New research sheds light on environments that lead to black hole merger events

New findings presented at the 2023 National Astronomy Meeting by Connar Rowan, a Ph.D. student at the University of Oxford, shed light on the circumstances that could lead to black hole merger events following the detection of gravitational waves. Since the initial detection of gravitational waves in 2015, scientists have been grappling with the challenge of determining their cosmic origins. The observable gravitational waves must originate from close-knit pairs of large, highly dense objects like black hole or neutron star binaries, given the immense distances they travel. While over 90 such detections have been made to date, the specific astrophysical environment that facilitates the emission of gravitational waves remains unknown.

One potential environment where black hole mergers could occur frequently is within quasars, which are powered by supermassive black holes and characterized by intense activity in their galactic nuclei. These quasars feature a dense gas disk that swirls around the supermassive black hole at speeds approaching that of light, resulting in exceptionally bright emissions.

Understanding the intricate interactions between stellar-mass black holes and the gas disk of a supermassive black hole necessitates sophisticated computer simulations. In their recent research, a team of astronomers from the University of Oxford and Columbia University focused on investigating the behavior of disk-embedded stellar-mass black holes. The study proposes that stellar-mass black holes could be drawn into the dense gas disks of quasars and be compelled into binary systems through gravitational interactions with both each other and the gas within the disks.

The team conducted high-resolution simulations of a gaseous quasar disk containing two stellar-mass black holes. The objective of these simulations was to observe whether the black holes would become trapped in a gravitationally bound binary system and potentially merge within the gas disk at a later stage. To replicate the intricate gas flows during these encounters, the simulations employed 25 million gas particles, necessitating approximately three months of computational running time for each simulation.

Illustration of the black hole binary formation mechanism. Two isolated black holes orbiting around a supermassive black hole encounter each other inside the large gas disc around the supermassive black hole. The gravitational interaction and with gas removes energy from the two black holes, allowing them to stay bound. Credit: Connar Rowan et al.

The simulations conducted in the study revealed that the presence of gas in the encounter between black holes has a significant impact. Normally, black holes would simply separate and move away from each other, but the gas in the disk causes them to slow down, resulting in their gravitational attraction keeping them in a bound binary system. This gravitational tugging occurs between the black holes themselves as well as with the massive gas streams in the disk and individual “mini” disks surrounding each black hole.

Additionally, the gas exerts a direct drag force, similar to air resistance, on the black holes as they consume gas along their path. This drag forces them to decelerate, while the gas absorbs their kinetic energy through gravitational interaction. As a result, the gas is violently expelled immediately after the encounter. The majority of simulations produced this outcome, confirming the earlier expectations that gas greatly facilitates the capture of black holes into bound pairs.

The researchers also discovered that the direction of the black holes’ orbit influenced their evolution. In half of the retrograde binaries (where the black holes orbit each other in the opposite direction of their orbit around the supermassive black hole), the black holes could approach each other closely, emitting significant gravitational waves and rapidly dissipating their orbital energy. This led to their abrupt merger.

Connar Rowan, the lead researcher, stated, “These simulations address two main questions: can gas catalyze black hole binary formation and, if so, can they ultimately merge even closer? For this process to explain the origin of the observed gravitational wave signals, both answers need to be yes.”

Professor Bence Kocsis, a co-author of the research paper, expressed excitement about the results, stating, “These findings are incredibly exciting as they confirm that black hole mergers within supermassive black hole disks are possible and may potentially account for many, if not most, of the gravitational wave signals we currently observe.”

Professor Zoltán Haiman of Columbia University, another co-author, added, “If a significant portion of the observed events, both present and future, is caused by this phenomenon, we should be able to directly associate quasars with gravitational wave sources in the sky.”

Source: Royal Astronomical Society

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