Thousands of relatively small black holes may be circling the supermassive black holes that lurk at the centres of galaxies. The idea would not only help explain how small black holes grow larger, it would also give us a new understanding of why supermassive black holes appear so bright.
The centres of galaxies are extraordinarily dense, so matter 鈥� including relatively small, or stellar-mass, black holes 鈥� tends to accumulate there. Some of this matter falls towards each galaxy鈥檚 central supermassive black hole, also known as an active galactic nucleus or AGN. This creates a glowing, hot ring of plasma around the black hole called an accretion disc. at Xiamen University in China and her colleagues modelled how stellar-mass black holes could end up in these discs and then alter their appearance.
鈥淪tars 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,鈥� says at Columbia University in New York, who was not involved in this 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, 鈥渓ike a fly trap鈥�.
Advertisement
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. 鈥淎t first, we thought that the heating due to stellar-mass black holes might play a minor role,鈥� says Zhou. 鈥淏ut the stellar-mass black holes can greatly heat up gas in the [AGN] accretion disc.鈥� This heating would be particularly noticeable in the outer areas of the disc, which could increase the radius of the disc鈥檚 glow by more than three times.
If this is happening, it could be a crucial insight into AGNs and how they form. 鈥淐urrently, we have a lot of 鈥榟oles鈥� in knowledge of the physics that create and evolve AGN discs,鈥� says at the Hebrew University of Jerusalem. 鈥淚mproving our understanding of AGN discs is crucial for understanding galactic evolution and history.鈥�
As importantly, it could explain how black holes grow via collisions. 鈥淏lack holes are very, very small, so the chance of them randomly meeting and colliding are very small in this universe with its vast distances,鈥� says at the University of Florida. 鈥淭he accretion disc acts as a sort of black hole assembly line where smaller black holes meet and collide to form more massive ones 鈥� this may be an important channel to merge black holes.鈥�
That process interests astronomers because while we have observed black holes with masses more than about 50 times that of our sun, the question of how they formed has remained unanswered for decades.
The most crucial part of this work is that the effects of the stellar-mass black holes on the accretion discs should be observable, says at Stony Brook University in New York. Detailed observations of the light from AGNs in different wavelengths may enable us to determine whether these stellar-mass black holes really are having the predicted effects, she says. So far, the observations do seem to match 鈥� AGNs that seemed too big and bright in previous data could potentially be explained by this type of heating.
arXiv
