While most of the binary black-hole mergers recently detected by NSF’s Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and by the Virgo detector in Italy are expected to consist of first-generation black holes formed from the collapse of stars, others might instead be of second — or even higher — generation, containing the remnants of previous stellar-mass black-hole mergers, according to a new review paper published in the journal Nature Astronomy.
An artist’s impression of binary black holes about to collide. Image credit: Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Australian National University.
“Gravitational-wave observations are revolutionizing the field of astronomy and our understanding of compact objects,” said University of Birmingham’s Dr. Davide Gerosa and Northwestern University’s Dr. Maya Fishbach.
“The prototypical gravitational-wave sources are merging binaries composed of black holes and neutron stars.”
“At the time of writing, more than 50 of these events have been detected by the Advanced LIGO and Virgo detector network.”
“What if, instead of being the direct product of stellar collapse, some of the observed black holes are remnants of previous black-hole mergers?”
In their review paper, the authors review theoretical findings, astrophysical modeling and current gravitational-wave evidence of such hierarchical mergers.
Among the LIGO/Virgo events, only a small fraction of systems are expected to be of hierarchical origin, with first-generation mergers comprising the bulk of the population.
“We believe that most of the gravitational waves so far detected are the result of first generation black holes colliding,” Dr. Gerosa said.
“But we think there’s a good chance that others will contain the remnants of previous mergers.”
“These events will have distinctive gravitational-wave signatures suggesting higher masses, and an unusual spin caused by the parent collision.”
According to the team, GW190521 — a gravitational-wave signal detected on May 21, 2019 — is the single most promising hierarchical-merger candidate to date.
“Understanding the characteristics of the environment in which such objects might be produced will also help narrow the search,” the researchers said.
“This must be an environment with a large number of black holes, and one that is sufficiently dense to retain the black holes after they have merged, so they can go on and merge again.”
“These could be, for example, nuclear star clusters, accretion disks — containing a flow of gas, plasma and other particles — surrounding the compact regions at the centre of galaxies, or globular clusters.”
D. Gerosa & M. Fishbach. Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures. Nat Astron, published online July 26, 2021; doi: 10.1038/s41550-021-01398-w