The Universe’s biggest black holes may be forged in violent mergers

The new research, led by Cardiff University, examined version 4.0 of LIGO-Virgo-KAGRA's Gravitational-Wave Transient Catalog (GWTC4), which contains 153 reliable detections of merging black holes.

Researchers focused on whether the largest black holes in the catalog could be "second-generation" objects. In this scenario, black holes formed from dying stars collide with each other, then merge again in dense stellar environments where stars are packed up to a million times more tightly than around our Sun.

The findings, published in Nature Astronomy, suggest the most massive black holes detected through gravitational waves belong to a separate class with a very different history from smaller black holes.

Gravitational Waves Reveal Two Black Hole Populations

"Gravitational-wave astronomy is now doing more than counting black hole mergers," explains lead author Dr. 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. This is exciting because we can use the information to test our understanding of how stars and clusters evolve in the Universe."

By analyzing the gravitational-wave signals, the team identified two distinct groups:

• a lower-mass population consistent with ordinary stellar collapse
• a higher-mass population whose spins appear exactly like those expected from hierarchical mergers in dense star clusters

Researchers say the spin behavior of the heavier black holes was especially revealing.

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

"Unlike the lower-mass systems we analyzed, which were generally slowly-spinning, the higher-mass systems are consistent with having more rapid spins, oriented in seemingly random directions. 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 catalogs."

Evidence for the Black Hole "Mass Gap"

The study also strengthens evidence for a mysterious "mass gap" predicted by astrophysicists for decades. According to this theory, stars above a certain size should explode so violently that they are destroyed completely instead of collapsing into black holes.

This would create a forbidden range where black holes formed directly from stars should not exist.

The researchers identified this transition in black holes with masses around 45 times greater than the Sun.

Dr. Antonini said: "In our study we find evidence for the long-predicted pair-instability mass gap -- a range of masses where stars are not expected to leave behind black holes at all. 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?

"The biggest black holes in the current sample seem to be telling us about cluster dynamics, not just stellar evolution.

"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."

Black Holes Could Help Scientists Study Nuclear Physics

The researchers say the discoveries may eventually help scientists investigate processes deep inside massive stars.

The team used the transition near the mass gap to study an important nuclear reaction linked to helium burning in stellar cores.

"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 Dr. Fani Dosopoulou, a research associate at Cardiff University.