Black holes, detected by their gravitational wave signal as they collide
with other black holes, could be the product of much earlier parent
collisions.
Such an event has only been hinted at so far, but scientists at the
University of Birmingham in the UK, and Northwestern University in the US,
believe we are getting close to tracking down the first of these so-called
‘hierarchical’ black holes.
In a review paper, published in Nature Astronomy, Dr. Davide Gerosa, of the
University of Birmingham, and Dr. Maya Fishbach of Northwestern University,
suggest that recent theoretical findings together with astrophysical
modeling and recorded gravitational wave data will enable scientists to
accurately interpret gravitational wave signals from these events.
Since the first gravitational wave was detected by the LIGO and Virgo
detectors in September 2015, scientists have produced increasingly nuanced
and sophisticated interpretations of these signals.
There is now fervent activity to prove the existence of so-called
‘hierarchical mergers’ although the detection of GW190521 in 2019 – the most
massive black hole merger yet detected – is thought to be the most
promising candidate so far.
“We believe that most of the gravitational waves so far detected are the
result of first generation black holes colliding,” says Dr. Gerosa. “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.”
Understanding the characteristics of the environment in which such objects
might be produced will also help narrow the search. 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, or accretion disks –
containing a flow of gas, plasma, and other particles – surrounding the
compact regions at the center of galaxies.
“The LIGO and Virgo collaboration has already discovered more than 50
gravitational wave events,” says Dr. Fishbach. “This will expand to
thousands over the next few years, giving us so many more opportunities to
discover and confirm unusual objects like hierarchical black holes in the
universe.”
Reference:
Gerosa, D., Fishbach, M. Hierarchical mergers of stellar-mass black holes and
their gravitational-wave signatures. Nat Astron (2021).
https://doi.org/10.1038/s41550-021-01398-w
Tags:
Space & Astrophysics