Holey Nomad
Intergalactic hiker
The boundary of the supermassive black hole in the galaxy NGC 1277.
The most massive black hole ever measured may be an intergalactic hitchhiker that escaped from one galaxy before getting captured by another. If this scenario, laid out in a paper posted February 18 at arXiv.org, is proven correct, it would be the first time astronomers have definitively spotted a black hole that was expelled from its original galactic home. Computer simulations of galaxy mergers suggest that some supermassive black holes can be nomads: When the galaxies' central black holes unite, they can emit an enormous surge of energy in one direction. That burst would rocket the newly formed black hole in the opposite direction, the simulations say, often with enough speed to escape the galaxy. Astronomers have scoured telescope images for signs of runaway black holes but have come up with only a few controversial possibilities. “We looked at a lot of objects and didn't find anything,†says Erin Bonning, an astronomer at Quest University Canada in Squamish, British Columbia. But last November, a study in Nature described a gargantuan black hole, 17 billion times the mass of the sun, at the center of a seemingly run-of-the-mill galaxy called NGC 1277 in the Perseus cluster 250 million light-years away. While most galaxies' central black holes make up about one-tenth of a percent of their total mass, NGC 1277's black hole accounts for 14 percent of the galactic mass. “That paper blew everyone's mind,†Bonning says. “It's an extraordinary black hole in an ordinary galaxy.†NGC 1277 and its black hole seemed such an odd couple that Bonning and her colleague Gregory Shields, of the University of Texas at Austin, began to question whether the two had evolved together. They studied images of the Perseus cluster and calculated the gravitational interactions of astronomical objects, trying to determine whether this black hole could have been tossed from another galaxy and then snapped up by NGC 1277.
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