Globular clusters may host multiple generations of black hole mergers

Supercomputer simulations show dense star clusters may host multiple generations of black hole mergers in which two coalesce to form a single, more massive hole followed by another merger, and another. Image: Northwestern Visualization/Carl Rodriguez

Supercomputer simulations that include relativistic effects indicate dense star swarms like the globular clusters that many, if not all, galaxies host likely serve as breeding grounds for successive generations of black hole mergers, resulting in more massive bodies than expected in the coalescence of two first generation holes.

“We think these clusters formed with hundreds to thousands of black holes that rapidly sank down in the centre,” Carl Rodriguez, an astrophysicist at the Massachusetts Institute of Technology and the Kavli Institute for Astrophysics and Space Research, said in a statement.

“These kinds of clusters are essentially factories for black hole binaries, where you’ve got so many black holes hanging out in a small region of space that two black holes could merge and produce a more massive black hole. Then that new black hole can find another companion and merge again.”

The Laser Interferometer Gravitational-Wave Observatory – LIGO – first detected gravitational waves from a binary black hole merger in 2015. If LIGO detects a binary black hole component with a mass greater than about 50 times that of the sun, the computer simulations carried out by Rodriquez and his team show it likely would originate in a dense stellar cluster as a result of multiple prior mergers.

“If we wait long enough, then eventually LIGO will see something that could only have come from these star clusters, because it would be bigger than anything you could get from a single star,” Rodriguez said.

Rodriquez and his team used a supercomputer at Northwestern University to simulate interactions within two dozen clusters containing between 200,000 and two million stars with a range of densities and compositions. The simulation modelled stellar evolution and gravitational interactions across 12 billion years, leading to the formation of black holes. The computer also modelled the trajectories of the holes.

“The neat thing is, because black holes are the most massive objects in these clusters, they sink to the centre, where you get a high enough density of black holes to form binaries,” Rodriguez says. “Binary black holes are basically like giant targets hanging out in the cluster, and as you throw other black holes or stars at them, they undergo these crazy chaotic encounters.”

Earlier studies based on Newtonian mechanics, which did not take into account gravitational waves, indicated black holes would seldom interact unless in very close proximity.

But taking relativistic effects into account, along with data from LIGO observations, Rodriquez and his colleagues found that about 20 percent of binary black holes in dense star clusters included at least one member formed in a previous merger. Some of those should have masses in the range of 50 to 130 times that of the sun – more than could be expected from a single star.

“My co-authors and I have a bet against a couple people studying binary star formation that within the first 100 LIGO detections, LIGO will detect something within this upper mass gap,” Rodriguez said. “I get a nice bottle of wine if that happens to be true.”