Supermassive black hole

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A supermassive black hole is the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses. Most, if not all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.

Supermassive black holes have properties which distinguish them from lower-mass classifications:

The average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) 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 by a much greater factor than the mass as a black hole grows. Thus, average density decreases for increasingly larger radii of black holes (due to volume increasing much faster than 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 traveling towards the black hole center would not experience significant tidal force until very deep into the black hole.

Formation

There are many 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

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.

Supermassive black hole hypothesis

Further information: Supermassive black hole hypothesis

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A because:

The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light hours from the center of the central object.
From the motion of star S2, we estimate the object's mass as 4.1 million solar masses.
We also know that the radius of the central object is significantly less than 17 light hours, because otherwise, S2 would either collide with it or be ripped apart by tidal forces. In fact, recent observations indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
The only known object which can pack 4.1 million solar masses into a volume that small is a black hole.

The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Grouphave provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole, based on data from the ESO and the Keck telescope. Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses, or about 8.2 × 10³⁶ kg.