Searching the Universe for Black Hole nurseries

Gravitational-wave researchers at the University of Birmingham have developed a new model that could help astronomers track down the origin of massive black hole systems in the Universe. 

Black holes are formed following the collapse of stars and possibly supernova explosions.  These colossally dense objects are measured in terms of the mass of our sun — referred to as solar masses. 

Typically, stars will only form black holes of up to 45 solar masses as instabilities in the gravitational collapse of stars seem to prevent the formation of more massive black holes. 

Consequently, a new model is needed to explain the existence of binary black hole systems with masses larger than about 50 solar masses. 

Current theories suggest that these more massive black holes are formed when smaller black holes merge.  This collision produces gravitational waves which can be observed by detectors such as LIGO, Virgo and the upcoming LISA. 

An aerial view of the VIRGO laser interferometer detector is searching for gravitational waves emitted by collisions of black holes (VIRGO)

Scientists believe that these ‘next generation’ black holes — made up of the merger of their ‘parents’ — might be the heavier black holes observable by LIGO and Virgo. 

Researchers from the University of Birmingham’s Institute for Gravitational Wave Astronomy, suggest in a new study, that future detection of multiple generations of black-hole mergers could allow us to figure the birthplace of this next generation of black holes. 

They have produced new calculations that could help astronomers better understand these mergers — and where to find them.  Their findings are published in the journal Physical Review D.

Dr Davide Gerosa, lead author of the paper explains: “Star clusters — groups of stars that are bound together by gravity — might act like black-hole ‘nurseries’, providing an ideal environment to grow generations of black holes. 

“But in order to know what type of star clusters are most likely to be capable of producing these, we first need to know something about the physical conditions that would be needed.” 

The solution to this puzzle lies in calculating the likely escape speed — velocity at which an object would need to be travelling to escape gravitational pull — a cluster needs to have to be able to host a black hole with a mass above 50 solar masses. 

As they merge, black holes receive recoils or kick velocity as gravitational waves are emitted.  The next generation of black holes can form only if their parents have not been ‘kicked out’ of the cluster, In other words, only if the escape speed of the cluster is large enough to prevent escape. 

The team calculated that observing black holes with mass above 50 solar masses would suggest that the cluster where they lived had an escape speed larger than about 50km/s.

Co-author Professor Emanuele Berti from Johns Hopkins University, explains: “Gravitational wave observations provide an unprecedented opportunity to understand the astrophysical settings where black holes form and evolve.  A very massive event would point towards a dense environment with large escape speed”. 

Where might you find these types of dense clusters? 

Many predictions for LIGO and Virgo so far concentrated on ‘globular clusters’ — spherical collections of about a million stars tightly bound together in the outskirts of galaxies.  The only problem here is the escape speed of these clusters is too low. 

LIGO and VIRGO have thus far concentrated on globular clusters such as the one above, but new research suggests that their escape velocity may be too low to host black hole nurseries (Getty Images)

According to the team’s research, this means that these globular clusters are unlikely to host multiple generations of black holes.  Astronomers will need to look further afield: nuclear star clusters — found towards the centre of some galaxies — are dense enough and might provide the type of environment needed to produce these objects. 

DR Gerosa concludes: “Gravitational-wave astronomy is revolutionizing our understanding of the Universe. 

“We are all waiting for upcoming results from LIGO and Virgo to put these and other astrophysical predictions to the test”.