Biggest black holes are born in busy star clusters in violent merging events

The most massive black holes in the Universe detected by the ripples they make in spacetime were not born directly from collapsing stars, according to a new study. These cosmic giants instead build up through a series of repeated and extremely violent collision events in very densely populated star clusters, an international team of researchers argue in an article published today in the journal Nature Astronomy.

Their study, led by Cardiff University, analysed version 4.0 of LIGO–Virgo–KAGRA’s Gravitational-Wave Transient Catalog (GWTC-4), containing 153 sufficiently confident black hole merger detections. The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) participated in this study through researchers at the Institute of Cosmos Sciences of the University of Barcelona (ICCUB).

The team wanted to test the idea that the heaviest black holes in GWTC-4 are second-generation objects, formed when earlier black holes merged and then merged again in the dense cores of star clusters, where stars can be packed up to a million times more tightly than in the Sun’s neighbourhood. Their findings probe the origins of the heaviest black holes detected by their gravitational waves, revealing two distinct populations.

“Gravitational-wave astronomy is now doing more than counting black hole mergers,” explains lead author Fabio Antonini from Cardiff University’s School of Physics and Astronomy. “It is starting to reveal how black holes grow, where they grow, and what that tells us about the lives and deaths of massive stars.”

“The ability to directly point to star clusters as the origin for these mergers opens up the exciting possibility to use gravitational waves as a completely new tool to learn about the formation and early evolution of dense star clusters that form in the early Universe,” says co-author Mark Gieles, ICREA research professor at the IEEC and ICCUB.

In the gravitational-wave data, the team identified, on the one hand, a lower-mass population consistent with ordinary stellar collapse and, on the other hand, a higher-mass population whose spins appear exactly like those expected if those black holes were formed by repeatedly merging with other black holes inside crowded groups of stars, rather than being born directly from single stars.

“What surprised us most was how clearly the high-mass black holes stand out as a separate population,” recalls co-author Isobel Romero-Shaw, Ernest Rutherford Fellow at Cardiff University. 

“Unlike the lower-mass systems we analysed, which were generally slowly-spinning, the higher-mass systems are consistent with having more rapid spins, oriented in seemingly random directions,” she adds. “This is the exact signature you would expect if black holes were repeatedly merging in dense star clusters. That makes the cluster origin much more compelling than it was with earlier catalogues.”

The study also provides the strongest evidence yet for a “mass gap”, where extremely massive stars explode catastrophically rather than collapsing into black holes. The long-predicted theory describes a forbidden mass range for black holes made directly from stars, where very massive stars are expected to be disrupted before they can form black holes.

The team pinpoints this range in a population of stellar-origin black holes 45 times the mass of the Sun and above. Antonini said: “In our study we find evidence for the long-predicted pair-instability mass gap. Gravitational-wave detectors have successfully found black holes that appear to sit in or near that gap, which we identify at around 45 solar masses. 

“So, the key question now is: are these black holes telling us that our models of stellar evolution are wrong, or are they being made in another way?,” Antonini wonders. “The biggest black holes in the current sample seem to be telling us about cluster dynamics, not just stellar evolution.”

He continues: “Above about 45 solar masses the spin distribution changes in a way that is hard to explain with normal stellar binaries alone but is naturally explained if these black holes have already been through earlier mergers in dense clusters.”

The team also used this transition to shed light on an important nuclear reaction involved in helium burning inside massive stars. “In the future, gravitational-wave data may help scientists study nuclear physics, because the mass limit set by pair instability depends on the nuclear reactions taking place in the cores of massive stars,” added co-author Fani Dosopoulou, a research associate at Cardiff University.

Lead Researcher at the IEEC

Mark Gieles

Institute of Space Studies of Catalonia (IEEC)
Institute of Cosmos Sciences (ICCUB)
E-mail: mgieles@ieec.cat, mgieles@icc.ub.edu