the extragalactic census.

For Further Reading

• The Hubble Atlas, by A. Sandage (Carnegie Institution of Washington 1961). This virtually defines the Hubble classification, and offers a stunning collection of galaxy photographs from the Mount Wilson and Palomar telescopes. It will soon be superseded by the Carnegie Atlas of Galaxies.
   
• The Color Atlas of Galaxies, by J.D. Wray (Cambridge 1988). The fruits of a long-term project to photograph galaxies through selected filters and produce composite color images. A wide range of galaxy types is illustrated, with care devoted to the color reproduction and its interpretation.
   
• Man Discovers the Galaxies, by R. Berendzen, R. Hart, and D. Seeley (Science History Publications 1976). From the recognition of galaxies through early distance measurements and the beginnings of modern observational cosmology.
   
• Galaxies, by Timothy Ferris (Sierra Club Books 1980). A classic coffee-table book full of beautiful galaxy images. Useful even for career astronomers who need to remind themselves of what drew them into galaxy study.
   
• Lonely Hearts of the Cosmos, by Denis Overbye. An engaging account of the quest for the cosmic distance scale.
   
• The Universe of Galaxies, edited by Paul Hodge (Freeman 1984). Articles excerpted from Scientific American, covering dark matter, galactic tides, clusters, and active galactic nuclei.
   
• Galaxies, by Paul Hodge.

Normal galaxies were first observed in the X-rays with the Einstein Observatory, the first X-ray telescope to observe the deep universe, launched by NASA in 1978 and developed under the leadership of the CfA HEA. This first look at the X-ray emission of galaxies revealed bright point-like sources and diffuse emission. With the Chandra X-ray Observatory we are now studying the X-ray emission of galaxies in exquisite details. We detect populations of luminous and variable point-like X-ray sources, diffuse emission from hot gas and in some cases emission from active nuclei.

In all galaxies near enough to be studied in detail with Chandra (and this includes galaxies as far as at least 20Mpc), we observe populations of X-ray sources, with characteristics (time variability and spectra/X-ray colors) consistent with those of the X-ray binary stars found in the Milky Way: these sources are powered by the outer layers of a normal star falling into a neutron star or a black hole; observing different types of galaxies (elliptical, spiral, starburst and interacting), we can begin to understand how these X-ray sources form and evolve and how their history is linked to the evolution of the parent galaxy. Sources are associated with the young stellar population of spiral arms, with the intermediate-age population of the stellar disks of spiral galaxies, and with the old stellar population of bulges, elliptical galaxies, and globular clusters. Typically, more luminous X-ray emission is found in sources associated with younger stellar populations, which are likely to be highly accreting young binary systems with early-type-star donors.

At the high luminosity end of these luminous sources are the Ultra-Luminous X-ray sources (ULXs); some scientists have suggested that ULX may be special sources, different from normal X-ray binaries, and harbor black holes of masses larger than 100 solar masses, which are unlikely to form from the evolution of normal massive stars, and could be the remnants of primordial black hole formation in the early universe. Although the jury is still out on ULXs, it is becoming more and more clear that the majority of these sources are just the extreme examples of normal X-ray binaries.

References

• "The Time-variable Ultraluminous X-Ray Sources of 'The Antennae'”
  - Fabbiano, G. 2006, Ann. Rev. A&A, 44, 32
   
• "X-Rays from Normal Galaxies" - Fabbiano, G. 1989, Ann. Rev. A&A, 27, 87
   
• "Chandra Observations of “The Antennae” Galaxies (NGC 4038/4039). III. X-Ray Properties and Multiwavelength Associations of the X-Ray Source Population"
  - Fabbiano, G. 2006, Ann. Rev. A&A, 44, 323

External Links

• NASA’s Chandra X-ray Observatory Website
• Harvard/Smithsonian Chandra X-ray Obsrevatory Website
• The Einstein Observatory (HEAO-2) - Goddard Space Flight Center & Smithsonian Astrophysical Observatory.
• Read more about NASA's High Energy Astrophysical Observatories (HEAO) missions from the NASA History Office in "The Star Splitters."
  (See p. 59 for Specific discussion on HEAO-2.)

Further Reading

• "Populations of X-Ray Sources in Galaxies" - Annual Review of Astronomy and Astrophysics, Vol. 44: 323-366 (Volume publication date September 2006).
   
• "X-Ray Properties of Black-Hole Binaries" Ronald A. Remillard, Jeffrey E. McClintock, Annual Review of Astronomy and Astrophysics. Volume 44, Page 49-92, Sep 2006.
   
• "Deep Extragalactic X-ray Surveys" by W.N. Brandt, G. Hasinger, Annual Review of Astronomy and Astrophysics. Volume 43, Page 827-859, Sep 2005.
   
• "Relativistic X-Ray Lines from the Inner Accretion Disks Around Black Holes" by J.M. Miller, Annual Review of Astronomy and Astrophysics. Volume 45, Page 441-479, Sep 2007.
   
• "Toward Understanding Massive Star Formation" by Hans Zinnecker, Harold W. Yorke, Annual Review of Astronomy and Astrophysics. Volume 45, Page 481-563, Sep 2007.
   

Preview Image

"M51: X-Rays from the Whirlpool"  A popular pair of interacting galaxies known as the Whirlpool debut here beyond the realm of visible light -- imaged at high energies by the orbiting Chandra X-ray Observatory.  The number of luminous x-ray sources, likely neutron star and black hole binary systems within the confines of M51, is unusually high for normal spiral or elliptical galaxies and suggests this cosmic whirlpool has experienced intense bursts of massive star formation. The bright cores of both galaxies, NGC 5194 and NGC 5195 (right and left respectively), also exhibit high-energy activity in this false-color x-ray picture showing a diffuse glow from multi-million degree gas. An expanded view of the region near the core of NGC 5194 reveals x-rays from a supernova remnant, the debris from a spectacular stellar explosion, first detected by earthbound astronomers in 1994.  (Source/Credit: A. Wilson (UMD) et al., CXC, NASA.)

