Supermassive black holes are black holes that are very massive, ranging from hundreds of thousands to billions of Solar masses. They are often found in the centers of galaxies. Quasars, which are one of the brightest objects in the sky, are powered by black holes accreting matter into a disk around them. From observations of quasars, there is evidence that supermassive black holes of order \(10^9 M_\odot\) have existed since before one billion years after the Big Bang. This is a very short time span considering the age of the Universe, and we do not yet know how these supermassive black holes were able to form so quickly.
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| A quasar with a supermassive black hole in the center. Image Credit: http://www.astro.caltech.edu/~stang/m31qsofiles/Quasar_Graham_11_12_5.jpg |
Astronomers have come up with three different ways supermassive black holes may have formed. The first scenario involves an accretion of gas left behind by first generation stars to form high mass black holes. However, there is a limit to how fast this accretion can happen, called the Eddington Limit. The Eddington limit is the point of equilibrium between the gravitational force that is attracting gas towards the black hole, and radiative pressure from the black hole's accretion disk. This is the point at which the fastest rate of accretion is achieved. In order for the black hole to gain sufficient mass within one billion years, it must accrete at the Eddington limit for its entire life time, which is unlikely to be the case.
A second scenario in which supermassive black holes may be formed is the merger of massive objects such as galaxies. Such mergers, however, are likely to cause expulsion of gas, which would only slow the process of accretion rather than increasing it.
Finally, the third scenario involves the collapse of supermassive stars, which have masses of order \(10^5 M_\odot\). In this case, due to the already existing mass from the supermassive star, it will take much less time for the black hole to reach a mass of \(10^9 M_\odot\) though the process of accretion. But to understand this scenario, it is important that we understand how supermassive stars are formed. Similar to regular-sized stars, supermassive stars form from the collapse of gas clouds. Normally, these gas clouds are made up of primarily molecular hydrogen, which keeps the gas clouds cool and causes them to fragment into clumps, which then turn into stars. For a supermassive star, however, these gas clouds should not fragment so that the entire gas cloud can collapse into one massive star. In order for this to happen, the gas cloud must not cool, which happens when its contents have low metallicity and no \(\text{H}_2\). This can occur at higher density and temperature so that \(\text{H}_2\) collides with H to form 3H.
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| Colliding protogalaxies Image Credit: http://astrobites.org/wp-content/uploads/2015/04/feature_im-320x260.jpeg |
We may be able to find such conditions in the high velocity collisions of protogalaxies, which are large gas clouds that will form galaxies. The high velocity collision a hot and dense region where molecular hydrogen and hydrogen reactions can take place. Such collisions, while infrequent, are frequent enough to explain quasars within the first billion years after the Big Bang. We can observe these interactions through the radiation released upon collision.
More information can be found in the original publication, titled "Direct collapse black hole formation via high-velocity collisions of protogalaxies," and the Astrobites summary can be found at http://astrobites.org/2015/04/09/protogalaxy-collisions-birthing-supermassive-black-holes/.
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