Supermassive black hole
A supermassive black hole (SMBH) is the largest type of black hole, on the order of hundreds of thousands to billions of solar masses. Most—and possibly all—galaxies are inferred to contain a supermassive black hole at their centers. In the case of the Milky Way, the SMBH is believed to correspond with the location of Sagittarius A*.
Supermassive black holes have properties which distinguish them from lower-mass classifications. First, the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some supermassive black holes. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. Also, the tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut traveling towards the black hole center would not experience significant tidal force until very deep into the black hole.
History of research
Donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a supermassive black hole. Thus, the first thoughts about supermassive black holes related to the center of the Milky Way. Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory. They discovered a radio source that emits synchrotron radiation; also it was found to be dense and immobile because of its gravitation. Therefore, the first discovered supermassive black hole exists in the center of the Milky Way.
The origin of supermassive black holes remains an open field of research. Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by accretion of matter and by merging with other black holes. There are, however, several hypotheses for the formation mechanisms and initial masses of the progenitors, or "seeds", of supermassive black holes. The most obvious hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter. Another model involves a large gas cloud in the period before the first stars formed collapsing into a “quasi-star” and then a black hole of initially only around ~20 solar masses, and then rapidly accreting to become relatively quickly an intermediate-mass black hole, and possibly a SMBH if the accretion-rate is not quenched at higher masses. The initial “quasi-star” would become unstable to radial perturbations because of electron-positron pair production in its core, and may collapse directly into a black hole without a supernova explosion, which would eject most of its mass and prevent it from leaving a black hole as a remnant. Yet another model involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first moments after the Big Bang. Formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical.
The difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enough volume. This matter needs to have very little angular momentum in order for this to happen. Normally, the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth. This is a major component of the theory of accretion disks. Gas accretion is the most efficient, and also the most conspicuous, way in which black holes grow. The majority of the mass growth of supermassive black holes is thought to occur through episodes of rapid gas accretion, which are observable as active galactic nuclei or quasars. Observations reveal that quasars were much more frequent when the Universe was younger, indicating that supermassive black holes formed and grew early. A major constraining factor for theories of supermassive black hole formation is the observation of distant luminous quasars, which indicate that supermassive black holes of billions of solar masses had already formed when the Universe was less than one billion years old. This suggests that supermassive black holes arose very early in the Universe, inside the first massive galaxies.
Currently, there appears to be a gap in the observed mass distribution of black holes. There are stellar-mass black holes, generated from collapsing stars, which range up to perhaps 33 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these regimes there appears to be a dearth of intermediate-mass black holes. Such a gap would suggest qualitatively different formation processes. However, some models suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.
Direct Doppler measures of water masers surrounding the nuclei of nearby galaxies have revealed a very fast Keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these lines to measure the mass and perhaps the spin of the black hole that powers the active galaxy's "engine".
Milky Way galactic center black hole
- The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light-hours (1.8×1013 m or 120 AU) from the center of the central object.
- From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses, or about 8.2×1036 kg.
- The radius of the central object must be less than 17 light-hours, because otherwise, S2 would collide with it. In fact, recent observations indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
- Any model of an astronomical object but a black hole is not foreseen to contain 4.1 million solar masses in this volume of space.
The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole, based on data from ESO's Very Large Telescope and the Keck telescope.
Supermassive black holes outside the Milky Way
Unambiguous dynamical evidence for supermassive black holes exists only in a handful of galaxies; these include the Milky Way, the Local Group galaxies M31 and M32, and a few galaxies beyond the Local Group, e.g. NGC 4395. In these galaxies, the mean square (or rms) velocities of the stars or gas rises as ~1/r near the center, indicating a central point mass. In all other galaxies observed to date, the rms velocities are flat, or even falling, toward the center, making it impossible to state with certainty that a supermassive black hole is present. Nevertheless it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole. The reason for this assumption is the M-sigma relation, a tight (nearly zero scatter) relation between the mass of the hole in the ~10 galaxies with secure detections, and the velocity dispersion of the stars in the bulges of those galaxies. This correlation, although based on just a handful of galaxies, suggests to many astronomers a strong connection between the formation of the black hole and the galaxy itself.
The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.1–2.3) × 108 (110-230 million) solar mass central black hole, significantly larger than the Milky Way's. The largest supermassive black hole in the Milky Way's neighborhood appears to be that of M87, weighing in at (6.4 ± 0.5) × 109 (~6.4 billion) solar masses at a distance of 53.5 million light-years. On 5 December 2011 astronomers discovered the largest super massive black hole yet found to be that of NGC 4889, weighing in at 21 billion solar masses at a distance of 336 million light-years away in the Coma constellation.
Some galaxies, such as Galaxy 0402+379, appear to have two supermassive black holes at their centers, forming a binary system. If they collided, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galactic mergers. The binary pair in OJ 287, 3.5 billion light-years away, contains the previous most massive black hole known (until the December 2011 discovery ), with a mass estimated at 18 billion solar masses. A supermassive black hole was recently discovered in the dwarf galaxy Henize 2-10, which has no bulge. The precise implications for this discovery on black hole formation are unknown, but may indicate that black holes formed before bulges.
