Black holes fascinate all of us. We are amazed that there can be an object which was made from ordinary matter yet is invisible to our eyes. We are shocked by the power of an object that can capture entire stars like a giant vacuum cleaner if we get too close to the surface, or event horizon. And finally we are taken aback by the concept that time itself slows down near to the event horizon (recall the plot in the recent movie titled, “Interstellar”).
We know black holes are made from individual stars in ordinary galaxies like the Milky Way. These so-called stellar mass black holes are very dense, and by interacting with other stars end up settling into the center where they collect together, or coalesce, into much larger objects called supermassive black holes.
Supermassive black holes also range in size. For example, the one at the center of our galaxy has a mass of about 10 million times the mass of the Sun, and they can also get much larger. The question is how much larger? We have observed supermassive black holes with masses up to about 10 billion solar masses. Interestingly, we measure this same upper limit on the mass whether the object is in the nearby or in the distant universe. This leads us to ask if supermassive black holes can exceed even this mass? Can such objects be eventually the size of the entire Milky Way, or even larger?
Astronomers this week have announced that there is a maximum size to a black hole that is set by certain physical processes taking place in the disk. They argue that a supermassive black hole can grow by accreting gas onto it at the rate of 1000 solar masses per year. But after a while, the gas gets so bunched up ‘waiting in line’ in this accretion disk that instead it starts forming stars. The stars in many cases are then able to avoid getting consumed.
Moreover, a mechanism appears to be activated that sends matter out of the accretion disk in a gigantic jet spewing gas out of the entire galaxy. These processes appear to cap the limit at a “mere” 10 billion solar masses. We are on what appears to be a stable orbit situated about two-thirds the way out from the center. We would appear to be forever safe from suffering this fate.