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Supermassive black holes might fill the universe.

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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, are believed to contain supermassive black holes at their centers.

supermassive black hole.
An artist's conception of a supermassive black hole & accretion disk. Credit: NASA/JPL-Caltech.

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, average 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 of supermassive black holes.

supermassive black hole.
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 RXJ 1242-11. Left: X-ray image, Right: optical image.

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 and prevent 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 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.

Doppler measurements of supermassive black holes.

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.

Milky Way galactic center black hole.

The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group have provided evidence that Sagittarius A* at the center of the Milky Way is the site of a supermassive black hole, based on data from the ESO and the Keck telescopes. Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses. Roughly, 8.154572 1036 kg.

Supermassive 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 31 and Messier 32. In a larger number of so-called active galaxies and 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 spiraling 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. As of November 2008(update), another binary pair, in OJ 287, contains the most massive black hole known, with a mass estimated at 18 billion solar masses,

Supermassive black hole mass and galaxy formation.

There is a correlation between the mass of the supermassive black hole and the mass of the spheroid that contains it (the bulge of spiral galaxies, and the whole galaxy for ellipticals). There is an even tighter correlation between the black hole mass and the velocity dispersion of the spheroid, the M-sigma relation. 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.

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