Image: Black hole
David A. Aguilar (CfA)
This artist's conception shows a rogue black hole floating near a globular star cluster on the outskirts of the Milky Way. New calculations by Ryan O'Leary and Avi Loeb suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way. Fortunately, the closest rogue black hole should reside thousands of light-years from Earth.
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updated 5/11/2009 4:50:48 PM ET 2009-05-11T20:50:48

Hundreds of relic black holes may be roaming the outskirts of the Milky Way galaxy trailing telltale streams of stars detectable from Earth, suggest astronomers in a new study.

The black holes are crash victims, ejected from their original host galaxies when worlds collided, a process that Ryan O'Leary and Abraham Loeb, with the Harvard-Smithsonian Center for Astrophysics, suspect was instrumental in building our own galaxy and probably many others.

"Our work was theoretical, but we have an idea of what these clusters would look like, Loeb told Discovery News.

The astronomers are combing through observations from the Sloan Digital Sky Survey in search of clusters of densely packed and relatively fast-moving stars that may prove their theory.

"Technologically, it's not difficult to detect these clusters," said Loeb. "The issue is really finding the needle in the haystack."

The research, which is to be published in Monthly Notices of the Royal Astronomical Society, builds on computer simulations showing how much energy would be needed to liberate a black hole from its host galaxy.

Manuela Campanelli at the Rochester Institute of Technology in New York said the specifics depend on the size of the colliding black holes, the angles at which they meet and their spin rates. The most dramatic crash would be a collision between two similarly sized black holes, which would send them careening across space at a velocity of 2,500 miles a second.

"That was a shock to the astronomical community, Campanelli told Discovery News.

The calculations show that black holes can easily be ejected from their host galaxies, she added.

O'Leary and Loeb believe this was common practice in the formative years of the Milky Way, as small dwarf galaxies crashed into each other. The ejected black holes would not have enough velocity to escape the gravity of the newly combined mass and should still be wandering the outer regions of the galaxy today.

"An observational discovery of this relic population ... would constrain the formation history of the Milky Way and the dynamics of black hole mergers in the early universe, the astronomers wrote.

"A similar population should exist around other galaxies, and may potentially be detectable in M31 and M33, they added.

Each black hole is estimated to contain the mass of between 1,000 and 100,000 suns.

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