At the center of almost every galaxy, there is a supermassive black hole, weighing millions if not billions of times the mass of our Sun. If a new paper is correct, around some of those enormous black holes there are many smaller ones, interacting and merging together.

As reported in Nature, the environment around supermassive black holes could be favorable for stellar-sized black hole collisions, like GW190521. This specific event was peculiar to research because it showed two black holes orbiting each other in a non-circular orbit (geometrically eccentric), something never seen before.

The merger was also the heaviest to date, and the first with a potentially light counterpart, creating a whole flurry of excitement and uncertainties. The international team argues that supermassive black holes could be crucial to make these mergers more likely.

“In these environments the typical velocity and density of black holes is so high that smaller black holes bounce around as in a giant game of billiards and wide circular binaries cannot exist,” co-author Professor Bence Kocsis from the University of Oxford, said in a statement.

However, the supermassive black hole is not enough. There are two other key players in facilitating the unusual merger of these black holes – especially since they believe they might not have been a first-time merger, but rather a repeated one.

“New studies show that the gas disk plays an important role in capturing smaller black holes, which over time move closer to the center and also closer to one other. This not only implies they meet and form pairs, but also that such a pair might interact with another, third, black hole, often leading to a chaotic tango with three black holes flying around,” added co-author Hiromichi Tagawa from Tohoku University.

Apart from gas, the other important property was geometry. The team considered collisions three-dimensionally at first – but once they scaled it down to a single plane of interaction, they discovered that it was a lot more likely to have events such as GW190521.

“But then we started thinking about what would happen if the black hole interactions were instead to take place in a flat disk, which is closer to a two-dimensional environment. Surprisingly, we found in this limit that the probability of forming an eccentric merger increases by as much as a 100 times, which leads to about half of all black hole mergers in such disks possibly being eccentric,” lead author Johan Samsing, from the Niels Bohr Institute at the University of Copenhagen, explained

“And that discovery fits incredibly well with the observation in 2019, which all in all now points in the direction that the otherwise spectacular properties of this source are not so strange again, if it was created in a flat gas disk surrounding a super massive black hole in a galactic nucleus.”

Black holes appear to have their own cosmic billiards game happening. The discovery of more events like this will help understand not just gravitational waves or stellar-sized black holes, but also the extreme environment around supermassive black holes.

“But how and where in our Universe do such black holes form and merge? Does it happen when nearby stars collapse and both turn into black holes, is it through close chance encounters in star clusters, or is it something else? These are some of the key questions in the new era of Gravitational Wave Astrophysics,” Samsing stated.