Introduction
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.
Chandra image of the spiral galaxy M83; the diamonds identify a new class of quasi-soft sources.
Chandra image of the spiral galaxy M83; the diamonds identify a new class of quasi-soft sources. (Source/Credit: NASA/CXC/SAO/R.DiStefano et al.)
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.
Chandra image of the elliptical galaxy NGC4697.
Chandra image of the elliptical galaxy NGC4697. (Source/Credit: NASA/CXC/SAO/R.DiStefano et al.)
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.)
Citation
Fabbiano, Pepi, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Normal Galaxies in X-rays." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 25, 2007].
<http://www.cosmosportal.org/articles/view/135908/>
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