Not one, but 20,000 black holes hiding in Milky Way's heart

Thousands of them lie hidden in the dark secretly guzzling gas

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The Milky Way may be teeming with tens of thousands of black holes lurking at its centre, according to a new study published on Wednesday.

A group of astrophysicists analysed X-rays emitted from the heart of the galaxy to hunt for black holes using data taken from NASA’s Chandra X-ray Observatory.

Black holes are greedy. As they suck up all the surrounding gas, the material spirals around it like a whirlpool, creating an accretion disk. The matter in the accretion disk is gobbled up in streams as it reaches the void’s centre. And as the disk spins, the frictional forces between the material causes it to heat up and emits X-ray radiation.

The researchers found 12 black holes with similar masses as the sun surrounding the supermassive black hole Sagittarius A* that lives in the galaxy’s center. But estimated that there could be up to 20,000 of them extending further out from Sagittarius A* based on calculations.

Chuck Hailey, first author of the paper published in Nature and a professor of physics at Columbia University, explained to The Register that not all black holes are easy to find.


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“Isolated black holes just don’t do much that would lend themselves to easy detection. That’s why we say they are black. Sure, they eat gas and dust, but they are very lousy at converting this fuel into X-rays, which we can detect," he explained.

"But black holes with a stellar companion make X-rays. The star dumps gas into a disk around the black hole, and that disk is good at making X-rays.”

The researchers predicted they were missing about 500 faint black hole binaries far out from the centre of the galaxy. Theories estimate that it is very rare for a black hole to capture a star, maybe only one out of 20 isolated black holes have a companion.

So that’s at least 20 isolated black holes for every binary black hole; if the lower estimate is about 500 binaries then there must be at least 10,000 isolated black holes.

There are only about five dozen known black holes in the entire galaxy stretching across 100,000 light years wide. “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," Hailey said.

Confirming these results will be difficult. But the team hope these results will be useful for astronomers studying gravitational waves. Since the ripples are emitted whenever black holes collide, or fall into the center of the galaxy, estimating the population of these black holes might help refine the estimates of the number of gravitational wave events.

“We would also like to use data available from another Chandra instrument, which we did not analyze, to look for more of these black hole binaries or maybe fainter ones. Black hole hunting is kind of fun in its own right too. It’s a bit contagious,” Hailey concluded. ®


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