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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