Black Holes Shouldn’t be Able to Merge, but Dozens of Mergers Have Been Detected. How Do They Do It?

Who knows what lurks in the hearts of some globular clusters? Astronomers using a collection of gravitational wave observatories found evidence of collections of smaller black holes dancing together as binaries in the hearts of globulars. What’s more, they’ve detected an increased number of gravitational wave events when some of these stellar-mass black holes crashed together.

The globular cluster NGC 6397 contains many stellar-mass black holes among its 400,000+ stars. It orbits the Milky Way at a distance of about 8,000 light-years from Earth. It has undergone core collapse, with a tightly packed core. Not only does that core contain stars, but also white dwarfs and neutron stars, indicating the aging stellar population. Image courtesy NASA/ESA/STScI.

Stellar-mass Black Holes in a Crowded Environment

Black holes are notoriously tricky objects. The stellar-mass types form when supermassive stars die and collapse on themselves. Typically, they shouldn’t be able to merge easily. Once black holes get fairly close together in binary pairs, they can settle into fairly stable orbits with each other. The situation changes, however, if they’re dancing together in a crowded environment. That actually describes globular clusters to a T. Those stellar agglomerations contain tens of thousands or even millions of stars packed together. Those stars are tightly gravitationally bound together, which creates a gravity “gradient” from the outside into the core. As aging supermassive stars in a globular die, some end up as stellar-mass black holes. Eventually, they sink to the core of the cluster. That’s called “mass segregation”. Eventually, they create a sort of “invisible dark core”.

Black holes in binary pairs in the cluster are most likely to merge. They get a little help from their neighbors along the way. Any nearby massive objects can remove orbital energy from the binary pair. Astronomers call these “dynamical interactions”. The loss of energy pushes them close together and affects the shape of the orbit to make it more elongated. That takes the black hole pair out of the stable orbit they’ve enjoyed.

If this is actually what’s happening, then the black holes pass closer and closer together under the effect of the gravitational interaction. Eventually, a merger occurs. That sets off gravitational waves that we can detect here on Earth. When two black holes are in such an elongated orbit, their gravitational wave signal has characteristic “fingerprints”. Those are clues that can be studied for clues to where the two objects met.

Learning from Black Hole Mergers in Globulars

A team of researchers led by Dr. Isobel Romero-Shaw (formerly of Monash University, now based at the University of Cambridge), with Professors Paul Lasky and Eric Thrane of Monash University, are working together to study those orbital shapes of the black hole binaries just before they merge. They find that some of the binaries observed by the LIGO-Virgo-KAGRA collaboration (a cooperative between three gravitational wave observatories) likely to have those elongated orbits. That indicates that the binaries collided in their densely populated star cluster core. These findings also indicate that a large chunk of the observed binary black hole collisions — at least 35% — could have been forged in such star clusters.

“I like to think of black hole binaries like dance partners”, said Dr. Romero-Shaw. “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. 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!”

There are a lot of these collisional dances to study. Since 2015, at least 85 pairs of black holes have crashed into each other and been detected by the LIGO-Virgo-KAGRA Collaboration. Based on this finding, astronomers in the consortium know these cosmic collisions happen pretty often. The next steps are to observe as many of these as possible, particularly with ever-improving instrumentation. As detector sensitivity improves, researchers should sense these gravitational wave events frequently—perhaps daily. The big question remains, however, what kicks off the final merger event? That’s what the teams are hoping to find out as they observe more of them in the hearts of globular clusters.

About the LIGO-Virgo-KAGRA Collaboration

Gravitational wave research into these kinds of mergers requires worldwide cooperation. That’s because multiple gravitational wave detectors can make it easier for verified events to be studied. The twin LIGO observatories in the United States work together with the Virgo facility in Italy and the KAGRA observatory in Japan. They carry out joint observations and analysis of resulting data and have worked together since 2010. The research team now expects to sense more mergers of binaries in globular clusters during the next LIGO-Virgo-KAGRA observing run, which begins in 2023.

For More Information

Black Hole Carnivals May Produce the Signals Seen by Gravitational Wave Detectors
Four eccentric mergers increase the evidence that LIGO–Virgo–KAGRA’s binary black holes form dynamically
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