Our Sun is part of a huge wheel-shaped collection of stars. On a dark night they form a glowing band across the sky--"The Milky Way" to ancient Greek observers, and to us, our galaxy. When you look at any part of that glow, you are looking through the wheel edge-on, and what you actually see is the light of many, many distant stars, whose light blends to a glow.

What holds the wheel together? Astronomers are still not sure (see further below), but have long suspected that a very massive black hole existed at the center of our galaxy, created early in the history of the universe. Their suspicion focused on a compact radio source, also found to emit x-rays, hidden behind dust clouds in the constellation of Sagittarius, the archer.

Now we know more. A large star in that region was found to orbit a dark concentration of mass, estimated at 3.7 million times the mass of our Sun (give or take 1.5 million). The laws of physics have ruled out any explanation but one--that this is indeed an enormous black hole.

Stars near a Black Hole

The public image of a "black hole" is rather menacing, a vortex sucking up anything that comes near, to be swallowed up and never to return. That is not so. The force of gravity near a black hole is indeed tremendous, but like the gravity pull of Sun, Earth or any other object, it obeys the conservation of energy. Any object attracted to a black hole gains velocity and energy, and these may get quite large. However, unless that object happens to be headed directly at the black hole (a relatively small target) it will just swing around and fly away again, like a comet making a pass at the Sun. The velocity acquired in falling towards the black hole also helps it escape.

The Star S2

The intense gravity of the black hole prevents any light from escaping it, and it is therefore invisible ("black" indeed!). Its vicinity, however, contains a fairly high density of stars (see image-left), including one big star--about 15 times the mass of the Sun and 7 times its radius--which was recently found to go around the center with an orbital period of only 15.2 years (see image "S2-Orbit" above). That star, designated S2 by astronomers, follows an ellipse which at its closest comes within about 124 astronomical units (1 AU=mean Sun-Earth distance) of the center of the galaxy (see illustration). At that time it speeds up to about 5000 km/sec--close to 2% of the velocity of light! Its side facing the black hole is somewhat closer to the black hole than the side facing away, and is therefore pulled more strongly; on a very close approach, such a difference could tear a star apart, but S2 would have to get some 70 times closer before that would happen, at a distance comparable to the orbital radius of Mars.

The observation of this orbit, reported by Rainer Schödel of the Max Planck Institute (Germany) and by his colleagues, is a triumph of Earth-based astronomy. Although S2 is much larger and brighter than the Sun, its visible light is obscured by dust and does not reach us. However, infra-red (IR) light emitted by S2 can penetrate, and a sophisticated IR camera was used, attached to an 8-meter (diameter) telescope of the Southern European Observatory in Chile. The giant mirror telescope overcame the image-blurring twinkling from the atmosphere by using "adaptive optics" with rapidly adjusting mirrors, and attained the incredibly fine resolution about 1/100 of a second of arc (note the scale on the graph above!).

Monster on a Restricted Diet

Far from being a voracious devourer of stars and of interstellar gas, "our own" black hole is rather benign. Gas falling into it causes X-rays to be emitted, but the emission is weak, apart from occasional "flares" thought to come from the capture of comet-sized chunks of matter. A report in "Science" (30 May 2003, page 1356) calls it "The Milky Way's Dark, Starving Pit" and also suggests an explanation. The galactic center is now inside an expanding bubble of gas, apparently created some 10-50,000 years ago when a supernova exploded nearby. The suggestion is that the front of the bubble sweeps away interstellar gas and keeps down the gas density surrounding the black hole.

So is this black hole what holds our galaxy together? Probably not. If it did, then the motion of stars around it would slow down with increasing distance, in accordance with Kepler's third law. The star S2 obeys Kepler's laws, and other stars near the center do so, too. However stars distant from the center do not slow down as much as expected, suggesting their motion is determined, not just by the attraction of the concentrated central mass, but also by some unseen "dark mass" spread out through the galaxy.

This very significant observation is further discussed at "The Doppler Effect - Rotating Galaxies and Dark Matter."

And by the way... The Greek legend about a divine mother's milk strewn across the sky is also the source of the word "galaxy," since "gala" in Greek means milk. The ancient Jewish name for the milky way was "river of fire" (nahar di nur).

Further Reading

• The article announcing the discovery (with scientific details): A Star in a 15.2 year orbit around the supermassive black hole at the center of the Milky Way, by R. Schödel et al (22 co-authors), Nature, vol. 419, p. 694-6, 17 October 2002. (PDF)
   
• A short report in the same issue, describing that work for general readers: Into the heart of darkness by Karl Gebhardt, Nature, vol 419, p. 675-6, 17 October 2002. (PDF)
   
• Note "The Black Hole at the center of Our Galaxy" by Fulvio Melia, published tby Princeton University--201 pp, $29.95, reviewed in "Science", 18 July 2003, p. 314. A more detailed discussion of the subject... and it is no more than a coincidence that the titles of the book and of this section are the same!

External Links

• "The Milky Way's Dark, Starving Pit" "Science" (30 May 2003, page 1356) calls it 
• "The Doppler Effect - Rotating Galaxies and Dark Matter" - David Stern's Phy6.
• X-rays to be emitted

Preview Image

This Chandra image shows our Galaxy’s center. The location of the black hole, known as Sagittarius A*, or Sgr A* for short, is arrowed. (Source/Credit: NASA/CXC/MIT/Frederick K. Baganoff et al.)

