A Black Hole Swarm

Like mosquitos hovering around a mountain, small black holes swarm around the supermassive black hole in the core of the Milky Way. According to a team led by Michael P. Muno (University of California, Los Angeles), there may be as many as 20,000 stellar-mass black holes lurking within a three-light-year-wide sphere surrounding the 3-million-solar-mass behemoth in our galactic center.

Stellar-mass black holes, which contain roughly 5 to 20 solar masses, often give themselves away when they are part of closely separated binary systems. Infalling matter from their companion star piles up in a hot accretion disk, which can produce outbursts of X-rays. Using NASA's Chandra X-ray Observatory, Muno's team caught seven of these outbursts over the past five years. Each was found within 75 light-years of the Milky Way's center, and surprisingly, four of them occurred within just 3 light-years of the core. Given the known X-ray behavior of these objects, this high spatial density suggests there are tens of thousands of black holes swarming in that small region of space — some 20 times more holes than expected.

Massive objects congregate in the Milky Way's core due to a process called dynamical friction. When a black hole has a close encounter with a star, the less-massive object (which is almost always the star) is flung away. As a result, the black hole loses some of its orbital energy and sinks a bit farther toward the core. In addition, if a black hole encounters a binary star system, it can eject one star, take its place, and end up as a detectable X-ray binary, as first proposed by Mark R. Morris (UCLA) in 1993. The stellar-mass black holes will fall toward the core, where they are eventually swallowed by the supermassive black hole, adding a few percent to its mass over several billion years.

If stars are kicked out from a supermassive black hole's immediate surroundings (a region just a few light-days across), there must be stellar-mass black holes there. According to Muno, astronomers might find them in the future with large ground-based telescopes because the black holes' gravity would perturb the orbital motions of massive stars seen whirling around the Milky Way's center. "It may take 20 or 30 years of careful monitoring, but it should be possible," says Muno, who presented his team's results on Monday at the American Astronomical Society conference in San Diego.