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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