October 4, 2023

Colliding black holes can hide in the light of super-bright quasars

    two small, bright dots representing merging black holes converge surrounded by a red swirl of gas and dust

two small, bright dots representing merging black holes converge surrounded by a red swirl of gas and dust

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

Supermassive black holes, which reside in the hearts of most, if not all, galaxies, can grow millions or even billions of times larger than that of the sun. Some of them are surrounded by discs of gas and dust that are heated to enormous temperatures, causing them to glow brightly. While some of this matter is funneled into the smoldering central supermassive black hole, other material is channeled through powerful magnetic fields to the poles of the black hole, where it is blown out at near light speed and also generates powerful light emissions.

These feeding supermassive black holes are called quasars. They can be so bright that they outshine the combined light of any star in the galaxy they are in. But new research suggests that the active galactic nuclei (AGN) that contain quasars may be hiding other small black holes, with masses between three and 10 times that of the sun, that grow by refracting and merging together.

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

Related: The gravitational wave background of the universe has been heard for the first time

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

“These simulations address two main questions: Can gas catalyze the formation of binary black holes, and if so, can they eventually 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 must be yes.”

Supermassive black hole big brothers can be such a hindrance

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

The simulations, each lasting three months, showed how gas in the disks slowed the speed of the stellar black holes.

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 also appeared to develop their own surrounding accretion disks, mini-versions of the supermassive black hole in which they are trapped.

five-panel diagram showing two small black holes converging near a supermassive black hole

five-panel diagram showing two small black holes converging near a supermassive black hole

The influence of the gas surrounding the supermassive black hole on the fusion process between the smaller black holes was also illustrated 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 black holes orbited the supermassive black hole.

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

“These results are incredibly exciting because they validate that black hole mergers can occur in supermassive black hole disks, potentially explaining many or perhaps most of the gravitational wave signals we observe today,” said study team member Bence Kocsis, an Oxford astrophysicist.

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

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