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
"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.
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- "The Black Hole at the Center of Our Galaxy" by Fulvio Melia, Princeton University Press, 2003.
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- 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.
"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.)