thumb|alt=|upright=1.5|The first direct image of a supermassive black hole, found in the galactic core of [[Messier 87.]]
A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space that nothing, not even light, can escape. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.
Two supermassive black holes have been directly imaged by the Event Horizon Telescope; these are Sagittarius A*, at the center of the Milky Way, and the black hole at the center of Messier 87, a giant elliptical galaxy.
Description
Supermassive black holes are classically defined as black holes with a mass above 100,000 () solar masses (); some have masses of . Supermassive black holes have physical properties that clearly distinguish them from lower-mass classifications. First, the tidal forces near the event horizon are significantly weaker for supermassive black holes. The tidal force on a body at a black hole's event horizon is inversely proportional to the square of the black hole's mass: a person at the event horizon of a black hole experiences about the same tidal force between their head and feet as a person on the surface of the Earth. Unlike with stellar-mass black holes, one would not experience significant tidal force until very deep into the black hole's event horizon.
It is somewhat counterintuitive that the density of an SMBH (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water. This is because the Schwarzschild radius (<math>r_\text{s}</math>) is directly proportional to its mass. 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, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have a lower average density.
The Schwarzschild radius of the event horizon of a nonrotating and uncharged supermassive black hole of around is comparable to the semi-major axis of the orbit of Uranus, or about 19 AU. Some astronomers refer to black holes of greater than as ultramassive black holes (UMBHs or UBHs), but the term is not broadly used. Possible examples include the black holes at the cores of TON 618, NGC 6166, ESO 444-46 and NGC 4889, which are among the most massive black holes known.
Some studies have suggested that the maximum natural mass that a black hole can reach, while being luminous accretors (featuring an accretion disk), is typically on the order of about . However, a 2020 study suggested even larger black holes, dubbed stupendously large black holes (SLABs), with masses greater than , could exist based on used models; some studies place the black hole at the core of Phoenix A in this category. Hubble's law showed that the object was located several billion light-years away, and thus must be emitting the energy equivalent of hundreds of galaxies. The rate of light variations of the source dubbed a quasi-stellar object, or quasar, suggested the emitting region had a diameter of one parsec or less. Four such sources had been identified by 1964.
In 1963, Fred Hoyle and W. A. Fowler proposed the existence of hydrogen-burning supermassive stars (SMS) as an explanation for the compact dimensions and high energy output of quasars. These would have a mass of about . However, Richard Feynman noted stars above a certain critical mass are dynamically unstable and would collapse into a black hole, at least if they were non-rotating. Fowler then proposed that these supermassive stars would undergo a series of collapse and explosion oscillations, thereby explaining the energy output pattern. Appenzeller and Fricke (1972) built models of this behavior, but found that the resulting star would still undergo collapse, concluding that a non-rotating SMS "cannot escape collapse to a black hole by burning its hydrogen through the CNO cycle".
Edwin E. Salpeter and Yakov Zeldovich made the proposal in 1964 that matter falling onto a massive compact object would explain the properties of quasars. It would require a mass of around to match the output of these objects. Donald Lynden-Bell noted in 1969 that the infalling gas would form a flat disk that spirals into the central "Schwarzschild throat". He noted that the relatively low output of nearby galactic cores implied these were old, inactive quasars. Meanwhile, in 1967, Martin Ryle and Malcolm Longair suggested that nearly all sources of extra-galactic radio emission could be explained by a model in which particles are ejected from galaxies at relativistic velocities, meaning they are moving near the speed of light. Martin Ryle, Malcolm Longair, and Peter Scheuer then proposed in 1973 that the compact central nucleus could be the original energy source for these relativistic jets. Dynamical evidence for a massive dark object was found at the core of the active elliptical galaxy Messier 87 in 1978, initially estimated at . Discovery of similar behavior in other galaxies soon followed, including the Andromeda Galaxy in 1984 and the Sombrero Galaxy in 1988. Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the Green Bank Interferometer of the National Radio Astronomy Observatory. They discovered a radio source that emits synchrotron radiation; it was found to be dense and immobile because of its gravitation. This was, therefore, the first indication that a supermassive black hole exists in the center of the Milky Way.
The Hubble Space Telescope, launched in 1990, provided the resolution needed to perform more refined observations of galactic nuclei. In 1994 the Faint Object Spectrograph on the Hubble was used to observe Messier 87, finding that ionized gas was orbiting the central part of the nucleus at a velocity of ±500 km/s. The data indicated a concentrated mass of lay within a span, providing strong evidence of a supermassive black hole.
