Rogue Black Holes Might Fill Our Galaxy

AUSTIN, Texas — Our home galaxy could be chockfull of rogue black holes that devour anything that crosses their paths, new computer simulations suggest.

Black holes are collapsed stellar corpses that trap all matter and light entering them, so they can't be seen directly. Instead, astronomers infer their existence by measuring their gravitational effects on other objects or by the radiation that shoots out of their chaotic environments.

When two black holes merge, under certain conditions the energy produced can kick the newly merged black hole clear out of its galaxy at jaw-dropping speeds, the simulations suggest. The masked fugitive is called a rogue black hole.

?Rogue black holes like this would be very difficult to spot,? said astronomer Kelly Holley-Bockelmann of Vanderbilt University, who presented the speculative results here at a meeting of the American Astronomical Society.

?Unless it's swallowing a lot of gas, about the only way to detect the approach of such a black hole would be to observe the way in which its super-strength gravitational field bends the light that passes nearby," she said.

Some mergers would also create gravitational waves, which would be strong enough to hurl the merged black hole at speeds as high as 2,485 miles per second (4,000 kilometers per second).

"This is much higher than anyone predicted. Even the average kick velocity of 200 kilometers per second is extremely high when compared to the escape velocities of typical astronomical objects," Holley-Bockelmann said. "We realized that basically any black hole merger would kick the new remnant out of a globular cluster, because the escape velocity is less than 100 kilometers per second."

Holley-Bockelmann and her team focused on intermediate black holes thought to weigh a few thousand times as much as the sun. Their existence is controversial, but they are thought to inhabit globular clusters, or crowds of 100,000 to a million ancient stars herded together by gravity.

They ran computer simulations of intermediate mass black holes as they merged with stellar-sized black holes, which typically pack the mass of a few suns. Paying close attention to the resulting "kick," they looked at a range of different-massed black holes with randomly selected spins and spin orientations.

Results showed that even if every globular cluster in our galaxy started out with an intermediate-sized black hole, only 30 percent of these would hold onto them through a merger.

Taking the speculation a step further, if the roughly 150 to 200 globular clusters known to reside in the Milky Way have spawned intermediate-sized black holes, 100 or more of them are probably wandering invisibly around our galaxy, the researchers conclude.

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