100 million black holes in Milky Way galaxy, new study says

The number of black holes of a certain mass per galaxy depends on the size of the galaxy, says co-author Manoj Kaplinghat, a UCI professor of physics and astronomy.
By David Sims | Aug 11, 2017
After conducting a year-and-a-half-long investigation to categorize and calculate the number of stellar-remnant black holes in the Milky Way galaxy, astronomers at the University of California, Irvine (UCI), have concluded they are far more numerous than expected.

The study is published in the current issue of Monthly Notices of the Royal Astronomical Society.

"We think we've shown that there are as many as 100 million black holes in our galaxy," said co-author James Bullock, UCI chair and professor of physics and astronomy, in a statement.

UCI began its inventory of black holes soon after the historic detection by the Laser Interferometer Gravitational-Wave Observatory (LIGO) of gravitational waves created by the collision of two massive blacks holes.

"Fundamentally, the detection of gravitational waves was a huge deal, as it was confirmation of a key prediction of Einstein's general theory of relativity," said Bullock. "But then we looked at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'"

The number of black holes of a certain mass per galaxy depends on the size of the galaxy, says co-author Manoj Kaplinghat, a UCI professor of physics and astronomy.

This is because larger galaxies containing a lot of heavier elements shed much of their mass over the course of their lives, so when they wind up in a supernova, only a small amount of matter is left to collapse in on itself. But a large star with a low metal content sheds less mass over time, so when it ends its life as a supernova, most of the mass goes into the black hole.

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