Disclaimer: This article is taken wholly from, or contains information that was originally published by, David P. Stern - "Educational Web Sites on Astronomy, Physics, Spaceflight and the Earth's Magnetism." Topic editors and authors for the Encyclopedia of the Cosmos may have edited its content or added new information. The use of information from David P. Stern should not be construed as support for, or endorsement by, that David P. Stern for any new information added by EoC personnel, or for any editing of the original content. The EoC has a specific working relationship with David P. Stern, and any changes to any of his content is to be done only with his approval or the approval of those appointed by him to represent his interests in this content.

The intracluster medium (ICM) is a hot ionized gas which fills the space between the galaxies in Galaxy Clusters (See Figure 1). The ICM is composed of gas which fell into the cluster and gas which has been removed from cluster galaxies. The gas expelled from galaxies has been enriched by elements produced inside stars, and the resulting metalicity of the ICM is about 1/3 that of our sun. The gas is removed from the galaxies either by galaxy scale winds powered by supernovae or by stripping due to the pressure of the ICM as the galaxies move through the cluster. About 2/3 of the baryonic matter in a cluster of galaxies is in the hot gas in the ICM and the rest is in the galaxies. The density is in the range 10^-3 to 10^-4 particles/cm³, and the temperature is roughly tenmillion degrees. This is hot enough that the gas emits X-rays via thermal Bremstrahlung radiation. There is also line emission due to the heavy elements in the gas.

Cooling Flows

In the central parts of the cluster, the gas is able to radiate enough of its energy that it should cool and drop out of the hot phase at rates ofabout 10-100 times the mass of our sun per year.It appears that much of the energy radiated by the gas is replaced by some form of energy input into the gas. Currently it is thought that the energy input is due to Radio Galaxies produced by the massive central galaxy.
(See Figure 2).

Preview Image

This optical and X-ray composite image shows Abell 2029, one of 26 galaxy clusters studied by Chandra, located one billion light years away. Visit "Chandra Discovery Sheds Light on Dark Energy" for full size version. Credit: Optical: National Optical Astronomy Observatory/Kitt Peak, X-ray: NASA/Chandra X-ray Center/IoA.

Starbust galaxies are either young or rejuvenated galaxies, with a star formation activity significantly larger than in our own Milky Way. These galaxies typically contain very luminous X-ray sources, including the Ultra-luminous X-ray sources (ULXs) that may harbor massive black holes.

A nearby example of starburst galaxy is M82, which hosts a number of variable ULXs; the most luminous of these ULXs reaches a peak luminosity in the range of 1041 erg s-1 and is the best and most discussed candidate for a black hole of mass higher than 100 solar masses. Hot X-ray emitting interstellar matter is also found in starburst galaxies. In M82 this hot gas appears to be escaping the nuclear region as a wind, visible above and below the stellar disk of this galaxy. Galaxy interaction or merging is frequently the trigger of a starburst episode.

A starburst galaxy is a galaxy experiencing a period of intense star forming activity. The burst occurs over a region a few thousand light years in diameter. The most popular theory for the cause of a starburst is that it is triggered by a close encounter or collision with another galaxy. This collision sends shock waves rushing through the galaxy. These shock waves push on giant clouds of gas and dust, causing them to collapse and form a few hundred stars. The massive stars use up their fuel quickly and explode as supernovas, which produce more shock waves and more star formation. In this way, a chain reaction of star formation and supernovas can sweep through the central region of a galaxy, where most of the gas is located. When most of the gas is used up or blown away by the explosions, the starburst ends.

External Links

• Chandra Observatory - Harvard.
• Chandra Observatory - NASA.
• "Great Observatories Present Rainbow of a Galaxy" - (An animation) M82 is shown in all its wavelength glory. Dissolving from Chandra X-ray Observatory images of three X-ray energy bands to images in three bands of the infrared spectrum taken by the Spitzer Space Telescope, and ending with the Hubble Space Telescope's visible- and near-infrared-light image. The three observatories' images were composited to reveal the galaxy's stars, as well as gas and dust features.  NASA/JPL Photo Journal Animation PIA08093.
• "Starburst Galaxies" - Chandra Observatory, Harvard.

Preview Image

"M82: Images From Space Telescopes Produce Stunning View of Starburst Galaxy" - Images from three of NASA's Great Observatories were combined to create this spectacular, multiwavelength view of the starburst galaxy M82. Optical light from stars (yellow-green/Hubble Space Telescope) shows the disk of a modest-sized, apparently normal galaxy.  Credit: X-ray: NASA/CXC/JHU/D.Strickland; Optical: NASA/ESA/STScI/AURA/The Hubble Heritage Team; IR: NASA/JPL-Caltech/Univ. of AZ/C. Engelbracht.  See also the animation "Great Observatories Present Rainbow of a Galaxy" (Source: Harvard-Smithsonian Center for Astrophysics/NASA by SAO.)

Galaxy interaction and merging is seen throughout the universe, and it is believed to be an important mechanism in galaxy formation and evolution; large elliptical galaxies may be the remnants of past mergers. The effect of interaction and merging can be seen in distorted and asymmetric galaxy morphologies and in the presence of tidal tails. Accompanying these morphological effects is also increased star formation and starburst activity, fostered by the shocks driven into the interstellar medium by the galaxy collision.

