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Do hundreds of black holes dot the Milky Way?

rogue black hole, milky way, astrophysicsBlack holes are thought to be common in the universe, with a supermassive gobbler lurking at the core of galaxies such as our own Milky Way. But might they also be found roaming outside the galactic centers?

A new study estimates that approximately 300 black holes may lurk throughout the Milky Way, remnants of the building blocks that came together billions of years ago to form the galaxy. What is more, those black holes may retain properties that would allow astronomers to identify them, providing an archaeological record of the galaxy's formation.

Ryan O'Leary, a graduate student at the Harvard-Smithsonian Center for Astrophysics (CfA), and CfA professor Avi Loeb conducted the research, set to be published in the Monthly Notices of the Royal Astronomical Society.

As galaxies merge, so do their respective central black holes. Previous studies have theorized that the final stage of black hole mergers in low-mass galaxies could release enough energy in the form of gravitational waves, or ripples in the fabric of space and time, to knock the resulting black hole right out of its parent galaxy. This recoil would not be powerful enough, however, to eject the black hole from the Milky Way's gravitational clutches, leaving these free-floating gobblers embedded within the Milky Way.

O'Leary and Loeb simulated the mergers of smaller galaxies in the Milky Way's history, finding that recoiled black holes, ranging in size from roughly 1,000 to a few hundred thousand times the mass of the sun, could exist in relatively large quantities in the Milky Way. (That is small relative to the supermassive black hole presumed to lurk at the Milky Way's core, which has a mass of some four million suns.) None would be particularly close by, the researchers say—at least a few thousand light-years away.

Critically, the study's authors write, each of these ejected black holes should be accompanied by a tight cluster of stars that it ripped from the core of its parent galaxy, and those stars could aid in identifying the rogue black holes. At such large distances, each of those clusters will likely appear as a single point of light, but O'Leary and Loeb suggest some distinguishing features that might aid in their identification, such as their high mass-to-light ratio, given the massive but nonluminous black hole at their center.

Image credit: David A. Aguilar (CfA)

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