The universe is filled with gravitational waves. The collisions of massive objects like black holes and neutron stars create many of them. Now, astronomers are wondering about the environment in which these catastrophic events occur. Turns out they might need to look at quasars.
The first detection of gravitational waves occurred in 2015. Since that time, astronomers have found another 90 points and are sure to detect many more. Identifying possible causes and environments is key to understanding the events that cause them. Quasars, for 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 promote gravitational waves.
Quasars are the heart of an active galactic nucleus. The engine that powers the quasar is a supermassive black hole. Where you have these black hole monsters, you also see dense disks of gas. Those disks spin around at close to the speed of light and are quite bright at different wavelengths of light.
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It turns out that if a stellar-mass black hole is pulled into a disk, it can be forced into a binary system with another black hole. The gravitational interaction between them also disturbs the gas in the surrounding disk. That gas could provide some kind of feedback that affects the orbits of black holes. Ultimately, such feedback can speed up their merger. That’s the idea behind simulations recently described in a presentation at a meeting of the Royal Astronomical Society. The article was published by Connar Rowan, a Ph.D. student at the University of Oxford in England.
Simulation of the merger of a black hole in a quasar disk
Rowan and a team of astronomers created their computer models to uncover the activity at the heart of a quasar. They wanted to explore their possible role in gravitational waves. «These simulations address two key questions: can gases catalyze black hole binary formation, and if so, can they eventually get even closer and merge?» he say. “For this process to explain the origin of the observed gravitational wave signals, both answers need to be yes.”
To get to those answers, the team simulated a disk of gas with 25 million particles and shaped it like it might exist around the central supermassive black hole at the center of a quasar. They also inserted two stellar-mass black holes to monitor their behavior in the disk. They wanted to see if two objects would be forced into a binary system bound by gravity. And, what would be the coercive mechanisms? Ultimately, they wanted to see if the two black holes would eventually merge. It took about 3 months for each simulation to come up with an answer.
Bence Kocsis, head of the GalNUC consortium that researches these active cores, says simulation is a valuable tool. «These results are extremely exciting as they confirm that black hole mergers in supermassive black hole disks can occur,» Kocsis said. «And they probably explain many or perhaps most of the gravitational wave signals we observe today.»
Stimulating simulation results
The results suggest a number of intriguing possibilities that are stimulating discussion in gravitational wave circles. First, the gas in the disk actually slows down the black holes during the encounter. They are actually trapped in orbit around each other even as they orbit the supermassive black hole together. Second, direct air resistance (similar to air resistance) also plays a role. The gas being swallowed by the black holes forces them to slow down. In response to the black hole’s absorption of kinetic energy through the gravitational interaction, the gas was violently ejected shortly after the collision. This result occurs in the majority of simulations and confirms previous expectations that gas greatly facilitates the capture of black holes into bonding pairs.
The third finding also suggests that the orbital direction of the black holes also plays a role. In binary systems where black holes orbit each other opposite to their orbits around the black hole, the black holes come close enough to generate gravitational waves. That basically slows them down enough to create an eventual catastrophic merger.
Simulations of the mergers of black holes in the vicinity of quasars provide a fascinating route for astronomers looking for additional sources of gravitational waves. «If a significant portion of observed events, today or in the future, are caused by this phenomenon, then we can see a direct link,» said Columbia University professor Zoltán Haiman. between quasars and the source of gravitational waves in the sky”. , a member of the team.
For more information
Quasar disks can host black hole collision events
GalNUC: Astrophysical dynamics and statistical mechanics of galactic nuclei
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