"Black Hole Messier 87" by Event Horizon Telescope / CC BY 3.0

Millions of Black Holes Are Hiding in Our Galaxy. Here’s How Astronomers Plan to Find Them.

Jul 31, 2019

By Rafi Letzter

It’s time to find all the missing black holes.

That’s the argument advanced by a pair of Japanese astrophysicists, who wrote a paper proposing a new search for millions of “isolated black holes” (IBHs) that likely populate our galaxy. These black holes, lost in the darkness, sip matter from the interstellar medium — the dust and other stuff floating between stars. But that process is inefficient, and a great deal of the matter gets expelled into space at high speeds. As that outflow interacts with the surrounding environment, the researchers wrote, it should produce radio waves that human radio telescopes can detect. And if astronomers can sift out those waves from all the noise that’s in the rest of the galaxy, they might be able to spot these unseen black holes.

“A naive way to observe IBHs is through their X-ray emission,” the researchers wrote in their paper, which has not yet been formally peer reviewed and which they made available July 1 as a preprint on arXiv.

Why is that? As black holes suck the matter from space, that matter at its fringes accelerates and forms what’s known as an accretion disk. The matter in that disk rubs against itself as it spins toward the event horizon— a black hole’s point of no return — spitting out X-rays in the process. But isolated black holes, which are small compared to supermassive black holes, don’t emit a great deal of X-rays this way. There simply isn’t enough matter or energy in their accretion disks to create large X-ray signatures. And past searches for IBHs using X-rays have failed to produce conclusive results.

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