Quasar discs could host black hole collision events

Four panels showing the stages of gravitational interactions between two black holes.
The key stages of the binary formation mechanism highlighted in the cartoon as snapshots from their moment in the simulations. The first panel shows the "mini"-discs around the isolated black holes before they encounter each other in panels 2 and 3 and become bound to one another. Afterwards the binary evolves slowly through gravitational interaction with a reformed mini disc, which rotates around both black holes.
Connar Rowan et al.

New research, in the wake of the gravitational wave discoveries, sheds light on the environments that could lead to black hole merger events. The work is presented this week at the 2023 National Astronomy Meeting by PhD student at the University of Oxford, Connar Rowan.

The first gravitational waves, predicted initially by Albert Einstein in 1916, were detected from Earth in 2015. However, determining their origin in the cosmos has been an open question. To be detectable across such vast distances, the gravitational waves we observe can only have come from pairs of large, highly dense objects in close proximity to each other, such as black hole or neutron star binaries. There have now been over 90 such detections, though the primary astrophysical environment that allows these objects to get close enough to emit gravitational waves remains a mystery.

One possible environment where black holes may undergo frequent mergers is in quasars. A quasar is a powerful active galactic nucleus powered by a supermassive black hole. A dense disc of gas swirls around a supermassive black hole close to the speed of light, resulting in extremely bright emissions.

The interactions of stellar-mass black holes with the gas disc of a supermassive black hole are highly complex and require sophisticated computer simulations to be understood. In the new research, the team of astronomers from the University of Oxford and Columbia University examined the behaviour of such disc-embedded stellar-mass black holes. The work suggests that stellar-mass black holes could be dragged into dense quasar gas discs and forced into binary systems by gravitational interactions with each other and the gas in the discs.

The team have performed high resolution simulations of the gaseous disc of a quasar containing two stellar-mass black holes. The aim of the simulation is to see if the black holes get captured into a gravitationally bound binary system and possibly merge at a later time within the gas disc. These simulations use 25 million gas particles to imitate the complex gas flows during the encounter, which requires a computational running time of around 3 months for each simulation.

The simulations show that the gas reduces the speed of the black holes during the encounter, so black holes that would normally simply fly apart remain gravitationally bound, trapped in orbit around each other while they both in turn orbit the supermassive black hole. This occurs through a mix of gravitational tugging between them and the massive gas streams in the disc and individual “mini” discs around the individual black holes.

In addition, the direct gas drag analogous to air resistance also plays a role where gas ‘eaten’ by the black holes along their path forces them to decelerate. In response to absorbing the black hole kinetic energy via gravitational interaction, the gas is violently ejected immediately following the encounter. This outcome occurs in the majority of the simulations and confirms the previous expectations that gas greatly facilitates the capture of black holes into bound pairs.

A series of cartoons showing the black hole binary formation mechanism.
Cartoon 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 with gas removes energy from the two black holes, allowing them to stay bound.
Connar Rowan et al.

It was also found that the direction of orbit of the black holes impacted how they evolved. In half of the retrograde binaries – binary systems where the black holes orbit each other in the opposite direction to their orbit around the supermassive black hole - the black holes could get close enough to produce significant gravitational waves and very rapidly dissipate their orbital energy via these wave emissions, merging very abruptly.

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

“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 Professor Bence Kocsis, co-author of the research paper.

“If a sizeable fraction of the observed events, either today or in the future, is caused by this phenomenon, we should be able to see a direct association between quasars and gravitational wave sources in the sky”, adds Professor Zoltán Haiman of Columbia University, another co-author of the research paper.

Media contacts

Ms Gurjeet Kahlon
Royal Astronomical Society
Mob: +44 (0)7802 877 700

Ms Megan Eaves
Royal Astronomical Society

Dr Robert Massey
Royal Astronomical Society
Mob: +44 (0)7802 877 699

Science contacts

Connar Rowan
University of Oxford

Dr Tjarda Boekholt
University of Oxford


Professor Bence Kocsis
University of Oxford

Professor Zoltán Haiman
Columbia University


Further information

Connar and his colleagues form the GALNUC research group at the University of Oxford.

Notes for editors

The NAM 2023 conference is principally sponsored by the Royal Astronomical Society (RAS), the Science and Technology Facilities Council (STFC) and Cardiff University.

About the Royal Astronomical Society

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The Science and Technology Facilities Council (STFC) is part of UK Research and Innovation – the UK body which works in partnership with universities, research organisations, businesses, charities, and government to create the best possible environment for research and innovation to flourish. STFC funds and supports research in particle and nuclear physics, astronomy, gravitational research and astrophysics, and space science and also operates a network of five national laboratories, including the Rutherford Appleton Laboratory and the Daresbury Laboratory, as well as supporting UK research at a number of international research facilities including CERN, FERMILAB, the ESO telescopes in Chile and many more.

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About Cardiff University

Cardiff University is recognised in independent government assessments as one of Britain’s leading teaching and research universities and is a member of the Russell Group - the UK’s most research intensive universities. The 2021 Research Excellence Framework found 90% of the University’s research to be world-leading or internationally excellent. Among its academic staff are two Nobel Laureates, including the winner of the 2007 Nobel Prize for Medicine, Professor Sir Martin Evans. Founded by Royal Charter in 1883, today the University combines impressive modern facilities and a dynamic approach to teaching and research. The University’s breadth of expertise encompasses: the College of Arts, Humanities and Social Sciences; the College of Biomedical and Life Sciences; and the College of Physical Sciences and Engineering. Its University institutes bring together academics from a range of disciplines to tackle some of the challenges facing society, the economy, and the environment. More at www.cardiff.ac.uk

Submitted by Gurjeet Kahlon on Mon, 03/07/2023 - 16:26