BY DR EMILY BALDWIN
Posted: 29 April, 2009
Hundreds of rogue black holes left over from the galaxy building days of the early Universe could be wandering loose in the Milky Way, say Harvard-Smithsonian astrophysicists.
The good news is that the Earth is safe, since the black holes are apparently skulking the perimeter of the Milky Way, thousands of light years away. These distant relics of the Milky Way’s past could provide vital information on the formation of our home Galaxy. “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,” says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics.
This artist's conception shows a rogue black hole floating near a globular star cluster on the outskirts of the Milky Way. Image: David A. Aguilar (CfA).
The black holes haven’t always been wandering the Galaxy alone, instead they began their lives in the centres of low-mass dwarf galaxies. Over billions of years these galaxies collided to form large Milky Way-sized galaxies, and at the same time the black holes would have merged into a single ‘relic’ black hole. In some cases, however, the gravitational radiation involved in the collision would have been enough to cause the black hole to recoil, kicking the monster out of the host dwarf galaxy at high speed, but without enough force to expel it from the galactic neighborhood completely.
Each with a mass of some 1,000 to 100,000 Suns, these black holes were banished to the edges of the Milky Way halo, the tell-tale signs of their existence exposed only when they feed on matter that strays too close to its jaws, or by a surrounding cluster of stars that the black hole grabbed from its host galaxy as it was ejected out of the system. “The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef,” says Ryan O’Leary, also from the Harvard-Smithsonian Center for Astrophysics. “Without the shining stars to guide our way, the black holes would be all but impossible to find.”
The clusters would be very small though, perhaps initially appearing solely as a single star in the sky, so astronomers would have to look closely at a spectrum to demonstrate that multiple stars were present, or study the motions of the individual stars, which would be moving rapidly under the influence of the black hole’s dominating gravity. “Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way’s halo,” says Loeb. “Now that we know what to expect, we can examine existing sky surveys for this new class of object.”
While an estimate of “hundreds” of rogue black holes has been made for our own Galaxy, the exact number depends on how many of the galactic building blocks contained black holes at their cores, and how those proto-galaxies merged together to form the Milky Way. Either way, finding and studying these relics will provide new and important clues about the history of our Galaxy.
A paper describing the results will be published in the Monthly Notices of the Royal Astronomical Society.