Hundreds — Perhaps Thousands — of Black Holes Occupy the Center of the Milky Way

Data from the Chandra X-ray Observatory suggests clusters of stellar-mass black holes surround Sagittarius A*, the supermassive black hole at the center of our galaxy.

An image of stellar-mass black holes surrounding Sagittarius A*, the supermassive black hole at the center of the Milky Way | Nature and Hailey, et. al.
An image of stellar-mass black holes surrounding Sagittarius A*, the supermassive black hole at the center of the Milky Way | Nature and Hailey, et. al.

Astronomers have long predicted that thousands of smaller stellar-mass black holes should exist in the area surrounding the big supermassive black holes at the center of large galaxies. However, searches of our own galactic center in the past couple of decades had not found any evidence of these black holes, ones that are just a few times heavier than our sun.

But now, a new search of archival data from the Chandra X-ray Observatory has finally yielded the discovery of a dozen such black holes grouped around Sagittarius A*, the supermassive black hole at the center of the Milky Way. The researchers say that the properties and distribution of these black holes suggest there should be hundreds — and perhaps thousands — of them within a parsec (approximately 3.3 light-years) of Sagittarius A*. Additionally, our galaxy should be representative of other similar galaxies, so they, too, likely have a comparable constituency of stellar-mass black holes.

“This result is really important because it was such an important but yet unconfirmed prediction,” said Columbia University’s Chuck Hailey, lead author of a paper published April 4 in the journal Nature.  “Our galaxy is pretty average, so all the other average galaxies out there are teeming with black holes, too.”

Illustration showing Sagittarius A* surrounded by a cloud of dust and gas within which are 12 small black holes. The takeout shows the 12 black are black hole binaries. | Columbia University

The black holes Hailey and his colleagues discovered come in the form of inactive or “quiescent” low-mass X-ray binary systems. These are binary star systems in which one of the components is a black hole and the other is usually a normal star. The gravity of the black hole pulls matter from the star, which releases gravitational energy as X-rays. A typical low-mass binary emits almost all of its radiation in X-rays and usually less than one percent in visible light.

But these X-ray objects are extremely hard to find.

“Black holes don't really emit much of any kind of radiation, except when they have a star as a companion, in which case they emit X-rays,” Hailey said in an email to Seeker. “People have searched for the large outbursts that sometimes occur in these systems, but they are very rare, so it hadn't yielded results.”

Hailey added that finding black hole binaries specifically in a relatively small area six light-years across is very difficult. The central region of our galaxy is a crowded place, filled with lots of other X-ray sources that aren’t black holes. Plus, there is also a lot of hot gas and dust crowding the field, so it’s a confusing mess.

“So, the black hole binaries are hiding there in a very noisy X-ray crowd,” Hailey said. “There are other esoteric ways to search for them, like when a black hole passes through a gas cloud it would leave an X-ray trail. But that never seemed to work out.”

Hailey said Chandra's capabilities were key to finding the black holes.

“It was absolutely crucial to have the high-angular resolution — the sharp eyes — of Chandra,” he said. “There are simply too many X-ray sources in the central region, and it is so far away, these X-ray sources would have all blurred together making an unanalyzable mess without such a good telescope.”

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Hailey and his team searched through 12 years of Chandra data for X-ray signatures of black hole, low-mass binaries in their inactive state and were able to find 12 within three light years, of Sagittarius A*.

But where there are low-mass binaries, there are also isolated black holes, too, and likely many more solitary black holes than binary systems.

Hailey explained that every now and then, an isolated black hole catches a passing star by gravitational attraction and makes a black hole binary that will emit X-rays.

“This is a rare process, though, happening maybe only 5 percent of the time,” he said. “So these black hole binaries give us a sense of both the size and distribution of the unseen, isolated black hole population, because the binaries will form right where the black holes are.”

In their paper, the team described it as low-mass, X-ray binary systems being “tracers of isolated black holes.”

The researchers say there must be anywhere from 300 to 500 black hole, low-mass binaries and about 10,000 isolated black holes in the area surrounding Sagittarius A*.

“The Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with little ones because we simply can't see their interactions in other galaxies,” Hailey said. “In a sense, this is the only laboratory we have to study this phenomenon."

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Stellar-mass black holes form when massive stars end their lives in supernova explosions. The Milky Way galaxy contains some 100 billion stars, and roughly one out of every thousand stars that form is massive enough to become a black hole. Therefore, our galaxy must harbor some 100 million stellar-mass black holes. Most of these are invisible to us, and before this new finding, only about a dozen others had been identified, and the nearest one is some 1,600 light-years from Earth.

While it has been exciting to finally confirm a long-held theory, Hailey said their findings are also very relevant for the exciting work going on in gravitational wave astronomy.

“Now that we have a good idea of the numbers and distributions of these black hole binaries with a normal star, theorists can more accurately infer the numbers and distribution of more exotic objects like black hole-black hole binaries,” he said. “Thus, we can refine our expectations for how many gravitational wave events in the centers of galaxies, associated with these types of objects, may occur.”