Orphan. There could be hundreds of rogue black holes hiding among the Milky Way's stars.

Credit: David Aguilar/CfA

Astronomers are now convinced that each of the hundreds of billions of galaxies in the cosmos formed a massive or supermassive black hole at its center. Researchers also theorize that when big galaxies collide--a relatively common occurrence--their central black holes eventually merge. Some observational evidence supports the idea (ScienceNOW, 4 March). A related theory holds that galaxies, such as our own, most often grow by absorbing smaller, satellite galaxies, such as the Large Magellanic Cloud, which orbits the Milky Way. However, not much tangible evidence supports the theory.

Now, two researchers from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, have proposed a way to detect evidence that a large galaxy--in this case, the Milky Way--has collided with a satellite galaxy. On the basis of computer simulations, theoretical astrophysicists Ryan O'Leary and Abraham Loeb hypothesize that sometimes the central black hole of a dwarf galaxy might remain independent after a galactic collision. What happens, they explain, is that the gravitational interaction between a supermassive black hole and a smaller galactic cousin can sometimes kick the smaller black hole out of the smaller galaxy's center--much as a black hole's intense gravity can sometimes produce huge jets of matter. The ejected black hole would not move fast enough to escape the galaxy's gravity entirely, but it would move faster than the background stars--something that makes it detectable, because it would also be dragging along a small cluster of surrounding stars.

O'Leary and Loeb contend in an upcoming issue of the Monthly Notices of the Royal Astronomical Society that there could be many such roving black holes in the Milky Way. To find them, the researchers suggest searching the galaxy for compact and relatively fast-moving clusters of old stars. Loeb and O'Leary are scanning the data archive of the Sloan Digital Sky Survey for possible candidates.

Astronomers Michael Disney of Cardiff University in the United Kingdom and Christopher Mihos of Case Western Reserve University in Cleveland, Ohio, call the proposal "an intriguing idea." Disney says that in principle it "could provide archaeological evidence from the early history of the Milky Way." The problem with the theory of galaxy formation involving mergers with dwarfs, he explains, is that "there is almost no observational evidence." So finding just one of the rogue black holes could strengthen the theory, Disney says.

The ability of two interacting black holes to kick one out into space "is turning out to be an important process in a wide variety of astronomical settings," says Mihos. "It gives the observers something to look for and the theorists something to keep scratching their heads over."