Do you wish to discover colliding black holes? Verify the disks round quasars

The universe is flooded with gravitational waves. Many of these are formed by collisions of massive objects such as black holes and neutron stars. Now astronomers are wondering in what environments these cataclysmic events occur. It turns out they may have to look for quasars.

The first detection of a gravitational wave came in 2015. Since then, astronomers have spotted another 90, and it’s certain more will be discovered. Determining their likely causes and environments is key to understanding the events that cause them. Quasars, with all their activity, seem like a good place to look. This is especially true for the types of black hole-black hole interactions that can trigger gravitational waves.

Artist’s impression of a quasar. They all have supermassive black holes at their hearts. Credit: NOIRLab/NSF/AURA/J. because Silva

A quasar is the heart of an active galactic core. The engine that powers the quasar is a supermassive black hole. Dense disks of gas are also seen where these black hole monsters are. Spinning at nearly the speed of light, these disks are quite bright in various wavelengths of light.

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It turns out that if a stellar-mass black hole is pulled into a disk, it could be forced into a binary with another black hole. Gravitational interactions between them also disrupt the gas in the surrounding disk. This gas could provide some kind of feedback that affects black hole orbits. Ultimately, such feedback could speed up their mergers. That’s the idea behind the latest simulations, described in a presentation at the Royal Astronomical Society meeting. The article was presented by Connar Rowan, a Ph.D. Student at Oxford University in England.

Simulating the merger of black holes in quasar disks

Rowan and a team of astronomers created their computer models to study what’s going on at the heart of a quasar. They wanted to explore their possible role in gravitational waves. “These simulations address two main questions: Can gas catalyze the formation of black hole binary systems, and if so, can they eventually get even closer and merge?” he said. “In order for this process to explain the origin of the observed gravitational-wave signals, both answers must be yes.”

The key stages of the binarization mechanism are highlighted in the cartoon as snapshots of their moment in the simulations. The first panel shows the “mini” disks around the isolated black holes before they meet and connect in panels 2 and 3. The binary then slowly evolves through gravitational interaction with a newly formed mini-disk rotating around both black holes. Credit Connar Rowan et al.

To get these answers, the team simulated and shaped a gas disk containing 25 million particles as it might exist around the central supermassive black hole at the heart of a quasar. They also inserted two stellar-mass black holes to track their behavior in the disk. They wanted to see if the two objects would be forced into a gravity-bound binary system. And what would be the coercive mechanisms? Eventually, they wanted to see if the two black holes would eventually merge. It took about three months for each simulation to provide an answer.

Bence Kocsis, leader of the GalNUC consortium studying these active nuclei, said the simulation is a valuable tool. “These results are incredibly exciting as they confirm that black hole mergers can occur in supermassive black hole disks,” Kocsis said. “And they potentially explain many, if not most, of the gravitational-wave signals we see today.”

Stimulating simulation results

The results reveal several intriguing possibilities that are fueling debate in gravitational-wave research circles. First, the gas in the disk actually reduces the speed of the black holes during an encounter. In fact, they remain locked in orbit around each other even as they orbit the supermassive black hole together. Second, direct gas drag (similar to air drag) also plays a role. The gas gobbled up by the black holes forces them to slow down. In response to the absorption of the black hole’s kinetic energy by gravitational interaction, the gas is violently ejected immediately after the encounter. This result occurs in most simulations and confirms previous expectations that gas greatly facilitates black hole trapping in bound pairs.

Here’s another way to look at the possible black hole mechanism in quasar dust disks. Two isolated black holes orbit a supermassive black hole. They meet inside the large gas disk. The gravitational interaction with the gas extracts energy from the two black holes, allowing them to remain bound. Courtesy of Connar Rowan et al.

The third finding also showed that the direction of the black hole’s orbit also played a role. In binary systems, where the black holes orbit each other in opposite directions to their orbit around the black hole, the black holes got close enough to trigger gravitational waves. This basically slowed them down enough to allow for a final, catastrophic merger.

The simulations of merging black holes near a quasar offer an interesting avenue for astronomers looking for other origins of gravitational waves. “If a significant fraction of the events observed now or in the future are caused by this phenomenon, then we should be able to see a direct connection between quasars and gravitational-wave sources in the sky,” said Columbia University professor Zoltán Haiman, a member of the team.

For more informations

Quasar disks could host black hole collision events
GalNUC: Astrophysical Dynamics and Statistical Mechanics of Galactic Nuclei

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