Black Holes Gone Rogue

July 07, 2009

It's Black Hole week here at Twisted Physics, primarily because my fodder file is choked to the brim with various collected items related to these gravitational bad boys of physics. How bad are they? Michael Jackson "Beat It" bad. In fact, a couple of months ago, scientists at the Harvard-Smithsonian Center for Astrophysics warned of rogue black holes rum amok in the Milky Way, ready to gobble up any random bits of matter that stumble into their path, just like the army of zombies in "Thriller."

Fortunately, our pretty blue planet doesn't hang out in those sorts of neighborhoods -- ours is more of a gated community type of planet, where the neighbors keep the lawns well-tended and everyone's kids attend private school. The closest rogue black hole should be several thousand light years away. And it's just a theoretical prediction right now. So these objects are mostly of interest to astrophysicists who like to walk on the wild side of their research now and then. Rogue black holes are remnants from the days when the early universe was just starting to form galaxies, like our Milky Way, so studying them could provide clues to the mechanisms underlying galaxy formation.

HSCfA's Ryan O'Leary and Avi Loeb say that rogue black holes probably started out with some small-time, juvie stuff: individual black holes lurking at the centers of tiny galaxies with very little mass, waiting for trouble to find them. Turf wars were almost inevitable in the roiling days of the early universe: these tiny galaxies would occasionally collide (rumble!), and every time this happened, the black holes at the center of each would join forces and merge to form a gang single "relic" black hole.

Where might we find these rogue black holes today? Probably in the outer reaches of the Milky Way. That's because when two black holes merges, they would emit a powerful kick of gravitational radiation and recoil in response. It would be strong enough to propel the new relic black hole to the outer edge of the galaxy, but not strong enough to leave the state completely. They should still be lurking there: hundreds of them, each with a mass ranging from 1000 to 1000,000 times the mass of our sun.

Not that we'd be able to see them. They're more like rogue ninjas in that respect, only visible in the act of consuming -- or, as astrophysicists call it, "accreting" -- matter. But Loeb and O'Leary think that there might be another telltale sign of a rogue black hole's existence: highly compact star clusters. Apparently rogue black holes travel with a posse of hangers-on. When the black hole recoils out of the dwarf galaxy, it takes a small retinue of stars with it, just those nearest to the Great Gobbler.

Such a cluster would be small enough, and dense enough, that it might look to us like a single star. Astronomers would need to study the spectrum of stars in existing sky surveys carefully to determine if there were one, or multiple stars. Either way, "The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef," says O'Leary. We can find a rogue black hole by the dense company it keeps.

Image: David Aguilar, Harvard-Smithsonian Center for Astrophysics.