The theory of how black holes grow

Astrophysicists in the US have developed a theoretical model for black holes to predict if growth by gas accretion or by merger is dominant.

It’s valid, they say, from the local Universe up to redshift 10 (roughly from the present day to about 13 billion years ago) – and it suggests that the main growth channel depends on the mass of the black hole and on redshift.

In the nearby Universe small black holes grow mostly by accretion, while very big black holes grow mostly via mergers. In the very-far-away Universe, there is a reversal: small black holes grow mostly by mergers, big black holes by accretion.

The model is the work of Avi Loeb and Fabio Pacucci from the Centre for Astrophysics Harvard & Smithsonian (CfA). They presented their results at the virtual 236th meeting of the American Astronomical Society and also published a paper in The Astrophysical Journal.

According to previous studies, black holes that grow mostly by accretion are predicted to rotate much faster on their axes than those that grow mostly via mergers.

“As the rate of rotation, or spin, fundamentally affects the way the region around a black hole shines, studying the main growth modality of black holes helps to provide us with a clearer picture of how bright these sources can be,” Pacucci says.

“We already know that matter falls toward the event horizon of black holes and, as it speeds up, it also heats up, and this gas starts to emit radiation. The more matter a black hole accretes, the brighter it is going to be; that’s why we’re able to observe far-away objects like supermassive black holes.

“They’re one billion times more massive than the Sun, and they are able to emit enormous amounts of radiation so we can observe them from even billions of light years’ distance.”

Loeb adds that even if their environment is gas-free, “black holes can grow in mass through galaxy mergers”.

“We believe that every galaxy contains a massive black hole at its centre, which regulates the formation of stars in their host,” says Pacucci.

“Understanding how black holes formed, grew and co-evolved with galaxies is fundamental to our understanding and knowledge of the universe, and with this study, we have one more piece of the puzzle.”

Future observations will test the new model, the researchers say, because the next generation of space-based X-ray and gravitational wave observatories, including Lynx, Athena, AXIS and LISA – the Laser Interferometer Space Antenna – will be able to detect most of the black holes investigated in this work, up to the very early Universe.