Scientists have discovered that the universe’s most massive black holes may form in the densest of stellar environments, or so-called globular clusters. It is in these clusters where violent collisions are common, suggesting a chaotic new origin for these cosmic titans of our cosmos.
Scientists pinpointed this potential birthplace of massive black holes by studying ripples in space and time — unified as a single entity called spacetime — otherwise known as gravitational waves. The waves were heard” on Earth by our highly sensitive gravitational wave detectors, the Laser Interferometer Gravitational-Wave Observatory (LIGO), KAGRA and Virgo. Gravitational waves were first predicted by Albert Einstein back in 1915 as part of his theory of gravity, known as general relativity. They are launched when powerful events such as the collision and merger of black holes set the very fabric of spacetime ringing.
The team behind this research analyzed 153 black hole merger detections contained in version 4.0 of LIGO–Virgo–KAGRA’s Gravitational-Wave Transient Catalog (GWTC4) with the aim of investigating if the heaviest black holes are formed by the repeated merger of successively larger black holes in dense stellar environments rather than directly from massive star collapses.
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“Gravitational-wave astronomy is now doing more than counting black hole mergers,” team leader Fabio Antonini from the U.K.’s Cardiff University said in a statement. “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.”
Mind the gap!
The team’s gravitational wave investigation into the origins of the most massive black holes revealed two distinct populations of black holes. Antonini and colleagues found a population of lower mass black holes that seem to have been born when massive stars died in supernova explosions and their cores underwent gravitational collapse. They also observed a population of black holes spinning in such a way that it indicates they formed via a chain of hierarchical mergers between smaller black holes in dense star clusters.
That is a revelation that shocked even the team behind this study.
“What surprised us most was how clearly the high-mass black holes stand out as a separate population,” team member Isobel Romero-Shaw of Cardiff University said. “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 catalogues.”
An artist’s concept of two black holes circling each other before merging. (Image credit: NASA)
The team’s research suggests evidence of a long theorized “mass gap” relating to the afterlife of stars. It suggests that the most massive stars don’t collapse to form black holes when they die, and rather undergo a supernova blast that obliterates them completely.
That, in turn, suggests there is a forbidden mass range for stellar-mass black holes born from collapsing stars, resulting from the fact that very massive stars are disrupted before a black hole can be created. The team believes this forbidden mass range begins with a mass of 45 times that of the sun. Black holes with masses greater than this, the researchers propose, are formed by mergers.
“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,” Antonini explained. “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 team’s findings could also reveal more about the death throes of the largest stars and how stellar bodies behave when jammed into regions millions of times denser than the cosmic backyard of the sun.
“The biggest black holes in the current sample seem to be telling us about cluster dynamics, not just stellar evolution,” Antonini said. “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.”
These results were published on Thursday (May 7) in the journal Nature Astronomy.