In a study titled “How Big Can a Black Hole Grow?” Andrew King from the University of Leicester explores supermassive black holes at the center of galaxies, around which are regions of space where gas settles into an orbiting disk.
This gas can lose energy and fall inwards, feeding the black hole. But these disks are known to be unstable and prone to crumbling into stars.
King calculated how big a black hole would have to be for its outer edge to keep a disk from forming, coming up with the figure of 50 billion solar masses.
The study suggests that without a disk, the black hole would stop growing, meaning 50 billion Suns would roughly be the upper limit. The only way it could get larger is if a star happened to fall straight in or another black hole merged with it.
King said: “The significance of this discovery is that astronomers have found black holes of almost the maximum mass by observing the huge amount of radiation given off by the gas disk as it falls in. The mass limit means that this procedure should not turn up any masses much bigger than those we know because there would not be a luminous disk.
“Bigger black hole masses are in principle possible — for example, a hole near the maximum mass could merge with another black hole, and the result would be bigger still. But no light would be produced in this merger, and the bigger merged black hole could not have a disk of gas that would make light.
“One might nevertheless detect it in other ways, for example as it bent light rays passing very close to it (gravitational lensing) or perhaps in future from the gravitational waves that Einstein’s general theory of relativity predicts would be emitted as it merged.”