We share the Milky Way with 100 million black holes

The Milky Way teems with black holes — about 100 million of them.

But there’s no reason to fear. “It may sound like a big number, but by astronomical standards, it’s a pretty small number,” says physicist Daniel Holz of the University of Chicago. The number of stars in the Milky Way, for example, is about a thousand times larger.

Scientists from the University of California, Irvine calculated the galaxy’s black hole population as part of a new census that estimates the numbers of cosmic chasms in galaxies big and small. The analysis, in press in Monthly Notices of the Royal Astronomical Society, quantified stellar-mass black holes, which form when a star collapses. Such objects can have masses tens of times that of the sun.

To draw up the celestial inventory, the researchers combined a variety of information about stars and galaxies. A star’s size and composition — the proportion of heavy elements it contains — determine whether it can form a black hole, and how big the black hole will be. And given a galaxy‘s size, scientists can estimate the number and properties of stars within, allowing researchers to deduce the number of black holes and their sizes.

Such stellar-mass black holes are a target of the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, which has detected three sets of gravitational waves from colliding black holes (SN: 6/24/17, p. 6). When LIGO made its first detection, some physicists thought the coalescing black holes were surprisingly large; each was about 30 times the mass of the sun (SN: 3/5/16, p. 6). This puzzle led scientists to propose exotic origins for LIGO’s black holes — for example, that they formed during the universe’s infancy, instead of from collapsing stars (SN: 9/3/16, p. 8).

But the new result indicates that, in the Milky Way alone, there are about 10 million black holes with masses at least that large. So “you don’t have to do anything particularly odd or unusual in order to explain the LIGO signal,” says physicist James Bullock, a coauthor of the study.

Astrophysicist Richard O’Shaughnessy of the Rochester Institute of Technology in New York isn’t surprised by the abundance of big black holes. But, he says, the new work may serve to placate researchers who thought LIGO’s large black holes were an oddity. “Hopefully this sort of presentation will make even the skeptics recognize that this is logical.”