A supermassive black hole is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun (M☉). Supermassive black holes are generally defined as black holes with a mass above 0.1 to 1 million M☉. Some astronomers have begun labeling black holes of at least 10 billion M☉ as ultramassive black holes. Most of these (such as TON 618) are associated with exceptionally energetic quasars. Some studies have suggested that the maximum mass that a black hole can reach, while being luminous accretors, is of the order of ~50 billion M☉. The radius of the event horizon of a supermassive black hole of ~1 billion M☉ is comparable to the semi-major axis of the orbit of planet Uranus.
Observational evidence indicates that almost every large galaxy has a supermassive black hole at the galaxy’s center. The Milky Way has a supermassive black hole in its Galactic Center, which corresponds to the location of Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.
Some of the best evidence for the presence of black holes is provided by the Doppler effect whereby light from nearby orbiting matter is red-shifted when receding and blue-shifted when advancing. For matter very close to a black hole the orbital speed must be comparable with the speed of light, so receding matter will appear very faint compared with advancing matter, which means that systems with intrinsically symmetric discs and rings will acquire a highly asymmetric visual appearance.
The origin of supermassive black holes remains an open field of research. Astrophysicists agree that black holes can grow by accretion of matter and by merging with other black holes. There are several hypotheses for the formation mechanisms and initial masses of the progenitors, or “seeds”, of supermassive black holes.
One hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter.
Another model hypothesizes that before the first stars, large gas clouds could collapse into a “quasi-star”, which would in turn collapse into a black hole of around 20 M☉. These stars may have also been formed by dark matter halos drawing in enormous amounts of gas by gravity, which would then produce supermassive stars with tens of thousands of solar masses. The “quasi-star” becomes unstable to radial perturbations because of electron-positron pair production in its core and could collapse directly into a black hole without a supernova explosion (which would eject most of its mass, preventing the black hole from growing as fast).
An alternative scenario predicts that large high-redshift clouds of metal-free gas, when irradiated by a sufficiently intense flux of Lyman-Werner photons, can avoid cooling and fragmenting, thus collapsing as a single object due to self-gravitation. The core of the collapsing object reaches extremely large values of the matter density, of the order of ~ 107 g/cm3, and triggers a general relativistic instability. Thus, the object collapses directly into a black hole, without passing from the intermediate phase of a star, or of a quasi-star. These objects have a typical mass of ~100,000 M☉ and are named direct collapse black holes.
Gravitation from supermassive black holes in the center of many galaxies is thought to power active objects such as Seyfert galaxies and quasars, and the relationship between the mass of the central black hole and the mass of the host galaxy depends upon the galaxy type.