Hundreds of massive black holes left over from the early universe may wander the Milky Way, according to new calculations.
These rogue black holes are thought to have originally lurked at the centers of tiny, low-mass galaxies. Over billions of years, those dwarf galaxies smashed together to form full-sized galaxies like the Milky Way.
The idea of such wandering black holes has been suggested before, but a new computer simulation calculated that hundreds of them should be left over, and predicted that they might now be shrouded by small star clusters.
"These black holes are relics of the Milky Way's past," said researcher Avi Loeb of the Harvard-Smithsonian Center for Astrophysics. "You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the early universe."
It appears that Earth is safe. The closest rogue black hole should reside thousands of light-years away.
Astronomers are eager to locate them for the clues they will provide about the formation of the Milky Way, since they are thought to date from the universe's galaxy-building days.
Back then, whenever two young galaxies with central black holes collided, their black holes would merge to form a single black hole. In the chaos of the merger, the black hole could be flung out toward the edges of the galaxy, the new computer model shows.
It predicts that hundreds of such black holes would still be around today in the outer reaches of the Milky Way, each containing the mass of 1,000 to 100,000 suns.
They would be difficult to spot on their own, though, because a black hole is not visible. They can be detected, however, when matter they're about to swallow is superheated as it accelerates inward.
Another telltale sign could mark a rogue black hole: a surrounding cluster of stars yanked from the dwarf galaxy when the black hole escaped. Only the stars closest to the black hole would be tugged along, so the cluster would be very compact.
These clusters are so small that each looks like a single star from far away. Thus, astronomers will have to use tricks to distinguish them, such as separating the light from the clusters into its component colors to discover the individual stars hiding inside.
"The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef," said Harvard-Smithsonian Center for Astrophysics' Ryan O'Leary, who co-wrote the paper. "Without the shining stars to guide our way, the black holes would be all but impossible to find."
The number of rogue black holes in our galaxy depends on how many of the early galaxy building blocks contained black holes at their cores, and how those proto-galaxies merged to form the Milky Way. Finding and studying them would provide new clues about the history of our galaxy.
Locating the star cluster signposts may turn out to be relatively straightforward.
"Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way's halo," Loeb said. "Now that we know what to expect, we can examine existing sky surveys for this new class of objects."
The research will be detailed in an upcoming issue of the journal Monthly Notices of the Royal Astronomical Society.
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