A recent study led by Yale Astronomy Professor Priya Natarajan and ESO post-doctoral fellow Ezequiel Treister (currently a Chandra fellow at the Institute for Astronomy, Hawaii) has shown that while black holes in the Universe can grow to extremely large masses (monster black holes that they term as Ultra-massive black holes) of about a billion or more times as massive as the sun, ultimately they stunt their own growth. Their work, backed up with observational evidence and theoretical arguments estimates a maximum for the mass of a black hole that can grow in the center of galaxies to be about 10 billion times the mass of the sun.
There has been mounting evidence for the key role that black holes might play in the process of galaxy formation; it appears now that they likely are the prima-donnas of this saga. Every galaxy in the Universe including our own appears to host a central black hole and the mass of this black hole is observed to be tightly coupled to the mass of the stars in the central region. This makes sense as interstellar gas is implicated in the formation of stars and in the feeding of the black hole. Black holes are found in either one of two states: alive or dead. Well fed black holes are active and under-nourished ones are dead. Active black holes are often very bright in the optical waveband; however we might not see all of them as the early Universe is a rather dusty place. Luckily, active black holes also emit significant X-ray radiation that can penetrate dust and so X-rays offer a more complete window into the population of active black holes in the Universe. What is puzzling and interesting according to these new findings is that it turns out that black holes ultimately limit their own growth. The work of Natarajan & Treister provides evidence of how this stunting mechanism can be inferred from observational data and how it might operate.
Black holes in the Universe start life out as small seeds and grow with time by feeding on gas. The gas supply to black holes is replenished with time by the merging of galaxies. However, it is not straightforward to feed and grow the black hole. Gas needs to funnel down all the way right to the center of the galaxy and needs to lose all its rotation in order to be swallowed by the hole. One of the most efficient ways to accomplish this is for the gas to settle down into an accretion disk right around the black hole. Using Natarajan's earlier work on the formation of black hole seeds in the very young Universe (roughly one hundred million years after the Big Bang), they predict that a small number of those seeds will indeed grow into monster black holes with masses of a few billion times the mass of the sun by today (13.7 Giga years later). One of the new results in this paper is the prediction of the existence of a handful of these monster black holes (UMBHs) in the nearby Universe - and indeed there are a few reported promising candidates in line with the numbers estimated by Natarajan & Treister. These monster black hole should live in the largest galaxies know, the bright elliptical galaxies that can be found in the centers of galaxy clusters. The key result though it that while a handful of these monster black holes do exist, there is a maximum to which black holes can grow at all epochs in the Universe. They establish this fact by looking carefully at the observed X-ray brightness of active black holes over cosmic time. The only way to match the observations at early times with late times is by having black holes stunt their own growth.
There are sound theoretical arguments that offer insight into how this stunting might occur. Prof. Natarajan worked out one of the possible mechanisms in her PhD thesis at the University of Cambridge. The mechanism she has investigated involves the destruction of the feeding accretion disk by the radiation from the black hole itself. There are several compelling physical reasons why this ought to be the case as has been proposed by many other groups.
The key implication of this result is that black holes play the starring role in galaxy formation by providing the switch that turns all the action off and as a consequence stunt their own growth. The next frontier is to understand better the physics of the formation of the earliest black hole seeds in the Universe in order to figure out who came first - the first stars or the first black hole seeds.