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→‎Formation: Removed the ‘supermassive’ since the "quasi-star” initially has only 20 solar masses
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[[Image:Supermassiveblackhole nasajpl.jpg|right|thumb|An artist's conception of a supermassive black hole and accretion disk.]]
[[Image:Supermassiveblackhole nasajpl.jpg|right|thumb|An artist's conception of a supermassive black hole and accretion disk.]]


There are many models for the formation of black holes of this size. The most obvious is by slow [[accretion (astrophysics)|accretion]] of matter starting from a black hole of stellar size. 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<ref>{{cite journal |last=Begelman |first=M. C. |authorlink= |coauthors=''et al.'' |year=2006 |month=Jun |title=Formation of supermassive black holes by direct collapse in pre-galactic haloes|journal=Monthly Notices of the Royal Astronomical Society |volume=370 |issue= 1|pages=289–298 |doi=10.1111/j.1365-2966.2006.10467.x |arxiv = astro-ph/0602363 |bibcode = 2006MNRAS.370..289B }}</ref>. 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 supermassive black hole as a remnant. Yet another model<ref>{{cite book |last=Spitzer |first=L. |authorlink= Lyman Spitzer|publisher=Princeton University Press |year=1987 |title=Dynamical Evolution of Globular Clusters |isbn=0-691-08309-6}}</ref> involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to [[theory of relativity|relativistic]] speeds. Finally, [[primordial black hole]]s may have been produced directly from external pressure in the first instants after the [[Big Bang]].
There are many models for the formation of black holes of this size. The most obvious is by slow [[accretion (astrophysics)|accretion]] of matter starting from a black hole of stellar size. 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<ref>{{cite journal |last=Begelman |first=M. C. |authorlink= |coauthors=''et al.'' |year=2006 |month=Jun |title=Formation of supermassive black holes by direct collapse in pre-galactic haloes|journal=Monthly Notices of the Royal Astronomical Society |volume=370 |issue= 1|pages=289–298 |doi=10.1111/j.1365-2966.2006.10467.x |arxiv = astro-ph/0602363 |bibcode = 2006MNRAS.370..289B }}</ref>. 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<ref>{{cite book |last=Spitzer |first=L. |authorlink= Lyman Spitzer|publisher=Princeton University Press |year=1987 |title=Dynamical Evolution of Globular Clusters |isbn=0-691-08309-6}}</ref> involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to [[theory of relativity|relativistic]] speeds. Finally, [[primordial black hole]]s may have been produced directly from external pressure in the first instants after the [[Big Bang]].


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, and explains the formation of [[accretion disc|accretion disks]].
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, and explains the formation of [[accretion disc|accretion disks]].

Revision as of 16:47, 2 April 2012

This article is about the astronomical object. For the song by Muse, see Supermassive Black Hole (song).
A gas cloud with several times the mass of the Earth is accelerating towards a supermassive black hole at the centre of the Milky Way.
Top: artist's conception of a supermassive black hole tearing apart a star. Bottom: images believed to show a supermassive black hole devouring a star in galaxy RX J1242-11. Left: X-ray image, Right: optical image.[1]

A supermassive black hole is the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way[2] (see Sagittarius A*), are believed to contain supermassive black holes at their centers.[3][4]

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 equal to, for stellar-mass black holes, or less than the density of water in the case of some supermassive black holes.[5] 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.

Formation

An artist's conception of a supermassive black hole and accretion disk.

There are many models for the formation of black holes of this size. The most obvious is by slow accretion of matter starting from a black hole of stellar size. 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[6]. 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[7] 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 instants after the Big Bang.

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, and explains the formation of accretion disks.

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[8] suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

Doppler measurements

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".

Such supermassive black holes in the center of many galaxies are thought to be the "engine" of active objects such as Seyfert galaxies and quasars.

Supermassive black hole hypothesis

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre.[9]

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A*[10] because:

  • 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.[11]
  • From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses.[12]
  • The radius of the central object must be significantly less than 17 light hours, because otherwise, S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations[13] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
  • Only a black hole is dense enough to contain 4.1 million solar masses in this volume of space.

The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group[14] have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole,[10] based on data from the ESO[15] and the Keck telescope.[16] Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses,[17] or about 8.2 × 1036 kg.