Antennae Galaxy – Prototypical Galaxy Merger

The prototypical galaxy merger is the Antennae galaxy, a merger of the two galaxies NGC4038 and NGC4039, which was observed several times with Chandra for a total exposure of nearly a week over a two-year period. These Chandra observations revealed a population of 120 X-ray sources, of which 14 are variable ULXs. The Antennae are filled with hot interstellar medium, which has a variety of temperatures in different parts of the galaxies and is enriched in metals (Ne, Mg, Si, Fe); the abundances in certain regions are much larger than in our own Solar neighborhood, and the relative amounts of these elements are consistent with production in the SNII explosions typical of a young stellar population rich in massive stars. Two galaxy-size loops of hot gas are also seen in the Antennae, embedded in a more tenuous diffuse hot halo. While the geometry of these features is yet to be explained, their presence suggest outflows that may disperse the metals in the inter-galactic space.

This montage of Chandra images (Left) shows a pair of interacting galaxies known as The Antennae. Rich deposits of neon, magnesium, and silicon were discovered in the interstellar gas of this system.

"3-Color, Full Field" – The top image, a wide field X-ray view, reveals spectacular loops of hot gas spreading out from the southern part of The Antenna into intergalactic space. Also shown are huge clouds of multimillion degree gas and bright point like sources due to neutron stars and black holes. The image is color coded so that low, medium and high energy X-rays appear as red, green and blue, respectively. Direct hits between stars are extremely rare when galaxies collide, but huge gas clouds can crash into each other at high speeds, creating shock waves that heat the clouds and the surrounding gas to millions of degrees.

"Diffuse Emmision" – In the closeup view on the lower left, also color coded by X-ray energies, the point sources have been taken out to emphasize the hot gas clouds in the central regions of The Antennae.

Collisions between the gas clouds may trigger a stellar baby boom. The most massive of these young stars race through their evolution in a few million years and explode as supernovas. Heavy elements manufactured inside these stars are blown away by the explosions that further heat the gas clouds and enrich them with heavy elements such as neon, magnesium, silicon and iron.

"Element Map" – The image at the lower right is processed and color-coded to show regions rich in iron (red), magnesium (green) and silicon (blue). These are the types of elements that form the ultimate building blocks for habitable planets.

Enrichment from supernovas occurs in all galaxies, but usually the new elements are observed in a highly diluted form as they are mixed up with the rest of the interstellar gas. This Chandra image is remarkable in that it shows clouds in which magnesium and silicon are 16 and 24 times as abundant as in the Sun.

As the enriched gas cools, a new generation of stars will form, and with them new planets. A number of studies indicate that clouds enriched in heavy elements are more likely to form stars with planetary systems. Several hundred million years from now, an unusually high number of planets may form in The Antennae.

Images of Interacting Galaxies

References

• "Antennae: Chandra Locates Mother Lode of Planetary Ore in Colliding Galaxies" - Harvard-Smithsonian Center for Astrophysics

External Links

• Chandra X-ray Observatory, Smithsonian Astrophysical Observatory: Harvard-Smithsonian Center for Astrophysics & NASA.
  
• Larger Collage of Interacting Galaxies from the Hubble Space Telescope.

Preview Image

"Antennae Galaxies’ Fertile Marriage" The Universe is an all-action arena for some of the largest, most slowly evolving and surprising processes known to mankind. A new picture taken by the Advanced Camera for Surveys (ACS), onboard the NASA/ESA Hubble Space Telescope, shows the best ever view of the Antennae galaxies - seemingly a violent clash between a pair of once isolated galaxies, but in reality a fertile marriage. (Source: NASA/ESA.)

Galaxies at high redshift often have peculiar shapes, in contrast to the well-developed spiral and elliptical galaxies seen in the local universe. At redshifts greater than 3 or 4, corresponding to when the universe was less than 2 billion years old, galaxies are primarily starburst systems.

Their shapes often resemble

• chains, in which big clumps of star formation are arranged in a linear pattern;
• doubles, with two primary clumps;
• tadpoles, with a large clump and a smooth tail or tail of smaller clumps;
• clump clusters, collections of clumps of star formation arranged in a disk-like pattern,
• in addition to spirals and ellipticals.

About a third of the ellipticals contain clumps indicating recent mergers.

The star-forming regions in the high redshift galaxies are much more massive than similar star-forming complexes in the local universe, indicating a more turbulent interstellar medium. It is likely that galaxies are built up through hierarchical assembly. The peculiar shapes may become spiral galaxies if they endure minor mergers, or may become ellipticals in major (more equal mass) mergers.

Examples of High Redshift Galaxies

Selection of four typical galaxies for each morphological type. 

Ultra Deep Field ("UDF") images were obtained with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) by S. Beckwith and coworkers in 2004 and are available on the Space Telescope Science Institute (STScI) archive.  Images are at i₇₇₅ band, with a line representing 05.

The distinguishing characteristics of the main types that we classified are as follows:

• Chain.—Linear objects dominated by several giant clumps and having no exponential light profiles or central red bulges.
   
• Clump cluster.—Oval or circular objects resembling chain galaxies in their dominance by several giant clumps and having no exponential profiles or bulges.
   
• Double clump.—Systems dominated by two similar clumps with no exponential profile or bulge.
   
• Tadpole.—Systems dominated by a single clump that is off-center from, or at the end of, a more diffuse linear emission.
   
• Spiral.—Galaxies with exponential-like disks, evident spiral structure if they have low inclination, and usually a bulge or a nucleus. Edge-on spirals have relatively flat emission from a midplane, and often extended emission perpendicular to the midplane, as well as a bulge.
   
• Elliptical.—Centrally concentrated oval galaxies with no obvious spiral structure.
   

External Links

• Advanced Camera for Surveys ("ACS") - Space Telescope Science Institute (STScI).
• Hubble Ultra Deep Field overview and related links - Space Telescope Science Institute (STScI).
• Space Telescope Science Institute (STScI) - Resources.
  
• The Ultra Deep Field catalog is available in plain-text on the STScI Web site at ftp://archive.stsci.edu/pub/hlsp/udf/acs-wfc/h_udf_wfc_V1_i_cat.txt.

