supermassive black hole

A supermassive black hole (SMBH) is a very large black hole, generally in the center of a galaxy. A black hole is considered "supermassive" if its mass is on the order of millions (or hundreds of thousands) of solar masses or more, sometimes cited as 10⁸ through 10⁹ as typical (as opposed to a stellar-mass black hole, likely to be about 5 to 50 solar masses). The largest observed as of 12/2019 is on the order of 40 billion solar masses. A SMBH has been detected and observed at the center of the Milky Way (named Sag A*), and it is thought that galaxies larger than dwarf galaxies generally host them. Such large black holes are considered the energy source for active galactic nuclei (AGNs), and galaxies without an AGN are presumed to host a dormant SMBH. Some methods of estimating the mass of SMBHs:

• stellar dynamics: observing stars orbiting it.
• gas dynamics: from spectral line shapes.
• reverberation mapping to determine the order-of-magnitude of its physical size.
• black hole shadow: as done for the M87 observation by the Event Horizon Telescope (EHT).

A binary SMBH (BSMBH) is a pair of co-orbiting SMBHs; they are thought to result from galaxy mergers. The terms supermassive black hole binary (SMBHB) and supermassive binary black hole (SMBBH) are used for the same pairing: no other astronomical object is of a mass-scale to be thought of as a SMBH companion.

There is a mystery regarding SMBH formation since the Eddington limit would seem to regulate their growth. Current SMBHs have had enough time to grow, but quasars at extreme redshifts suggest a power source that only an SMBH can provide, which would have had insufficient time to grow, and furthermore, there have not been observations suggesting some kind of atypical rapid growth in process.

Regarding terms, SBH is occasionally used to mean SMBH, but perhaps more often to mean "stellar black hole". SMBH unfortunately might be used or read to mean "stellar mass" or "solar mass", but its use as "supermassive" appears pretty consistent.