Supermassive black hole
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A supermassive black hole is a black hole with a mass of an order of magnitude between 105 and 1010 solar masses. Most, if not all galaxies, including the Milky Way, contain supermassive black holes at their centers.
Supermassive black holes have properties which distinguish them from their relatively low-mass cousins:
- The average density of 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 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, 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 results from scientific definitions and does not necessarily manifest as a real physical property.
- 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 travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.
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[edit] Formation
There are several 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 of supermassive black hole formation involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses or larger. The star would then become unstable to radial perturbations due to 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 preventing it from leaving a supermassive 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 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 objects. 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.
[edit] Doppler measurements
Direct Doppler measures of water masers surrounding the nucleus 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. The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group[2] provided evidence that Sagittarius A* is the supermassive black hole residing at the center of the Milky Way based on data from the ESO[3] and the Keck telescopes.[4] Our galactic central black hole is calculated to have a mass of 3.7 million solar masses.[5]
[edit] Supermassive black holes outside the Milky Way
In May 2004, Paolo Padovani and collaborators announced their discovery of 30 previously hidden supermassive black holes outside the Milky Way. Their discovery also suggests there are at least twice as many of these black holes as previously thought. It is currently believed that every galaxy contains a supermassive black hole at its center, with most of them being in an "inactive" state not accreting much matter. In contrast, it is thought that black holes are not to be found in the center of globular clusters, although it's believed that some of them, like M15 in Pegasus and Mayall II in the Andromeda Galaxy have central black holes with a mass in the order of magnitude of 104 solar masses in their center.
Some galaxies, Galaxy 0402+379 for example, have two supermassive black holes, forming a binary system. Should these collide, the event would create strong gravitational waves.
On January 8, 2008, the discovery of the most massive known black hole was announced at the American Astronomical Society meeting. It is reported to have a mass of 18 billion solar masses, and is known as OJ287.[6]
[edit] Supermassive black hole mass and galaxy formation
There appears to be a link between the mass of the supermassive black hole in the center of a galaxy and the morphology of the galaxy itself. This manifests as a correlation between the mass of the spheroid (the bulge of spiral galaxies, and the whole galaxy for ellipticals) and the mass of the supermassive black hole. There is an even tighter correlation between the black hole mass and the velocity dispersion of the spheroid. The explanation for this correlation remains an unsolved problem in astrophysics.
[edit] See also
- Active galactic nucleus
- Double Helix Nebula
- Galaxy
- Galactic center
- Magnetospheric eternally collapsing object
- Neutron star
- Quasar
- Sagittarius A*
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[edit] Further reading
- Fulvio Melia (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge University Press. ISBN 978-0-521-81405-8.
- Fulvio Melia (2007). The Galactic Supermassive Black Hole. Princeton University Press. ISBN 978-0-691-13129-0.
[edit] References
- ^ Chandra :: Photo Album :: RX J1242-11 :: 18 Feb 04
- ^ UCLA Galactic Center Group
- ^ ESO - 2002
- ^ http://www.keckobservatory.org/news/old_pages/andreaghez.html
- ^ UCLA Galactic Center Group
- ^ SPACE.com - Colossal Black Hole Shatters the Scales
- Julian H. Krolik (1999). Active Galactic Nuclei. Princeton University Press. ISBN 0-691-01151-6.
[edit] External links
- 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 orbiting star (533 KB MPEG Video)
- Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours ESO, October 21, 2002
- Early Black Holes Grew Up Quickly
- Fulvio Melia's website
- Images, Animations, and New Results from the UCLA Galactic Center Group
- Washington Post article on Supermassive black holes