Further Reading

• About SPITZER - Spitzer will be the final mission in NASA's Great Observatories Program - a family of four orbiting observatories, each observing the Universe in a different kind of light (visible, gamma rays, X-rays, and infrared).
• "Galaxy Morphologies in the Hubble Ultra Deep Field: Dominance of Linear Structures at the Detection Limit" - The Astrophysical Journal, 631:85–100, 2005 September 20. (This is the source of this EoC article.)
• "Hubble Ultra Deep Field" - Wikipedia.
  

Preview Image

"Spitzer and Hubble Team Up To Find 'Big Baby' Galaxies in the Newborn Universe" – This image demonstrates how data from two of NASA's Great Observatories, the Spitzer and Hubble Space Telescopes, are used to identify one of the most distant galaxies ever seen. This galaxy is unusually massive for its youthful age of 800 million years. (After the Big Bang, the Milky Way by comparison, is approximately 13 billion years old.)

• Left / Hubble Ultra Deep Field - The galaxy, named HUDF-JD2, was pinpointed among approximately 10,000 others in a small area of sky called the Hubble Ultra Deep Field. This is the deepest images of the universe ever made at optical and near-infrared wavelengths.
   
• Upper Right / Hubble visible-light view - A blow-up of one small area of the Hubble Ultra Deep Field is used to identify where the distant galaxy is located (inside green circle). This indicates that the galaxy's visible light has been absorbed by traveling billions of light-years through intervening hydrogen.
   
• Center Right / Hubble near-infrared view - The galaxy was detected using Hubble's near infrared camera and multi-object spectrometer. But at near-infrared wavelengths it is very faint and red.
   
• Bottom Right / Spitzer infrared view - The Spitzer infrared array camera, easily detects the galaxy at longer infrared wavelengths. The instrument is sensitive to the light from older, redder stars which should make up most of the mass in a galaxy. The brightness of the infrared galaxy suggests that it is quite massive.
   

Distant Galaxy in the Hubble Ultra Deep Field
Left	Upper Right	Center Right	Bottom Right

The original composite plus four images that comprise the Preview Image are presented here at reduced size. Click on each image to view larger image. Click caption below image to view original (largest) images. (Source: NASA - Spitzer Space Telescope.)

Introduction

On equivalent spatial scales, galaxy clusters are the largest perturbations to the cosmic matter density. Gravitational potential wells set by dark matter are populated with galaxies and intracluster baryons, including stars and gas.

General Characteristics

• R~1-2 Mpc (Megaparsec) in radius.
• M~10¹³-10¹⁵M_{sol} in mass.
• There exist anywhere from two to 1000's of galaxies in a cluster.
• The Milky Way resides in the Local Group which includes the Large and Small Magellanic Clouds.
• Observable in a large range of wavelengths, including the optical, X-ray, and radio; these correspond with a suite of labeling parameters, which relate to the underlyling mass distribution; the characterization of the mass function.

• Contain evolution of large-scale structure as tracers of the cosmic matter distribution.
• Laboratories for testing of smaller-scale astrophysics.

Preview Image

This rich galaxy cluster, catalogued as Cl 0024+17, is allowing astronomers to probe the distribution of dark matter in space. The blue streaks near the center of the image are the smeared images of very distant galaxies that are not part of the cluster. The distant galaxies appear distorted because their light is being bent and magnified by the powerful gravity of Cl 0024+17, an effect called gravitational lensing. Dark matter cannot be seen because it does not shine or reflect light. Astronomers can only detect its influence by how its gravity affects light. By mapping the distorted light created by gravitational lensing, astronomers can trace how dark matter is distributed in the cluster. While mapping the dark matter, astronomers found a dark-matter ring near the cluster's center. The ring's discovery is among the strongest evidence that dark matter exists. This Hubble observation was taken in November, 2004 by the Advanced Camera for Surveys. (Source: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University))

The morphology-density relation describes how different types of galaxies tend to be arranged in clusters.

In general, bulge-dominated early-type galaxies, Ellipticals and S0s, preferentially inhabit the central, densest areas of galaxy clusters. Meanwhile, disk-dominated late types tend to be scattered in the more sparsely populated regions of these clusters. This relation is valid for wide variations in shape and richness of clusters. However, this correlation of galaxy shape and the environment it inhabits changes as we look back in time, highlighting some important physical mechanisms at work in clusters and painting a richly dynamic tableau of galaxy evolution as a whole.

In the Local Universe (z ~ 0)

In the early 1980s, it was established that the distribution of galaxy morphologies varied smoothly from the densest regions to the outskirts. In counting the numbers of elipticals, S0s, and spiral galaxies, one can expect to find that each of these galaxies respectively comprises roughly 50%, 40%, and 10% of the total population in the innermost part of the clusters. Outside the cluster, ellipticals and S0s each account for 10% of the number of galaxies, while spirals are 80% of this low density environment population. Again, we see this trend regardless of the differences in individual cluster richness.

At Intermediate Redshift (z ~ 0.5-1)

Despite the powerful morphological segregation we observe locally, this relationship does not hold as a function of redshift. While a similar morphology-density relation is present in centrally concentrated, regular clusters at z ~ 0.5 (about 5 billion years in the past), it is nearly absent in more irregular clusters. More importantly, there is a fundamental discrepancy between the overall number of S0s at this distance and the number we observe locally. There appears to be twice as many of these galaxies in clusters today as there were at z ~ 0.5, whose scarcity is accompanied by a proportionate increase in the spiral population. This situation is more exaggerated as we look back to z ~ 1 (8-9 billion years ago), where it becomes difficult to distinguish galaxies by appearance.

