Size does matter, and even black holes cannot grow indefinitely. According to a new piece of research, black holes at the center of galaxies can only reach a mass of about 50 billion times that of the Sun before they disrupt their feeding cycle and stop gaining weight.
Supermassive black holes (SMBHs) exist at the core of most galaxies and tend to be surrounded by a disc of gas, which acts as their main source of food. If the gas loses energy it falls inwards towards the black hole. This accretion disc can be easily disrupted, which interrupts the feeding cycle, stunting the black hole's growth.
Professor Andrew King of the University of Leicester has looked at the details of the arrested development in SBMHs. The results are presented in a letter (available on Arxiv) that has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
“I was looking at a different question altogether, and then noticed that a formula which a colleague and I had worked out almost 10 years ago for a different purpose directly gave the mass limit,” King told IFLScience. “In those days the largest measured black hole masses were much smaller (about a billion times the Sun I think) than now, so the question of a limit of about 50 billion Suns was less urgent.”
As observational techniques have improved, we have been able to discover larger SMBHs, with the heaviest currently on record having a mass of about 40 billion Suns. So, understanding how the size of these monsters increases is now a lot more important; hypotheses can be tested against observations. Black holes can't be seen directly, so we need to look at other signs of their presence. We observe black holes by looking at the light coming off the disc around them or the powerful particle jets excited by the magnetic field.
According to King’s calculations once an SMBH reaches the mass of 50 billion suns, the energy it emits is enough to break up the disc. An idealized disc (one that spins in perfect agreement with the SMBH) could lead to a much higher mass (270 billion suns) but it is highly unlikely to form that way. The letter suggests that it’s possible for SMBHs to exceed the 50 billion limit if they are the result of two black holes merging rather than accretion from the in-falling material.
If these enormous black holes exist, they would be incredibly rare, and we would not be able to spot them with traditional methods.
“Probably the most likely way is to observe such a hole via gravitational lensing – light from a still more distant object (usually a quasar) would be bent by the hole’s gravity,” added King. “There is no reason in principle why we could not do this – we just need to be lucky.”
[H/T: New Scientist]