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A 'supermassive black hole' is a black hole with a mass of an order of magnitude between $10^5$ and $10^\{10\}$ (hundreds of thousands and tens of billions) of solar masses. It is currently thought that most, if not all galaxies, including the Milky Way, contain supermassive black holes at their galactic centers.

Contents

Compared to lower-mass black holes

Formation

Doppler measurements

Supermassive black holes outside the Milky Way

Supermassive black hole mass and galaxy formation

See also

Further reading

References

External links

Compared to lower-mass black holes

Supermassive black holes have some interesting properties which distinguish them from relatively low-mass cousins:

★ The average density of a supermassive black hole 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.

★ 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.

Formation

Black holes of this size can form in several ways. The most obvious is by slow accretion of matter (starting from a black hole of stellar size). Another method of producing a supermassive black hole involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses and up. The star then becomes unstable to radial perturbations due to electron-positron pair production in its core, and may collapse directly into a black hole with no supernova. Yet another method involves a dense stellar cluster which undergoes core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, it may be possible to construct primordial black holes directly from external pressure in the first instants of the Big Bang.
The problem in forming a supermassive black hole is getting enough matter in a small enough volume. This matter needs to have nearly all of its angular momentum removed in order for this to happen. The process of transporting this angular momentum outwards appears to be the constraining factor in black hole growth, and leads to the formation of accretion disks.
Observationally, there currently appears to be a gap in the population distribution of black holes. There are stellar mass black holes, generated from collapsing stars, which range up to perhaps 10 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these regimes appears to be a dearth of objects. However, some models 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 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 3.8 million solar masses.

Supermassive black holes outside the Milky Way

In May 2004, Paolo Padovani and other leading astronomers 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, there do not appear to be black holes in the center of globular clusters, although it's believed that some of them, like M 15 in Pegasus and Mayall 2 in the Andromeda Galaxy have central black holes with a mass in the order of magnitude of $10^4$ 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.

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.

See also

★ Active galactic nucleus

★ Double Helix Nebula

★ Galaxy

★ Galactic center

★ Magnetospheric eternally collapsing object

★ Micro black hole

★ Neutron star

★ Quasar

★ Sagittarius A
★

Further reading

★ The Edge of Infinity. Supermassive Black Holes in the Universe, Fulvio Melia, , , Cambridge University Press, 2003, ISBN 978-0-521-81405-8

★ The Galactic Supermassive Black Hole, Fulvio Melia, , , Princeton University Press, 2007, ISBN 978-0-691-13129-0

References

1. http://chandra.harvard.edu/photo/2004/rxj1242/
2. http://www.astro.ucla.edu/~ghezgroup/gc/?
3. http://www.eso.org/outreach/press-rel/pr-2002/pr-17-02.html
4. http://www.keckobservatory.org/news/old_pages/andreaghez.html


★ Active Galactic Nuclei, Julian H. Krolik, , , Princeton University Press, 1999, ISBN 0-691-01151-6

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

★ 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