Possible Explanations

Clearly, a large-scale transformation of spiral galaxies to S0s must take place between z~1 and z~0, or from around 8-9 billion years ago to the present. The increasing prominence of gas-poor, dynamically relaxed early-types in cluster cores with time is likely a product of external agents that discourage the maintenance of delicate late-type spiral structure. These include full blown galaxy merging, high speed, ephemeral encounters between galaxies (“galaxy harassment”), ram pressure stripping of interstellar gas by the hot, ionized intracluster medium (ICM), and the gravitational stresses of the cluster environment itself. As a whole, these factors favor the eventual extinction of star formation and the appearances characteristic of early-type galaxies. However, the relative importance of all these processes is still uncertain, and individual scenarios admittedly provide some contradictions to observations, as seen below.

Galaxy mergers

While it is possible that two merging spirals can ultimately produce an elliptical galaxy, galaxies generally move too fast in clusters for them to “stick” efficiently. Still, this process may play a small role in producing a slight increase of elliptical galaxies over time.

Galaxy Harassment

These relatively minor events are more common than galaxy mergers; any particular galaxy in a sufficiently dense environment can be expected to interact with about 5 others in a period of a billion years. Minor interactions such as these are heavily dependent on a galaxy’s environment- galaxies in sparsely populated cluster outskirts do not enjoy the frequency of encounters that denser cluster interiors would. Unfortunately, while this accounts for the morphology-density relation in individual clusters, it does not explain why the relation exists for smaller, less dense clusters where harassment would not be important.

Ram Pressure Stripping

Like harassment, the potential strength of ram pressure stripping scales with cluster density- higher density environments have more ICM, and thus could strip away more of a galaxy’s gas and give it the subdued star formation rate of an S0. While this mechanism is an appealing solution, it cannot account for the actual morphological transformation of a spiral to an S0, only the quenching of its star formation. There are also problems with the long timescales required for this process. And like harassment, ram pressure stripping fails to justify the morphology-density relation across all densities of clusters.

Isolated Evolution

The efficacy of harassment and ram pressure stripping cannot be completely discounted in all cases. But many recent studies indicate that tidal interactions between individual galaxies and interactions between galaxies and the cluster’s gravity are dominant players in spiral-S0 transformations. In this case, galaxies naturally evolve on their own, from late to early type on the Hubble tuning fork, no matter where they are. Their proximity to the cluster center, with concomitant severity of ram pressure stripping and harassment, helps determine how rapidly this occurs.

Conclusion

It is worthwhile to note that all these trends listed are most pronounced for the most luminous galaxies observed. At faint magnitudes, the morphology-density relation is actually weaker than has been described. Since luminous and massive early-type galaxies are expected to dominate the highest density regions much more than smaller galaxies, this is to be expected.

Ultimately, the issue of equivalent population ratios for all types of local clusters, as well as the fundamental disk/bulge ratios of spirals and S0s, and how these galaxy components are transformed, remains unresolved. Further work remains in elucidating this problem.

Preview Image

The "Ghost of Mirach" galaxy is shown in ultraviolet as seen by NASA's Galaxy Evolution Explorer. The Ghost of Mirach—a galaxy called NGC 404—seen as the whitish spot in the center of the images. Mirach is a red giant star that looms large in visible light. Because NGC 404 is lost in the glare of this star, it was nicknamed the Ghost of Mirach. But when the galaxy is viewed in ultraviolet light, it comes to "life," revealing a never-before-seen ring. This ring, seen in blue, contains new stars—a surprise considering that the galaxy was previously thought to be, essentially, dead. The field of view spans 55,000 light years across. The Ghost of Mirach is located 11 million light-years from Earth. The star Mirach is very close in comparison—it is only 200 light-years away and is visible with the naked eye. (Image Credit: NASA/JPL-Caltech/DSS.)

This uniquely beautiful patchwork image, with its myriad of brightly coloured galaxies, shows the Chandra Deep Field South (CDF-S), arguably the most observed and best studied region in the entire sky. The CDF-S is one of the two regions selected as part of the Great Observatories Origins Deep Survey (GOODS), an effort of the worldwide astronomical community that unites the deepest observations from ground- and space-based facilities at all wavelengths from X-ray to radio. Its primary purpose is to provide astronomers with the most sensitive census of the distant Universe to assist in their study of the formation and evolution of galaxies.

The new image released by ESO combines data obtained with the VIMOS instrument in the U- and R-bands, as well as data obtained in the B-band with the Wide-Field Imager (WFI) attached to the 2.2 m MPG/ESO telescope at La Silla, in the framework of the GABODS survey.

The newly released U-band image – the result of 40 hours of staring at the same region of the sky and just made ready by the GOODS team – is the deepest image ever taken from the ground in this wavelength domain. At these depths, the sky is almost completely covered by galaxies, each one, like our own galaxy, the Milky Way, home of hundreds of billions of stars.

Galaxies were detected that are a billion times fainter than the unaided eye can see and over a range of colours not directly observable by the eye. This deep image has been essential to the discovery of a large number of new galaxies that are so far away that they are seen as they were when the Universe was only 2 billion years old.

In this sea of galaxies – or island universes as they are sometimes called – only a very few stars belonging to the Milky Way are seen. One of them is so close that it moves very fast on the sky. This "high proper motion star" is visible to the left of the second brightest star in the image. It appears as a funny elongated rainbow because the star moved while the data were being taken in the different filters over several years.

Notes

Because the Universe looks the same in all directions, the number, types and distribution of galaxies is the same everywhere. Consequently, very deep observations of the Universe can be performed in any direction. A series of fields were selected where no foreground object could affect the deep space observations (such as a bright star in our galaxy, or the dust from our Solar System). These fields have been observed using a number of telescopes and satellites, so as to collect information at all possible wavelengths, and characterise the full spectrum of the objects in the field. The data acquired from these deep fields are normally made public to the whole community of astronomers, constituting the basis for large collaborations.

