Supermassive Black Holes: Study Finds Some "Wander" Through Galaxies

Black holes put human life into perspective very quickly; they make it easy to ponder your tiny existence in the vastness of space. But it seems, sometimes, black holes need to ponder and wander on their own, an astronomer at Yale University tells Inverse.

In a new study published Tuesday in The Astrophysical Journal Letters, researchers describe the odd habits of “wandering supermassive black holes.” Black hole stans know that supermassive black holes exist at the center of galaxies, right? Well, sometimes, it turns out supermassive black holes “wander” throughout their galaxy solemnly listening to Elliott Smith’s Either/Or, something dark and contemplative. (Not that I’m judging; it’s a good album. I’m just saying a person can only handle so much raw emotion at once. Maybe black holes have a different capacity for depth.)

“A wandering supermassive black hole is a black hole of at least 1 million solar masses that exists away from the center of its host galaxy,” the study’s lead author Michael Tremmel, an astronomer at Yale, tells Inverse. “In the specific case of our simulations, we define such wanderers as being at least 2,000 light-years from the center of the galaxy. In most cases, our simulations predict these black holes are actually significantly farther even than that.”

Indeed, after running computer simulations, Tremmel and his team concluded that galaxies similar in mass to our own probably host many supermassive black holes, and perhaps some of them like to wander about. The researchers used a cosmological simulation program called Romulus.

It’s still not entirely clear what causes SMBH to form, but Tremmel says he and his team have a solid idea.

“You can think of these black holes as ‘failed’ mergers between two supermassive black holes,” he explains. “In some cases, after galaxies merge, their respective supermassive black holes will come together and merge themselves. In the case of these ‘wanderers,’ the smaller galaxy was destroyed by the larger galaxy in such a way as to leave its supermassive black hole far from the center of the larger galaxy where it will not be able to sink efficiently and merge with the central supermassive black hole.”

Black hole merger

Tremmel hopes his team’s research can serve as a stepping stone for other models that will predict supermassive black hole merger rates. In future research, he’ll investigate how we can actually “find” some of them hiding away in the Milky Way.

“I think the most direct line of future research on wandering black holes is to see how we might be able to infer their presence in our galaxy or other nearby massive galaxies (like Andromeda, for example),” he says. “They do not accrete gas, so they do not glow brightly like many of the black holes we are able to ‘see.’ Likely, we will have to look at their gravitational influence on surrounding gas and stars, but since the density of stars and gas is more sparse far from the center of a galaxy, this will be difficult.”

But still, forming from “failed mergers”? That’s a pretty rough way to get started. No wonder why they’re doomed to wander along the galaxy.

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