A supermassive black hole, known as Sagittarius A* (Sgr A*), resides at the center of the Milky Way galaxy. Findings presented yesterday at the annual meeting of the American Astronomical Society in San Diego, Calif., indicate that it, in turn, could be surrounded by a swarm of smaller black holes that have accumulated over billions of years. The results suggest that the smaller black holes will eventually fall into the supermassive black hole, increasing its size and helping astronomers better understand how such black holes grow.

Michael Muno of the University of California at Los Angeles and his colleagues analyzed data collected by NASA's Chandra X-ray Observatory as the instrument monitored the region around Sgr A*. The team sifted through data from all the x-ray sources situated within 70 light-years of Sgr A*, searching for those that had characteristics of black holes and neutron stars in binary systems and found four sources within just three light-years of the central black hole. Initial models had suggested there was less than a one-in-five chance that even one such binary would be so close to Sgr A*. "The observed high concentration of these sources implies that a huge number of black holes and neutron stars have gathered in the center of the galaxy," Muno says. Indeed, the team estimates that up to 10,000 or more stellar-mass black holes and neutron stars could be swarming around Sgr A*.

The findings are the first evidence to date of a dense stellar graveyard that has been theorized for years and indicate that the black holes and neutron stars will slowly be swallowed up by Sgr A* at a rate of about one every million years, leaving the central supermassive black hole about 3 percent more massive in a few billion years. A detailed report describing the results has been submitted to Astrophysical Journal Letters.