The supermassive black holes at the centers of galaxies can capture smaller black holes. They end up in orbit around the supermassive giant. This gives the small black holes the opportunity to grow. Plus, it makes the supermassive black holes look even bigger and brighter.
The supermassive black holes at the centers of galaxies may be orbiting by thousands of relatively small black holes. That would explain how small black holes get bigger. It would also give us new insight into why supermassive black holes appear so bright.
The centers of galaxies have an extremely high density and therefore a strong attractive force of gravity. Matter – including ‘stellar’ black holes, whose masses are comparable to that of stars – therefore tends to accumulate there.
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Some of this matter falls to the central supermassive black hole in the galaxy. Such a vibrant system center is also called one active galactic nucleus (AGN). This creates a so-called accretion disk around the black hole. This is a red-hot ring of plasma.
Astronomer Shuying Zhou of Xiamen University in China and her colleagues have used computer models to calculate how small black holes can end up in such disks and how that changes them.
Collisions
‘Stars and black holes buzz around in a swarm around the central supermassive black hole. They keep breaking through the thin accretion disks,” says astronomer Zoltan Haiman of Columbia University in New York, who was not involved in this work. The collisions drain energy from the stars and black holes. Ultimately, they are slowed down so much that they are captured and stuck in the disc, he says, ‘like in a flytrap’.
Zhou and her colleagues discovered that each supermassive black hole must be surrounded by thousands of these small black holes. Each of those small black holes develops its own small accretion disk, further heating the plasma and gas around the AGN. “At first we thought that heating by stellar black holes would play a small role,” says Zhou. ‘But the stellar black holes can enormously heat the gas in the AGN accretion disk.’
This heating should be especially noticeable in the outer areas of the disk. This can increase the range of the disk’s glow by more than three times.
Assembly line
If this process does indeed occur, it could be crucial to our understanding of AGNs and how they form. “We currently have many ‘gaps’ in our knowledge of the physics that determines how AGN disks form and evolve,” says astronomer Shmuel Gilbaum of the Hebrew University of Jerusalem. ‘Improving our understanding of AGN disks is crucial for understanding the history and evolution of the universe.’
This could also explain how black holes keep getting bigger thanks to collisions. “Black holes are very small, so the chance of them accidentally meeting and colliding in this vast universe is extremely small,” says Imre Bartos of the University of Florida. ‘The accretion disk acts as a kind of conveyor belt for black holes. There, smaller black holes can meet and collide to form heavier black holes. This could be an important source of mergers.’
This process interests astronomers because they have observed black holes with a mass more than fifty times that of our Sun. How these black holes formed has been an unanswered question for decades.
Observations
The most important part of this work is that the effects of small black holes on the accretion disks must be observable, says astronomer Rosalba Perna of Stony Brook University in New York. Detailed observations of the light from AGNs in different wavelengths can help us determine whether these stellar black holes indeed have the predicted effects, she says.
So far, the observations appear to agree with the predictions. AGNs that appeared larger and brighter than thought possible could be explained by this heating process.
