Harvard-Smithsonian astronomers have observed a gigantic black hole speeding away from its home galaxy like a pebble from a slingshot. The runaway black hole is a curiosity that provides key support for Albert Einstein’s nearly century-old theory of general relativity—and a rare glimpse of a cosmic eviction unfolding in our universe.
Galaxies, including ours, have black holes at their centers. When two galaxies collide and merge, the black holes at their centers generally follow suit. The idea that merging black holes could, under the right conditions, be kicked powerfully out of the center of their galaxy was an idea that followed from Einstein’s prediction that there are gravitational waves, ripples in the fabric of space. But without the ability to detect such waves directly, scientists could only “see” them by scouring catalogs of galaxies to see if they could find an example of such a rare phenomenon in action.
”These were theoretical calculations—people predicted a black hole can get a kick that will expel it from the center of the merged galaxy, but we had not seen evidence for it,” said Avi Loeb, chairman of the astronomy department at Harvard University. “What would be a smoking gun would be to see a black hole not in the center” of the galaxy.
The search for such a galaxy lit on a region of space about four billion light years away, in a galaxy known as Cosmos ID 42. There, using telescopes—including NASA’s Hubble Space Telescope, ground-based observatories, and the Chandra X-ray observatory—the researchers saw what appeared to be a black hole barreling away from its home address at several million miles per hour.
The new finding, to be published in the Astrophysical Journal later this month, opens up plenty of new questions, said Francesca Civano, a post-doctoral researcher at the Harvard-Smithsonian Center for Astrophysics who led the work. Are there other errant black holes, ejected from their homes and wandering the universe? What effect would losing a black hole have on the continued evolution of the galaxy?
And, of more Earthly concern, what of our own galaxy? Loeb pointed out that the Milky Way has a black hole at its center, too. And the Milky Way is on a collision course with the neighboring Andromeda galaxy, which has its own black hole.
“This story of two black holes coming together will play out in our own neighborhood,” Loeb said. “In a few billion years.”