Supermassive black holes outside the Milky Way

It is now widely accepted that the center of nearly every galaxy contains a supermassive black hole.[18][19] The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation,[20] strongly suggests a connection between the formation of the black hole and the galaxy itself.[18]

The explanation for this correlation remains an unsolved problem in astrophysics. It is believed that black holes and their host galaxies coevolved between 300-800 million years after the Big Bang, passing through a quasar phase and developing correlated characteristics, but models differ on the causality of whether black holes triggered galaxy formation or vice versa, and sequential formation cannot be excluded. The unknown nature of dark matter is a crucial variable in these models.[21][22]

The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.1–2.3) × 108 solar mass central black hole, significantly larger than the Milky Way's.[23] 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 solar masses at a distance of 53.5 million light years.[24][25] 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.[26]

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.[27] 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 [28]), with a mass estimated at 18 billion solar masses.[29] 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.[30]

On March 28, 2011, a supermassive black hole (SMBH) was for the first time seen tearing a mid-size star apart.[31] 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.[32][33] 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.

See also

References

  1. ^ Chandra :: Photo Album :: RX J1242-11 :: 18 Feb 04
  2. ^ Schödel, R. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ 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.
  4. ^ 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.
  5. ^ 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. {{cite journal}}: Invalid |ref=harv (help)
  6. ^ Begelman, M. C. (2006). "Formation of supermassive black holes by direct collapse in pre-galactic haloes". 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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  7. ^ Spitzer, L. (1987). Dynamical Evolution of Globular Clusters. Princeton University Press. ISBN 0-691-08309-6.
  8. ^ Winter, L.M. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  9. ^ "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
  10. ^ a b Henderson, Mark (December 9, 2008). "Astronomers confirm black hole at the heart of the Milky Way". London: Times Online. Retrieved 2009-05-17.
  11. ^ Schödel, R. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Ghez, A. M. (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. {{cite journal}}: Check |arxiv= value (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ Ghez, A. M. (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  14. ^ UCLA Galactic Center Group
  15. ^ ESO - 2002
  16. ^ http://www.keckobservatory.org/news/old_pages/andreaghez.html
  17. ^ http://www.skyandtelescope.com/news/27621359.html
  18. ^ a b 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.
  19. ^ Richstone, D.; et al. (January 13, 1997). "Massive Black Holes Dwell in Most Galaxies, According to Hubble Census". 189th Meeting of the American Astronomical Society. Retrieved 2009-05-17. {{cite web}}: Explicit use of et al. in: |first= (help)
  20. ^ Merritt, D.; Ferrarese, Laura (2001-01-15). "The MBH-σ Relation for Supermassive Black Holes". The Astrophysical Journal. 547 (1). The American Astronomical Society.: 547:140–145. arXiv:astro-ph/0008310. Bibcode:2001ApJ...547..140M. doi:10.1086/318372.
  21. ^ Robert Roy Britt (2003-07-29). "The New History of Black Holes: 'Co-evolution' Dramatically Alters Dark Reputation".
  22. ^ "Astronomers crack cosmic chicken-or-egg dilemma". 2003-07-22.
  23. ^ Bender, Ralf (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ Gebhardt, Karl; Thomas, Jens (2009). "The Black Hole Mass, Stellar Mass-to-Light Ratio, and Dark Halo in M87". The Astrophysical Journal. 700 (2): 1690–1701. Bibcode:2009ApJ...700.1690G. doi:10.1088/0004-637X/700/2/1690. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  25. ^ Macchetto, F.; Marconi, A.; Axon, D. J.; Capetti, A.; Sparks, W.; Crane, P. (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. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  26. ^ Overbye, Dennis (2011-12-05). "Astronomers Find Biggest Black Holes Yet". The New York Times.
  27. ^ D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." http://relativity.livingreviews.org/Articles/lrr-2005-8/
  28. ^ Two most massive black holes as of December 2011
  29. ^ Shiga, David (10 January 2008). "Biggest black hole in the cosmos discovered". NewScientist.com news service.
  30. ^ Kaufman, Rachel (10 January 2011). "Huge Black Hole Found in Dwarf Galaxy". National Geographic. Retrieved 1 June 2011.
  31. ^ "Astronomers catch first glimpse of star being consumed by black hole". The Sydney Morning Herald. 2011-08-26.
  32. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  33. ^ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)

Further reading

External links

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