Using the Very Long Baseline Array to observe Messier 106, Miyoshi et al. (1995) were able to demonstrate that the emission from an H<sub>2</sub>O maser in this galaxy came from a gaseous disk in the nucleus that orbited a concentrated mass of , which was constrained to a radius of 0.13 parsecs. Their ground-breaking research noted that a swarm of solar mass black holes within a radius this small would not survive for long without undergoing collisions, making a supermassive black hole the sole viable candidate. Accompanying this observation which provided the first confirmation of supermassive black holes was the discovery of the highly broadened, ionised iron Kα emission line (6.4 keV) from the galaxy MCG-6-30-15. The broadening was due to the gravitational redshift of the light as it escaped from just 3 to 10 Schwarzschild radii from the black hole.
On April 10, 2019, the Event Horizon Telescope Collaboration released the first horizon-scale image of a black hole, in the center of the galaxy Messier 87. In 2020, the Nobel Prize in Physics was awarded jointly to Andrea Ghez and Reinhard Genzel "for the discovery of a supermassive compact object at the centre of our galaxy"'. This was considered the first definitive confirmation that Sagittarius A* is indeed a supermassive black hole.
Formation
thumb|An artist's conception of a supermassive black hole surrounded by an accretion disk and emitting a [[relativistic jet.]]
The origin of supermassive black holes remains an active 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. Independently of the specific formation channel for the black hole seed, given sufficient mass nearby, it could accrete to become an intermediate-mass black hole and possibly a SMBH if the accretion rate persists. and ULAS J1342+0928, are hard to explain so soon after the Big Bang. Some postulate they might come from direct collapse of dark matter with self-interaction.
First stars
The early progenitor seeds may be black holes of that are left behind by the explosions of massive stars and grow by accretion of matter. 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.
Before the first stars, large gas clouds could collapse into a "quasi-star", which would in turn collapse into a black hole of around . 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 . 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). A 2018 theory proposes that SMBH seeds were formed in the very early universe each from the collapse of a supermassive star with mass of around .
Direct-collapse and primordial black holes
Large, high-redshift clouds of metal-free gas, when irradiated by a sufficient 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 matter density, of the order of about , 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 about and are named direct collapse black holes.
A 2022 computer simulation showed that the first supermassive black holes can arise in rare turbulent clumps of gas, called primordial halos, that were fed by unusually strong streams of cold gas. The key simulation result was that cold flows suppressed star formation in the turbulent halo until the halo's gravity was finally able to overcome the turbulence and formed two direct-collapse black holes of and . The birth of the first SMBHs can therefore be a result of standard cosmological structure formation.
Primordial black holes (PBHs) could have been produced directly from external pressure in the first moments after the Big Bang. These black holes would then have more time than any of the above models to accrete, allowing them sufficient time to reach supermassive sizes. Formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical.
The formation of a supermassive black hole requires a relatively small volume of highly dense matter having small angular momentum. 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. This is a major component of the theory of accretion disks. Gas accretion is both the most efficient and the most conspicuous way in which black holes grow. The majority of the mass growth of supermassive black holes is thought to occur through episodes of rapid gas accretion, which are observable as active galactic nuclei or quasars.
Observations reveal that quasars were much more frequent when the Universe was younger, indicating that supermassive black holes formed and grew early. A major constraining factor for theories of supermassive black hole formation is the observation of distant luminous quasars, which indicate that supermassive black holes of had already formed when the Universe was less than one billion years old. This suggests that supermassive black holes arose very early in the Universe, inside the first massive galaxies.
thumb|An artist's impression of stars born in winds from supermassive black holes.
Maximum mass limit
There is a natural upper limit to how large supermassive black holes can grow. Supermassive black holes in any quasar or active galactic nucleus (AGN) appear to have a theoretical upper limit of physically around for typical parameters, as anything above this slows growth down to a crawl (the slowdown tends to start around ) and causes the unstable accretion disk surrounding the black hole to coalesce into stars that orbit it. A study concluded that the radius of the innermost stable circular orbit (ISCO) for SMBH masses above this limit exceeds the self-gravity radius, making disc formation no longer possible. At masses just below the limit, the disc luminosity of a field galaxy is likely to be below the Eddington limit and not strong enough to trigger the feedback underlying the M–sigma relation, so SMBHs close to the limit can evolve above this. This is in spite of it does not usually envisage that PBHs could be necessarily in the SLAB range, to which several studies, however, stated that PBHs cannot have initial mass higher than ~, in principle their mass could be anything up to the horizon mass at the time of matter-radiation equality, which is of the order of . As it was also mentioned that PBHs could be good candidates for being the seeds of SMBHs at the center of supergiant galaxies, the distinction between the two classes of explanation is not clear-cut. In addition to the host bulge mass, there is a scaling relation between the mass of the supermassive black hole and the concentration of stars within the bulge.
