What is a Supermassive Black Hole (definition, research and history)?
Maarten Schmidt of the radio source 3C 273, in 1963, initiated the journey to the discovery of these extraoadinary phenomenon, supermassive black holes. Hydrogen emission lines had been red shifted, indicating the object was travelling from Earth, located several billion light-years away, suggesting it was emitting the energy equivalent to hundreds of galaxies.
In 1963, Hoyle and Fowler proposed the existence of hydrogen burning supermassive stars (SMS) to explain the fundamental nature of quasars. However, Feynman noted stars above a certain critical mass are unstable and susceptible to collapse into a black hole. Fowler stated that they would likely undergo a series explosion oscillations, a consequence of collapse, resulting in the energy output trend. Appenzeller and Fricke built models and concluded that a non-rotating 0.75×106 M☉ SMS “cannot escape collapse to a black hole by burning its hydrogen through the CNO cycle”.
Salpeter and Zeldovich proposed in 1964 that matter falling onto a massive compact object would explain the properties of quasars. It would require a mass of around 108 M☉. Lynden-Bell noted in 1969, that the infalling gas would form a flat disk that spirals into the central Schwarzschild throat. Ryle and Longair stated that most sources of extra-galactic radio emission could be explained by a model in which particles are ejected from galaxies at relativistic velocities. This is speculating that they are travelling at close to the speed of light.
Wolfe and Burbidge noted in 1970 that the large velocity dispersion of the stars in the nuclear region of elliptical galaxies could only be explained by a large mass concentration at the nucleus. The behaviour could only be explained by a massive black hole with up to 1010 M☉, or a large number of smaller black holes with masses below 103 M☉. Evidence for a massive dark object was found at the core of the active elliptical galaxy Messier 87 in 1978, initially estimated at 5×109 M☉.
Lynden-Bell and Rees hypothesised in 1971 that the centre of the Milky Way galaxy would contain a massive black hole. Sagittarius A* was discovered and named in 1974, by Balick and Brown.
In 1994 the Faint Object Spectrograph on the Hubble was used to observe Messier 87. The data indicated a concentrated mass of (2.4±0.7)×109 M☉ lay within a 0.25″ span, providing strong evidence of a supermassive black hole. Their ground-breaking research noted that a swarm of solar mass black holes within a radius this small would not survive without undergoing collisions, making a supermassive black hole the likely conclusion.
On April 10, 2019, the first horizon-scale image was revealed, of a black hole, in the centre of the galaxy Messier 87. In February 2020, astronomers reported that a cavity in the Ophiuchus Supercluster, originating from a supermassive black hole, is a result of the largest known explosion since the Big Bang.
Supermassive black hole within the Milky Way
It is likely, through a culmination of evidence that the Milky Way galaxy has a supermassive black hole at its centre, in a region called Sagittarius A*. The evidence is detailed below:
- The S2 star follows an orbit with a period of 15.2 years and a pericentre (closest distance) of 17 light-hours (1.8×1013 m or 120 AU) from the centre of the subject.
- From the motion of S2, the object’s mass can be estimated as 4.1 million M☉
- No known astronomical object other than a black hole can contain 4.1 million M☉ in this volume of space.
Infrared observations of bright flare activity near Sagittarius A* show orbital motion of plasma with a period of 45±15 min at a separation of six to ten times the gravitational radius of the SMBH. This emission is consistent with a circularised orbit of a polarised hot spot on an accretion disk in a strong magnetic field.
Classical data for the presence of black holes is offered by the Doppler effect, a phenomenon where light from orbiting matter is red-shifted when receding, or blue-shifted when advancing. For matter near to a black hole the orbital velocity is close to the speed of light. Receding matter will appear very faint, thus systems with symmetric discs will acquire a highly asymmetric visual appearance. Telescope resolution, currently cannot confidently verify predictions accurately enough.
Observation of the lower, non-relativistic velocities of matter, orbiting further out from anticipated black holes is realistic. Direct Doppler measures of water masers surrounding the nuclei of nearby galaxies have revealed a very fast Keplerian motion. Currently, the only known objects that can pack enough matter in such a small space are black holes.
Formation of supermassive black hole
The origin of supermassive black holes is an open question. There are several hypotheses for the formation mechanisms and initial masses of the progenitors. These are agreed to be the starting point of supermassive black holes.
One model hypothesises that before the first stars, large gas clouds could collapse into a quasi-star, which would then collapse into a black hole of around 20 M☉. The quasi-star becomes unstable to radial perturbations because of electron-positron pair production in its core.
Another theory suggests 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 object collapses directly into a black hole, without passing from the intermediate phase of a star, or of a quasi-star. These are called direct collapse black holes.
Independently of the formation of 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.
Formation of a supermassive black hole requires a relative 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.
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. 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 billions of solar masses 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.
Black holes that spawn from dying stars have masses 5–80 M☉. The minimal supermassive black hole is approximately a hundred thousand solar masses, also called intermediate-mass black holes. Some models suggest that ultraluminous X-ray sources (ULXs) may be black holes.
Ultramassive black holes
There is an upper limit to their size. An ultramassive black holes (UMBHs), which is at least ten times the size of most supermassive black holes, appear to have a theoretical upper limit of around 50 billion solar masses, as anything above this slows growth down to a crawl and causes the unstable accretion disk surrounding the black hole to coalesce into stars that orbit it.
Big bang and supermassive black holes
Astronomers have detected the signal linked to a violent collision of two black holes that merged to form a gigantic black hole. “It’s the biggest bang since the Big Bang observed by humanity,” said Caltech physicist Alan Weinstein, who was part of the discovery team.
They are classified in two relative sizes, small, named after stellar black holes, formed when a star collapses and are approximately 5 miles in radius. And there are supermassive black holes potentially billions times bigger, around which entire galaxies revolve.
Previously, it was speculated that star collapses could only create stellar black holes more than 50 times the mass of our sun. In 2019 two detectors picked up a signal that was the merger of the two black holes, signifying each black hole was approximately 66 times the mass of our sun and the other was almost 85 times the mass. This was termed the first ever intermediate black hole, at 151 times the mass of the sun.
Contrary to this theory, research can’t quite explain how merged black holes, would collide with so many others to merge again. There is a possibility that supermassive black holes were formed in the immediate aftermath of the Big Bang.
Size of a supermassive black hole
Currently the largest known black hole, powering the quaser, TON618, has a mass of 66 billion solar masses. The sheer size and enormity of TON 618 questions existing theories to the limits. “It’s surprising that little attention has been paid to the possible existence of stupendously massive black holes until now, because they could exist in principle,” study co-author Kühnel, a theoretical cosmologist at Ludwig Maximilian University in Munich, told Space.com.
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