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 water. This is because the Schwarzschild radius is directly proportional to mass, such that density is inversely proportional to the square of the mass.
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.
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 instances 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 (ULX's) may be black holes from this missing group.
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 provided evidence that Sagittarius A* is the supermassive black hole residing at the center of the Milky Way based on data from the ESO and the Keck telescopes. Our galactic central black hole is calculated to have a mass of 3.6 million solar masses.
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.
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.
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