On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart. That is, according to astronomers, the only likely explanation of the observations that day of sudden X-ray radiation and the follow-up broad-band observations. The source was previously an inactive galactic nucleus, and from study of the outburst the galactic nucleus is estimated to be a SMBH with mass of the order of a million solar masses. This rare event is assumed to be a relativistic outflow (material being emitted in a jet at a significant fraction of the speed of light) from a star tidally disrupted by the SMBH. A significant fraction of a solar mass of material is expected to have accreted onto the SMBH. Subsequent long-term observation will allow this assumption to be confirmed if the emission from the jet decays at the expected rate for mass accretion onto a SMBH.
In 2012, astronomers reported an unusually large mass of approximately 17 billion solar masses for the black hole in the compact, lenticular galaxy NGC 1277, which lies 220 million light-years away in the constellation Perseus. The putative black hole has approximately 59 percent of the mass of the bulge of this lenticular galaxy (14 percent of the total stellar mass of the galaxy). However this conclusion was called into question by a re-analysis of the data in 2013, which concluded that a much smaller black hole, or none at all, could fit the observations. On February 27th astronomers used the NuSTAR satellite to accurately measure the spin of a supermassive black hole for the first time, reporting that the surface was spinning at almost the speed of light. 
Supermassive black holes in fiction
- Active galactic nucleus
- Black hole
- Central massive object
- Galactic center
- General relativity
- Hypercompact stellar system
- List of most massive black holes
- M-sigma relation
- Neutron star
- Sagittarius A*
- Chandra :: Photo Album :: RX J1242-11 :: 18 Feb 04
- Antonucci, R. (1993). "Unified Models for Active Galactic Nuclei and Quasars". Annual Reviews in Astronomy and Astrophysics 31 (1): 473–521. Bibcode:1993ARA&A..31..473A. doi:10.1146/annurev.aa.31.090193.002353.
- Urry, C.; Padovani, P. (1995). "Unified Schemes for Radio-Loud Active Galactic Nuclei". Publications of the Astronomical Society of the Pacific 107: 803–845. arXiv:astro-ph/9506063. Bibcode:1995PASP..107..803U. doi:10.1086/133630.
- Schödel, R.; et al. (2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature 419 (6908): 694–696. arXiv:astro-ph/0210426. Bibcode:2002Natur.419..694S. doi:10.1038/nature01121. PMID 12384690.
- Celotti, A.; Miller, J.C.; Sciama, D.W. (1999). "Astrophysical evidence for the existence of black holes". Class. Quant. Grav. 16 (12A): A3–A21. arXiv:astro-ph/9912186. doi:10.1088/0264-9381/16/12A/301.
- Melia 2007, p. 2
- Begelman, M. C.; et al. (Jun 2006). "Formation of supermassive black holes by direct collapse in pre-galactic haloed". Monthly Notices of the Royal Astronomical Society 370 (1): 289–298. arXiv:astro-ph/0602363. Bibcode:2006MNRAS.370..289B. doi:10.1111/j.1365-2966.2006.10467.x.
- Spitzer, L. (1987). Dynamical Evolution of Globular Clusters. Princeton University Press. ISBN 0-691-08309-6.
- "Biggest Black Hole Blast Discovered". ESO Press Release. Retrieved 28 November 2012.
- Winter, L.M.; et al. (Oct 2006). "XMM-Newton Archival Study of the ULX Population in Nearby Galaxies". Astrophysical Journal 649 (2): 730–752. arXiv:astro-ph/0512480. Bibcode:2006ApJ...649..730W. doi:10.1086/506579.
- "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month" by Eisenhauer et al, The Astrophysical Journal, 628:246-259, 2005
- Henderson, Mark (December 9, 2008). "Astronomers confirm black hole at the heart of the Milky Way". London: Times Online. Retrieved 2009-05-17.
- Schödel, R.; et al. (17 October 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way". Nature 419 (6908): 694–696. arXiv:astro-ph/0210426. Bibcode:2002Natur.419..694S. doi:10.1038/nature01121. PMID 12384690.
- Ghez, A. M.; et al. (December 2008). "Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits". Astrophysical Journal 689 (2): 1044–1062. arXiv:astro-ph/0808.2870. Bibcode:2008ApJ...689.1044G. doi:10.1086/592738.
- Milky Way's Central Monster Measured
- Ghez, A. M.; Salim, S.; Hornstein, S. D.; Tanner, A.; Lu, J. R.; Morris, M.; Becklin, E. E.; Duchêne, G. (May 2005). "Stellar Orbits around the Galactic Center Black Hole". The Astrophysical Journal 620 (2): 744–757. arXiv:astro-ph/0306130. Bibcode:2005ApJ...620..744G. doi:10.1086/427175.
- UCLA Galactic Center Group
- ESO - 2002
- "| W. M. Keck Observatory". Keckobservatory.org. Retrieved 2013-07-14.
- "Dusty Surprise Around Giant Black Hole". ESO Press Release. Retrieved 21 June 2013.
