Some black hole mergers happen in chaotic star cluster 'carnivals'

An illustration of a horde of black holes located within a star cluster.
An illustration of a horde of black holes located within a star cluster. (Image credit: ESA/Hubble/N. Bartmann)

Some black hole collisions may occur in densely packed, "carnival-like" star clusters, scientists have discovered. The finding hints at what binary black hole systems looked like before the black holes merged and what may trigger such violent events.

The clues to the origins of black hole collisions were discovered within the gravitational waves that such mergers send rippling through the very fabric of space-time, as first predicted by Albert Einstein's theory of general relativity.

For the new research, scientists investigated the orbital shapes of black hole binary systems before the two black holes spiraled together and merged.

Related: How dancing black holes get close enough to merge

They discovered that some of the black hole pairs that the LIGO-Virgo-KAGRA collaboration — a group of laser interferometers located in the U.S., Italy and Japan, respectively — detected in gravitational waves had highly flattened or elliptical orbits. These flattened orbits resembled those of long-period comets, like Halley's Comet, rather than an orbit of a planet like Earth, which implies that the black hole merger that released the gravitational waves could have occurred in dense star clusters

The findings suggest that, of the 85 black hole mergers detected by LIGO-Virgo-KAGRA since 2015, at least 35% happened in star clusters. 

"I like to think of black hole binaries like dance partners," Isobel Romero-Shaw, a physicist at the University of Cambridge who led the study, said in a statement

"When a pair of black holes evolve together in isolation, they're like a couple performing a slow waltz alone in the ballroom. It's very controlled and careful; beautiful, but nothing unexpected," she said. "Contrasting to that is the carnival-style atmosphere inside a star cluster, where you might get lots of different dances happening simultaneously; big and small dance groups, freestyle, and lots of surprises!"

The findings could help astronomers determine where the black hole merger occurred and what causes such mergers.

How do black holes pair up?

Black holes form when massive stars run out of fuel for nuclear fusion. As fusion ceases, so does the outward energy that supports stars against the inward pressure of their own gravity. This imbalance causes the stars' cores to undergo gravitational collapse, and as they rapidly fall inward, outer material is violently ejected, triggering a supernova blast that's energetic enough to push away any material around the newly formed black hole.

As a result, it should be difficult for two black holes to form in close enough proximity to spiral together and merge within 13.8 billion years, the age of the universe.

One way black holes could work around this obstacle to eventually merge would be by forming in highly populated areas of space, such as the hearts of dense star clusters. In such clusters, black holes could start far apart and then get pushed together by two potential mechanisms. 

In the first possible scenario, called "mass segregation," the most massive objects in a cluster would sink to the bottom of a gravitational potential well at the heart of a cluster. This would cause black holes from all areas of the star cluster to move toward its middle; because black holes emit no light, such clusters have invisible, dense and dark cores.

Another possible merger mechanism, called "dynamical interactions," suggests that if two black holes form a binary and begin orbiting each other at a great distance within a star cluster, the interaction between the pair can be influenced by other objects within that cluster. This would result in orbital energy being stripped from the binary black holes, causing them to spiral closer together. 

Both mechanisms involve black hole binaries in star clusters, but they could be identified by the influence they have on the binaries' characteristics, including the shapes of their orbits. 

This means studies like those conducted by Romero-Shaw and her team may be able to distinguish between these merger mechanisms when the gravitational wave detectors of the LIGO-Virgo-KAGRA collaboration begin their third operating run in 2023.

The resumption of detector activity follows a sensitivity upgrade that could help the detectors spot gravitational waves from black hole mergers as frequently as once per day.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.