Supermassive black hole
A supermassive black hole is a
black holewith a massof an order of magnitude between 105 and 1.8x 1010 solar masses. Most if not all galaxies, including the Milky Way["Seeing a Star Orbit around the Supermassive Black Hole at the centre of the Milky Way", R. Schödel, et al., Nature, Vol 419, pp. 694-696, October 16, 2002] , are believed to contain supermassive black holes at their centers. [Cite journal
volume = 31
issue = 1
pages = 473–521
last = Antonucci
first = R.
title = Unified Models for Active Galactic Nuclei and Quasars
journal = Annual Reviews in Astronomy and Astrophysics
date = 1993
doi = 10.1146/annurev.aa.31.090193.002353] [Cite journal
volume = 107
pages = 803–845
last = Urry
first = P.
coauthors = Paolo Padovani
title = Unified schemes for radioloud AGN
journal = Publications of the Astronomical Society of the Pacific
date = 1995
doi = 10.1086/133630]
Supermassive black holes have properties which distinguish them from their relatively low-mass cousins:
densityof a supermassive black hole (measured as the mass of the black hole divided by its Schwarzschild volume) can be very low, and may actually be lower than the density of air. This is because the Schwarzschild radiusis 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, and mass merely increases linearly, the volume increases at a greater rate than mass. Thus, density decreases for increasingly larger radii of black holes. One should be aware however that this phenomenon results from scientific definitions and does not necessarily manifest itself as a real physical property.
tidal forces in the vicinity of the event horizonare significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.
Direct Doppler measures of water masers surrounding the nucleus of nearby galaxies have revealed a very fast
keplerianmotion, 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 mappinguses 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. The Max Planck Institute for Extraterrestrial Physicsand UCLAGalactic Center Group [ [http://www.astro.ucla.edu/~ghezgroup/gc/ UCLA Galactic Center Group ] ] provided evidence that Sagittarius A*is the supermassive black hole residing at the center of the Milky Waybased on data from the ESO[ [http://www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html ESO - 2002 ] ] and the Keck telescopes. [http://www.keckobservatory.org/news/old_pages/andreaghez.html] Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses. [http://www.skyandtelescope.com/news/27621359.html]
upermassive black holes outside the Milky Way
There are a handful of galaxies aside from the Milky Way in which the presence of a supermassive black hole can unambiguously be inferred from the motion of stars or gas near the center. These include two other galaxies in the
Local Group, Messier 31and Messier 32. In a larger number of so-called active galaxiesand quasars, the presence of a supermassive black hole is implied by the "activity" of the nucleus, i.e. by the emission of large amounts of radiation, presumably from gas that is spiralling in to the black hole. It is currently believed that the majority of bright galaxies contain a supermassive black hole but that most are in an "inactive" state not accreting much matter. Currently, there is no compelling evidence for massive black holes at the centers of globular clusters, dwarf galaxies, or smaller stellar systems.
At least one galaxy,
Galaxy 0402+379, appears to have two supermassive black holes at its center, forming a binary system. Should these collide, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galaxy mergers [D. Merritt and M. Milosavljevic (2005). "Massive Black Hole Binary Evolution." http://relativity.livingreviews.org/Articles/lrr-2005-8/] .
* cite book
author=Julian H. Krolik
title=Active Galactic Nuclei
publisher=Princeton University Press
id =ISBN 0-691-01151-6
* [http://www.hubblesite.org/go/blackholes 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
* [http://chandra.harvard.edu/photo/2002/0157/0157_composite.jpgImages of supermassive black holes]
* [http://antwrp.gsfc.nasa.gov/apod/image/0210/mwcentre_eso_big.jpgNASA images of supermassive black holes]
* [http://arxiv.org/abs/0808.2624 Is there a Supermassive Black Hole at the Center of the Milky Way? (arxiv preprint)]
* [http://www.einstein-online.info/en/spotlights/milkyway_bh/index.html The black hole at the heart of the Milky Way]
* [http://www.eso.org/outreach/press-rel/pr-2002/video/vid-02-02.mpg ESO video clip of orbiting star] (533 KB MPEG Video)
* [http://www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours]
ESO, October 21, 2002
* [http://www.universetoday.com/am/publish/early_black_holes_grew_quickly.html Early Black Holes Grew Up Quickly]
* [http://www.astro.ucla.edu/research/galcenter/ Images, Animations, and New Results from the UCLA Galactic Center Group]
* [http://www.washingtonpost.com/wp-dyn/content/article/2007/10/30/AR2007103002073.html?nav=most_emailed Washington Post article on Supermassive black holes]
* [http://www.orbitsimulator.com/gravity/articles/mwblackhole.html A simulation of the stars orbiting the Milky Way's central massive black hole]
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