Another well-known empirical correlation exists between the mass of the supermassive black hole and the stellar velocity dispersion <math>\sigma</math> of the bulge, known as the M–sigma relation. Recent observations show that the M–sigma relation steepens at high black hole masses, a behavior that can be understood in terms of the Virial theorem.
Furthermore, the environment immediately surrounding the supermassive black hole in fainter galaxies is frequently populated by a nuclear star cluster. The co-existence of these two types of central massive objects was quantified in 2009, with a mathematical mass scaling relation between the supermassive black hole and the nuclear star cluster subsequently discovered in 2016. This relation has important implications for understanding the co-evolution of galactic nuclei and for predicting rates of extreme mass ratio inspiral (EMRI) events.
An AGN is now considered to be a galactic core hosting a massive black hole that is accreting matter and displays a sufficiently strong luminosity.
Mergers and recoiled SMBHs
The interaction of a pair of SMBH-hosting galaxies can lead to merger events. Dynamical friction on the hosted SMBH objects causes them to sink toward the center of the merged mass, eventually forming a pair with a separation of under a kiloparsec. The interaction of this pair with surrounding stars and gas will then gradually bring the SMBH together as a gravitationally bound binary system with a separation of ten parsecs or less. Once the pair draw as close as 0.001 parsecs, gravitational radiation will cause them to merge. By the time this happens, the resulting galaxy will have long since relaxed from the merger event, with the initial starburst activity and AGN having faded away.
thumb|Candidate SMBHs suspected to be recoiled or ejected black holes
The gravitational waves from this coalescence can give the resulting SMBH a velocity boost of up to several thousand km/s, propelling it away from the galactic center and possibly even ejecting it from the galaxy. This phenomenon is called a gravitational recoil. The other possible way to eject a black hole is the classical slingshot scenario, also called slingshot recoil. In this scenario first a long-lived binary black hole forms through a merger of two galaxies. A third SMBH is introduced in a second merger and sinks into the center of the galaxy. Due to the three-body interaction one of the SMBHs, usually the lightest, is ejected. Due to conservation of linear momentum the other two SMBHs are propelled in the opposite direction as a binary. All SMBHs can be ejected in this scenario. An ejected black hole is called a runaway black hole. or a spectroscopic binary nature of a quasar/AGN is seen as evidence for a recoiled black hole.
Candidate recoiling black holes include NGC 3718, SDSS1133, 3C 186, E1821+643 and SDSSJ0927+2943. and objects around RCP 28. Runaway supermassive black holes may trigger star formation in their wakes. A linear feature near the dwarf galaxy RCP 28 was interpreted as the star-forming wake of a candidate runaway black hole. Later it was however found that this feature is likely a bulge-less edge-on galaxy. A study using JWST spectroscopy did however find more evidence for this object being produced by a runaway black hole.
Hawking radiation
Hawking radiation is black-body radiation that is predicted to be released by black holes, due to quantum effects near the event horizon. This radiation reduces the mass and energy of black holes, causing them to shrink and ultimately vanish. If black holes evaporate via Hawking radiation, a non-rotating and uncharged stupendously large black hole with a mass of will evaporate in around . for the supermassive black hole in Sgr A* at the center of the Milky Way. However, the resolution provided by presently available telescope technology is still insufficient to confirm such predictions directly.
What already have been observed directly in many systems are the lower non-relativistic velocities of matter orbiting further out from what are presumed to be black holes. Direct Doppler measures of water masers surrounding the nuclei 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 active galaxies.
In the Milky Way
thumb|Inferred orbits of six stars around supermassive black hole candidate Sagittarius A* at the Milky Way [[Galactic Center]]
Evidence indicates that the Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, 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 ( or 120 AU) from the center of the central object. or about .
- The radius of the central object must be less than 17 light-hours, because otherwise S2 would collide with it. Observations of the star S14 indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
- No known astronomical object other than a black hole can contain in this volume of space.
On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.