Science & Technology

There May Be Tens of Thousands of Black Holes in the Center of the Milky Way

Our place here on Earth is pretty comfortable. It's a comfortable distance from the sun, which is a comfortable size and located in a comfortable spot in the Milky Way galaxy. But at the center of our galaxy, things get pretty dicey. That's where you'll find a supermassive black hole with the heft of 4 million suns. It's called Sagittarius A*, and for decades, scientists believed that it — and every black hole at the center of every galaxy — conjured a particular type of gravitational pull that formed a "cusp" of stars and black holes around its perimeter. That's all been theoretical, however, since we've never actually observed this crowd of objects around Sagittarius A*. But for research published in April 2018, a team of researchers figured out just how to spot this black-hole party — and what they found was awe inspiring.

Ring Around the SMBH

The theoretical crowd of objects around a supermassive black hole is known as a "density cusp" or a "Bahcall–Wolf cusp," named for the researchers who first pointed out the strong possibility of its existence. That early research was mostly just math: the physicists knew how stars and black holes of a certain size interact with each other, and when they crunched the numbers they concluded that supermassive black holes must be ringed by dense clusters of stars — and, by extension, more black holes, since stars turn into black holes at the end of their lives. Last year, researchers finally found solid evidence for a cusp of stars around our own galaxy's black-hole center.

But even though we've spotted those clusters of stars, black holes are infamously difficult to see. They are black, after all. They should be there right alongside those stars, but we haven't found them.

"There are only about five dozen known black holes in the entire galaxy — 100,000 light-years wide — and there are supposed to be 10,000 to 20,000 of these things in a region just six light-years wide that no one has been able to find," said Chuck Hailey, co-director of the Columbia Astrophysics Lab and lead author on the study in a press release. "There hasn't been much credible evidence."

Searching in the Darkness

That's not for lack of trying. In the past, researchers have tried to find these black holes by searching for what are known as black-hole binaries: black holes that either began their lives paired with the gravity of a high-mass star or captured a high-mass star when it passed by. Those stars tend to blow off their outer layers to form an accretion disk around the black hole, which heats up and releases a powerful emission of X-rays. When scientists spot one of these X-ray emissions, they know there's probably a black hole involved.

"It's an obvious way to want to look for black holes," Hailey said, "but the Galactic Center is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years."

Instead, Hailey and his team looked for something more reliable: the weak, but consistent X-rays emitted by the marriage of a black hole and a low-mass star. "If we could find black holes that are coupled with low mass stars and we know what fraction of black holes will mate with low mass stars, we could scientifically infer the population of isolated black holes out there," Hailey said. To do that, they searched through data collected by the Chandra X-ray Observatory and found 12 of these binaries within 3 light-years of Sagittarius A*.

From the measurements they made of this dozen, they extrapolated that there must be anywhere from 300 to 500 of the same kinds of black-hole binaries in the area surrounding the supermassive black hole — and roughly 10,000 isolated black holes.

This is huge, according to Hailey. It's especially important for the new but growing area of gravitational wave detection, since knowing how many black holes there are makes it easier to predict how many gravitational waves might result from them. "All the information astrophysicists need is at the center of the galaxy," he says.

Sagittarius A* in 60 Seconds

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Written by Ashley Hamer April 29, 2018