Astronomers find evidence of more than 10000 black holes near the center of the Milky Way

Artist's illustration of the Milky Way seen from above.

Image: NASA/JPL-Caltech

By Miriam Kramer2018-04-05 14:38:53 UTC

The heart of the Milky Way is stacked with black holes.

According to a new study published this week in the journal Nature, the center of our galaxy appears to play host to more than 10,000 relatively small black holes that we have gone undiscovered until now.

These black holes could help us explain the history of the Milky Way and understand other galaxies on a grand scale.

Some of these black holes — objects so dense that light can't even escape them — interact with stars and the supermassive black hole called Sagittarius A* (pronounced "Sagittarius A-Star") that functions as the core of the Milky Way.

"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," Columbia University astrophysicist and lead author of the study Chuck Hailey said in a statement.

"In a sense, this is the only laboratory we have to study this phenomenon."

Astronomers have long thought that black holes were lurking in that part of the galaxy, but this study marks the first evidence of them.

These black holes formed after massive stars collapsed in on themselves, but that just marks the beginning of a black hole's life.

The extremely dense objects are actually thought to migrate toward the center of the galaxy, explaining why scientists have found these objects near the middle of our Milky Way, about 26,000 light-years away from our part of the galaxy.

"They [black holes or massive stars] are typically more massive than the other individual objects around them, and the cumulative effect of gravitational interactions between objects over the long life of a galaxy is that more massive objects tend to end up near the center of the system," astronomer Brooke Simmons, who wasn't involved in the study, said via email.

"That includes black holes, so having lots of black holes at the center of a galaxy is a fundamental consequence of gravity in these large systems made up of billions of individual objects with a variety of different masses," Simmons said.

Scientists spotted the black holes thanks to data collected by the Chandra X-ray Observatory space telescope.

The research team went on a hunt for binary black hole systems, which include a star and a black hole trapped in a cosmic dance.

Occasionally, binaries burst out with a flurry of activity, emitting bright X-rays out into the universe, but most of the time, they give off just a low level of radiation.

The scientists hunted for that lower level of radiation in the Chandra data to find these systems.

"Isolated, unmated black holes are just black — they don't do anything. So looking for isolated black holes is not a smart way to find them..." Hailey said.

"But when black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable. 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."

By using this method, scientists were able to find about a dozen black hole binaries within 3 light-years of Sagittarius A*, allowing them to extrapolate that there are more than 10,000 black holes in the middle of our galaxy.

So the next time you find yourself looking up into the sky in a dark, light pollution free part of the Earth and you happen to look toward the center of our cloudy, packed galaxy, just think about all those black holes swirling around out there and everything we have yet to find.

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