Observations in the U-band, that is, at the boundary between visible light and ultraviolet are challenging: the Earth's atmosphere becomes more and more opaque out towards the ultraviolet, a useful property that protects people's skin, but limiting to ground-based telescopes. At shorter wavelengths, observations can only be done from space, using, for example, the Hubble Space Telescope. On the ground, only the very best sites, such as ESO's Paranal Observatory in the Atacama Desert, can perform useful observations in the U-band. Even with the best atmospheric conditions, instruments are at their limit at these wavelengths: the glass of normal lenses transmits less UV light, and detectors are less sensitive, so only instruments designed for UV observations, such as VIMOS on ESO's Very Large Telescope, can get enough light.

The VIMOS U-band image, which was obtained as part of the ESO/GOODS public programme, is based on 40 hours of observations with the VLT. The VIMOS R-band image was obtained co-adding a large number of archival images totaling 15 hours of exposure. The WFI B-band image is part of the GABODS survey.

ESO Press Officer: Dr. Henri Boffin - +49 89 3200 6222 - hboffin@eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

BL Lacertae (BL Lac or S4 2200+420) was known to be variable in the visible part of the electromagnetic spectrum from as early as 1929, and because of its stellar appearance it was originally thought to be a VARIABLE STAR. It is located in the constellation Lacerta, the lizard (RA (J2000) = 22h 02m 43.29s, DEC (J2000) = +42^o 16’ 39.98”). Observations in the late 1960s showed it to have highly variable radio emission as well.

BL Lac was found to be an extragalactic object with the measurement of its redshift (z = 0.069, based on the detection of very weak emission lines) in the early 1970s. During the same period, it was also understood to be an unusual type of ACTIVE GALACTIC NUCLEUS (AGN). Thus, BL Lacertae is the prototype of the class of AGN known as BL LACERTAE OBJECTS (also called BL Lacs). Collectively with some quasars, such as 3C273 and 3C279, they are known as ‘Blazars’. These objects are characterized as having bright nuclei with strongly polarized optical emission and large variability in all wavelengths. Their nonthermal radio-to-gamma-ray continua are thought to be emitted by a relativistic jet oriented close to the line-of-sight. BL Lac objects make up a small subset of AGNs, with about 350 known at present.

BL Lac is an important member of the class because it is relatively nearby, such that details can be well studied. The galaxy surrounding the active nucleus, or host galaxy, is a giant ELLIPTICAL GALAXY, typical of the host galaxies around other BL Lac objects.

Radio observations show BL Lac to exhibit SUPERLUMINAL MOTION, indicative of material being ejected at relativistic velocities from the nucleus. Observations by the Compton Gamma-Ray Observatory showed BL Lac to be a strong gamma-ray source, together with many other blazers. The rapid variability timescales and high luminosities observed at these energies indicate that the gamma rays are produced in a very compact region of the jet.

The nuclear emission in BL Lac is nonthermal and produces a continuum spectrum consisting of two components. The radio-to-ultraviolet spectrum is produced by synchrotron emission with peak power in the infrared-optical region, while the x-ray-to-gamma-ray spectrum is produced by inverse Compton emission. These two components are present in all BL Lacs objects.

BL Lac objects, as a class, are characterized by optical spectra which are featureless or which have extremely weak lines (less than 5Å equivalent width). This property makes the determination of their distances difficult. BL Lac itself is no exception. In the mid-1990s, however, strong, broad and variable emission lines (e.g. H-alpha, equivalent width = 7.3Å luminosity ~2x10⁴¹ erg cm^-2 s^-1) appeared in the spectrum of BL Lac. Current thinking is that the lines appear when the variable continuum emission is low. Furthermore, the presence of emission lines indicates the presence of a radiation field external to the jet, which may play an important role in the jet energetics (providing seed photons for the inverse Compton emission responsible for the gamma-rays).

Bibliography

• Catanese M et al 1997 Detection of gamma rays with E > 100MeV from BL Lacertae Astrophys.J. 480 562-7
• Pesce J E, Falomo R and Treves A 1995 Environmental properties of BL Lac objects Astron.J. 110 1554-63

• Sambruna R M, Ghisellini G, Hooper E, Koolgaard R I, Pesce J E and Urry C M 1999, ASCA and contemporaneous ground-based observations of the BL Lacertae objects 1749+096 and 2200+420 (BL Lac) Astrophys.J. 515 140-52
• Urry C M and Padovani P 1995 Unified schemes for radio-loud active galactic nuclei Publ.Astron.Soc.Pacific 107 803-45
• Vermeulen R C, Ogle P M, Tran H D, Browne I W A, Cohen M H, Readhead A C S, Taylor G B and Goodrich R W 1995 When is BL Lac not a BL Lac? Astrophys.J.Lett. 452 L5-8

Things have been rather frenzied this summer, to say the least. I'm quickly learning that the once-venerated HST is a dilapidated geriatrics ward of telescopy, one whose shoddy WFPC2 camera I can only imagine was MacGuyvered from popsicle sticks, dryer lint, and unsold copies of Deep Blue Something's latest album. Because, well, we're never really going to run out of those, now are we?

But there has been one very positive development in the last few months. As a serious Dude of Outreach here, I've had a chance to look into the Hayden Planetarium's latest version of their Digital Universe Atlas. This runs on Partiview, visualization software that allows you the thrill of aimless, haphazard flight without the disappointment of waking up from that amazing dream or crashing from that Friday night post-rave mescaline high.

Or in my case, it saves me from doing grownup work and lets me play with astronomical pornography all day.