- Merritt, David (2013). Dynamics and Evolution of Galactic Nuclei. Princeton, NJ: Princeton University Press. p. 23. ISBN 9780691158600.
- King, Andrew (2003-09-15). "Black Holes, Galaxy Formation, and the MBH-σ Relation". The Astrophysical Journal Letters 596: L27–L29. arXiv:astro-ph/0308342. Bibcode:2003ApJ...596L..27K. doi:10.1086/379143.
- Ferrarese, Laura; Merritt, David (2000-08-10). "A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies". The Astrophysical Journal (The American Astronomical Society) 539 (1): L9–12. arXiv:astro-ph/0006053. Bibcode:2000ApJ...539L...9F. doi:10.1086/312838.
- Bender, Ralf; et al. (2005-09-20). "HST STIS Spectroscopy of the Triple Nucleus of M31: Two Nested Disks in Keplerian Rotation around a Supermassive Black Hole". The Astrophysical Journal 631 (1): 280–300. arXiv:astro-ph/0509839. Bibcode:2005ApJ...631..280B. doi:10.1086/432434.
- Gebhardt, Karl; Thomas, Jens (August 2009). "The Black Hole Mass, Stellar Mass-to-Light Ratio, and Dark Halo in M87". The Astrophysical Journal 700 (2): 1690–1701. arXiv:0906.1492. Bibcode:2009ApJ...700.1690G. doi:10.1088/0004-637X/700/2/1690.
- Macchetto, F.; Marconi, A.; Axon, D. J.; Capetti, A.; Sparks, W.; Crane, P. (November 1997). "The Supermassive Black Hole of M87 and the Kinematics of Its Associated Gaseous Disk". Astrophysical Journal 489 (2): 579. arXiv:astro-ph/9706252. Bibcode:1997ApJ...489..579M. doi:10.1086/304823.
- Overbye, Dennis (2011-12-05). "Astronomers Find Biggest Black Holes Yet". The New York Times.
- Major, Jason. "Watch what happens when two supermassive black holes collide". Universe today. Retrieved 4 June 2013.
- D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." http://relativity.livingreviews.org/Articles/lrr-2005-8/
- Two most massive black holes as of December 2011
- Shiga, David (10 January 2008). "Biggest black hole in the cosmos discovered". NewScientist.com news service.
- Kaufman, Rachel (10 January 2011). "Huge Black Hole Found in Dwarf Galaxy". National Geographic. Retrieved 1 June 2011.
- "Astronomers catch first glimpse of star being consumed by black hole". The Sydney Morning Herald. 2011-08-26.
- Burrows, D. N.; Kennea, J. A.; Ghisellini, G.; Mangano, V.; et al (Aug 2011). "Relativistic jet activity from the tidal disruption of a star by a massive black hole". Nature 476 (7361): 421–424. arXiv:1104.4787. Bibcode:2011Natur.476..421B. doi:10.1038/nature10374.
- Zauderer, B. A.; Berger, E.; Soderberg, A. M.; Loeb, A.; et al (Aug 2011). "Birth of a relativistic outflow in the unusual γ-ray transient Swift J164449.3+573451". Nature 476 (7361): 425–428. arXiv:1106.3568. Bibcode:2011Natur.476..425Z. doi:10.1038/nature10366.
- Remco C. E. van den Bosch, Karl Gebhardt, Kayhan Gültekin, Glenn van de Ven, Arjen van der Wel, Jonelle L. Walsh, An over-massive black hole in the compact lenticular galaxy NGC 1277, Nature 491, pp. 729–731 (29 November 2012) doi:10.1038/nature11592, published online 28 November 2012
- Emsellem, Eric (2013). "Is the black hole in NGC 1277 really overmassive?". Monthly Notices of the Royal Astronomical Society 433 (3): 1862–1870. arXiv:1305.3630. Bibcode:2013MNRAS.433.1862E. doi:10.1093/mnras/stt840. Retrieved 31 August 2013.
- Fulvio Melia (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge University Press. ISBN 978-0-521-81405-8.
- Laura Ferrarese and David Merritt (2002). "Supermassive Black Holes". Physics World 15 (1): 41–46. arXiv:astro-ph/0206222. Bibcode:2002astro.ph..6222F.
- Fulvio Melia (2007). The Galactic Supermassive Black Hole. Princeton University Press. ISBN 978-0-691-13129-0.
- Merritt, David (2013). Dynamics and Evolution of Galactic Nuclei. Princeton University Press. ISBN 978-0-691-12101-7.
- Julian Krolik (1999). Active Galactic Nuclei. Princeton University Press. ISBN 0-691-01151-6.
- Black Holes: Gravity's Relentless Pull Award-winning interactive multimedia Web site about the physics and astronomy of black holes from the Space Telescope Science Institute
- Images of supermassive black holes
- NASA images of supermassive black holes
- The black hole at the heart of the Milky Way
- ESO video clip of stars orbiting a galactic black hole
- Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours ESO, October 21, 2002
- Images, Animations, and New Results from the UCLA Galactic Center Group
- Washington Post article on Supermassive black holes
- A simulation of the stars orbiting the Milky Way's central massive black hole