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.
The key was finding a giant galaxy, the kind that could support a 17-billion-solar-mass black hole, about 325,000 light-years away from NGC 1277. Bonning and Shields propose that this galaxy, called NGC 1275, is the product of a galactic merger that took place billions of years ago. The merging galaxies’ black holes, each about 10 billion times the mass of the sun, orbited each other at nearly the speed of light until they united. Then the scenario from the computer simulations played out: Energy released from the merger flung away the newly formed black hole.
Bonning and Shields suggest that the black hole spent a few billion years whizzing through intergalactic space at about 4.5 million kilometers per hour. It had some companions for its journey: a posse of millions of orbiting stars trapped by the black hole’s intense gravitational pull. Finally the black hole made a close pass to NGC 1277, and over hundreds of millions of years, the galaxy reeled it in to its center.
Bonning and Shields submitted this version of events to Astrophysical Journal Letters and they expect plenty of scrutiny. Avi Loeb, a theorist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., praises Bonning and Shields’ creativity but notes that each of the steps that they describe does not occur frequently in the universe. “Several rare events together are unlikely,” he says. “I would think that there are more likely ways of achieving the same result.”
Shields notes that NGC 1275’s current black hole is slightly smaller than scientists expected, perhaps suggesting that the galaxy had to rebuild from scratch after losing its previous black hole. He and Bonning hope to perform a computer simulation of their proposed scenario. “It was a lot of fun to work on,” Shields says. “Now we need to convince astronomers, including myself, that it’s true.”