Astrophysics
The accretion discs of supermassive black holes may provide a place for mini black holes to grow
THOUSANDS of relatively small black holes may be circling the supermassive ones that lurk at the centres of galaxies. If so, it would not only help explain how small black holes grow larger, it would also help us understand why supermassive versions appear so bright.
The centres of galaxies are extraordinarily dense, so matter – including relatively small, or stellar-mass, black holes – tends to accumulate there. Some of this matter can fall towards a galaxy’s central supermassive black hole, in which case it is known as an active galactic nucleus, or AGN. This creates a glowing, hot ring of plasma around the black hole called an accretion disc. Shuying Zhou at Xiamen University in China and her colleagues have modelled how stellar-mass black holes could end up in these discs and change their appearance.
“Stars and black holes zoom around in a three-dimensional swarm in the nucleus of the galaxy, around the supermassive black hole, and keep crashing through the thin accretion discs,” says Zoltan Haiman at Columbia University in New York, who wasn’t involved in the work. The crashes take energy away from the stars and black holes, and eventually they have so little that they are captured by the disc, he says, “like a fly trap”.
Zhou and her colleagues found that a supermassive black hole should be surrounded by thousands of these small black holes, and that each of them would develop its own tiny accretion disc, further heating up the plasma and gas around the AGN (arXiv, doi.org/msx5).
“At first, we thought that the heating due to stellar-mass black holes might play a minor role,” says Zhou. “But the stellar-mass black holes can greatly heat up gas in the [AGN] accretion disc.” This heating would be particularly noticeable in the outer a