And let me tell you, this stuff is pretty awesome. NotThe Dark Knight awesome, sure, but at least this won't leave you sobbing silently into your pillow from a deluge of unmitigated angst and cinematic despair afterward. I pieced together a tour of the Universe for this month's Astrofest, and seeing those galaxies spinning around in a polarized eyeglasses-induced 3D perspective is almost as cool as punching out that vapid animatronic gopher that sells lottery tickets on TV here would be. Seriously, Pennsylvania, I'm sure you can tax poor people in a less sadistic way.

The salient point, however, is that the software is completely FREE. Which, to a grad student, is the most beautiful word in the English language. Unless of course, it's a preface to “free colonoscopy,” or something.

Anyway, here's a quick, abbreviated selection of screen shots, similar to what I would show in a typical presentation:

Our lovely home in the cosmos- the Milky Way, surrounded by Large and Small Magellanic clouds, a smattering of boring dwarf galaxies, and a superimposed spherical grid indicating the maximum extent of Patrick Swayze's creepiness.

The Andromeda galaxy and its dwarf companions, looking back on the Milky Way. Looks kind of lonely and bitterly ostracized out here, doesn't it? It's giving us the shifty eye out there, hovering in the distance, endlessly plotting its unspeakable revenge. Kinda like Gargamel from the Smurfs, when you think about it.

The view from the interior of the Virgo cluster, a metropolis of 2,000 + galaxies, Alicia Silverstone's career, and where your puppy actually went when your parents told you he went to go live on a farm Upstate.

The clumpy distribution of galaxies in our universe on larger scales. Any aliens out here will still be blissfully unaware of the XFL and The Magic Hour with Magic Johnson for another ten million years. But the clock for them is ticking.

Galaxies in the 2MASS catalog demonstrating their preferential distribution into city-like clusters (red dots), highway-like filaments (green-yellow dots), and nearly empty voids. The so-called “Great Attractor” is up at the top right, a spot in the universe drawing in the nearest 100 million galaxies at speeds from 600-1000 km/s. This is a place of enormous gravitational significance, matched only by Michael Flatley's 1996 post-Riverdance ego.

Here we've zoomed out to scales of tens of billions of light years, or a volume of the universe encompassing just about everyone who would rather drive a flaming pitchfork into his spinal cord than see Space Chimps. Note the honeycomb pattern of clusters, filaments and voids, along with our incomplete coverage of the sky.

One of the trippier overlays: the WMAP cosmic microwave background map placed in the distance at the epoch of recombination. In the foreground are Sloan-imaged quasars, then more normal galaxies and well-defined large scale structure as you move towards the present at the bottom right. This splotchy color scheme is also what I saw when I slammed my head into a door a week ago.

What I personally find even more exciting is the enormous potential this software has for high school- introductory level astronomy education. This package has some rudimentary statistics commands and visualization procedures for picking out subcategories (e.g. galaxy morphology, luminosity, redshift...) and doing actual analysis while you're looking right at your universe. You can spin clusters around while looking at the morphology-density relationship for galaxies, zoom out and do a census based on any neophyte-applicable observable, or comb through the Great Wall of galaxies. And that's just for the “Extragalactic” shell. The “Milky Way” variant has a striking tableau of accurately rendered nebulae, molecular clouds HI, OII, and HII regions, and the galaxy viewed from the inside in radio to gamma wavelengths. Plus, you can visualize where all our extrasolar planets have been lurking in space.

I've been waiting for something like this for a long time. Well, that and for microwaves to stop having that damn “timed cook” button on them. I mean come on, you're a microwave, it's cooking something for a specified length of time is what you do. I shouldn't have to press an extra button for this.

So definitely check this stuff out! The 300+ page manuals can seem a little daunting, but a lot of the commands they go over are mighty useful. And stay tuned for what's hopefully the final installment of the Unappreciated Topics in Astronomy series, provided I can find my kneecap-busting crowbar and track down my remaining interviewees. You either talk about astronomy, or get Nancy Kerriganed. That's just how I roll.
 

Galaxies are now known to contain Supermassive Black Holes in their centers. In most galaxies (perhaps 90 percent) the central black hole has very little observable effect on the galaxy. In the remaining about 10 percent, a supply of gas becomes available, perhaps in Interacting Galaxies and Mergers. When the gas sinks to within a distance of a few light years, the black hole pulls gas into an accretion disk roughly the size of our solar system. The gas becomes heated to high temperatures as it falls into the accretion disk.

The accretion disk can be very luminous and in the brightest sources (called quasars) can out shine all the stars in the galaxy. The objects in which the black hole is accreting gas in this way are said to have Active Galactic Nuclei (AGN). In addition to the photons from the accretion disk, AGN may also generate winds or outflows. In fact outflows seem to be a general phenomenon associated with disks around compact objects, especially young stellar objects. In about 10 percent of AGN called Radio Galaxies, almost exclusively in large elliptical galaxies, the outflows emit synchrotron emission at radio wavelengths.

Most if not all galaxies may go though an Active Galactic Nuclei (AGN) phase at some point. The life time of the AGN phase is likely to be in the range 10 to 100 million years.

The supply of gas which feeds the accretion disk can have two additional effects. Dense gas and dust around the accretion disk causes our view of the AGN to be blocked in about 2/3 of the objects. In these cases, we do not see the light from the accretion disk directly. The light which is absorbed by the dust is typically re-radiated in the infrared part of the spectrum. In addition, the gas can also form stars, some times quite rapidly, producing a “starburst”.

Active Galactic Nuclei are often found in Starburst Galaxies.

Preview Image

Spiral galaxy NGC7742 which contains a bright AGN. (Source: The Hubble Heritage Team (AURA/STScI/NASA) )