for National Geographic News
Merging galaxies that forcefully eject supermassive black holes have theoretically created a whole new class of astronomical object—and now scientists think they know how to find them.
Black holes that get kicked out should carry with them clusters of nearby stars, a new study says.
These stars can act as signposts and can reveal details about the now galaxy-less black hole's past life.
In theory, hundreds of massive black holes left over from the age of galaxy formation could be lurking in the nearby universe.
"Every such black hole that's ever been kicked out is still potentially observable, and that's very encouraging," said lead study author David Merritt of the Rochester Institute of Technology.
"It's not quite what anybody has seen so far," he said. "We're just talking about what they would look like if you were to find them."
Observations made with the Hubble Space Telescope and re-examined with ground telescopes have come close, Merritt said.
"People are now just starting to make the kind of observations to see these kinds of things, if they are there."
Most galaxies are thought to harbor black holes at their centers that are millions to billions of times the mass of our sun.
When galaxies merge, their respective supermassive black holes start to coalesce in a process that creates a spurt of gravitational waves.
If the waves are strong enough, the kick they provide should drive the newly merged black hole outside the host galaxy, simulations from the past few years suggest.
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