Colliding black holes could hide in the light of superbright quasars

two small, bright dots representing merging black holes come together surrounding by a red swirl of gas and dust
A simulaton shows how a black hole merger would proceed in the enviroment around a quasar, with the main image representing the moment before the black holes collide. (Image credit: Connar Rowan et al.)

Collisions between stellar-mass black holes may occur in the violent environments created around their larger, feeding counterparts, according to a new study.

Supermassive black holes, which are found at the hearts of most, if not all, galaxies can grow to masses millions or even billions of times that of the sun. Some of them are surrounded by disks of gas and dust that are heated to tremendous temperatures, causing them to glow brightly. While some of this matter is funneled to the feasting central supermassive black hole, other stuff is channeled by powerful magnetic fields to the poles of the black hole, where it is blasted out at near-light speed, also generating powerful emissions of light.

These feeding supermassive black holes are called quasars. They can be so bright that they outshine the combined light of every star in the galaxy that hosts them. But new research suggests that the active galactic nuclei (AGN) that quasars sit within could hide other diminutive black holes, with masses between three and 10 times that of the sun, that are growing by smashing together and merging.

In fact, the quasars and their environments themselves could actually be driving the merger process between smaller black holes, which can be detected here on Earth via the ripples in the fabric of space-time they create called gravitational waves. 

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That was the conclusion of a team of scientists, including Oxford University physics graduate student Connar Rowan, who created a sophisticated computer simulation to mimic the complex interactions between stellar-mass black holes and the gas disk around a supermassive black hole.

"These simulations address two main questions: can gas catalyze black hole binary formation, and if so, can they ultimately get even closer and merge?" Rowan said in a statement. "For this process to explain the origin of the observed gravitational wave signals, both answers need to be yes."

Supermassive black hole big brothers can be such a drag

Examining the simulated environment around a quasar comprised of 25 million gas particles used to imitate the complex gas flows during the encounter, the team saw that stellar-mass black holes could be dragged into dense gas disks. These black holes could then be forced into binary systems as a result of their gravitational influence on each other and that of the gas in the disk.

The simulations, each of which lasted three months, showed how gas in the disks slowed the speed of the stellar-mass black holes, meaning even if these objects would usually fly apart, they instead remained gravitationally bound to each other.

This left them trapped in orbit around each other as binary black holes and also trapped in orbit around their supermassive black hole "big brother." The smaller black holes were also found to develop their own surrounding accretion disks, like mini-versions of the supermassive black hole that has them trapped. 

A diagram showing how two isolated black holes are drawn together by a supermassive black hole and merge in its violent shadow. (Image credit: Connar Rowan et al.)

The influence of the gas around the supermassive black hole on the merger process between the smaller black holes was also exemplified by the fact that the simulations showed the violent ejection of gas immediately after the black holes merged. 

The team also discovered another effect on how the black hole binary evolved over time — the direction in which the stellar-mass black holes were orbiting the supermassive black hole.

In half the systems in which the black holes orbit the supermassive black hole opposite to its rotation — retrograde motion — the black holes would get close enough to produce significant gravitational wave emissions. Because gravitational waves carry angular momentum away from the binary, this causes the two black holes to spiral together faster and merge very abruptly. 

"These results are incredibly exciting as they validate that black hole mergers in supermassive black hole discs can happen, and possibly explain many or perhaps most of the gravitational wave signals we observe today,” said research team member Bence Kocsis, an astrophysicist at Oxford. 

The research was presented by Rowan at the Royal Astronomical Society's 2023 National Astronomy Meeting, which was held in Cardiff, Wales between July 3 and July 7.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.