Read our COVID-19 research and news.

Do Solo Black Holes Roam the Universe?

Even gravitational monsters can get the heave-ho. Two mysterious bright spots in a disheveled, distant galaxy suggest that astronomers have found the best evidence yet for a supermassive black hole being shoved out of its home.

If confirmed, the finding would verify Einstein's theory of general relativity in a region of intense gravity not previously tested. The results would also suggest that some giant black holes roam the universe as invisible free floaters, flung from the galaxies in which they coalesced. Although loner black holes may be an entity that has to be reckoned with, they would still be rare, notes theorist Laura Blecha of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Blecha, along with observational astronomer Francesca Civano of Harvard-Smithsonian and their colleagues took detailed x-ray observations of the distant galaxy CID-42, nearly 4 billion light-years from Earth. The team focused on the region after a Hubble Space Telescope survey revealed two compact visible-light sources within the starlit body: one of them at or near the galaxy's center, the other offset from the core. Because CID-42 appears to be the remnant of two giant galaxies that collided relatively recently, it seemed likely that one or both of the compact sources were supermassive black holes.

Observations with NASA's Chandra x-ray Observatory revealed that only one of the compact visible-light sources—a blob that lies about 8000 light-years from the galaxy's estimated center—emits x-rays. The high-energy radiation is believed to be a sign that this blob is a supermassive black hole munching away on surrounding gas.

Going rogue. This simulation reveals how a black hole might be expelled from a galaxy, like CID-42, which recently underwent a major collision—a merger of two galaxies that each had its own supermassive black hole. When this galaxy collision occurred, the supermassive black holes in the center of each galaxy also collided.
Credit: L. Blecha

Blecha, who simulated collisions between galaxies that contain black holes, says the findings suggest two key possibilities. The sources could be two supermassive black holes brought together by the collision of the galaxies that formed CID-42. In that case, the central compact source, like its off-center black hole sibling, would emit x-rays. The absence of x-rays detected by Chandra would then have to be explained by a high concentration of dust that hid the high-energy radiation from the flying observatory.

Alternatively, says Blecha, the central source could simply be a new hot spot of star formation, ignited by the recent galactic merger and truly devoid of any x-ray emission. The off-center x-ray-emitting source would then be a single black hole several million times as heavy as the sun, which is being expelled from the galaxy at about 2000 kilometers per second.

That speed matches the velocity expected to be imparted by the asymmetric emission of gravitational waves—ripples in spacetime that Einstein predicted would be generated when heavy objects like black holes are accelerated. If a black hole emits more gravitational waves in one direction than another, the body will recoil in the opposite direction with a speed fast enough to escape the galaxy, the team will report in two papers in the 10 June issue of The Astrophysical Journal.

"This is the best candidate that people have seen for a recoiling supermassive black hole," says theoretical astrophysicist Cole Miller of the University of Maryland, College Park, who was not part of the study. If the recoil explanation holds up, the study "would offer observational confirmation that general relativity is a good description even in a region of strong gravity."

Follow-up observations could help distinguish between the two exotic models. For instance, if the two bright visible-light sources are a pair of supermassive black holes that will ultimately merge, their pattern of visible-light emission might vary over time scales of a few years to a few decades, says Miller. Radio telescope studies could help pinpoint the location of the sources within the galaxy and indicate whether the more centrally located blob behaves as some supermassive black holes do—emitting a relatively strong radio signal—or more like a region of intense star formation.

Although it's tempting to think of the cosmos as filled with invisible rogue black holes flung out of their home galaxies and ready to gobble anything that comes near, homeless supermassive black holes would in fact be a rare commodity, both Miller and Blecha note. Every massive galaxy that astronomers have searched for a central black hole has contained one, indicating that the beasts aren't kicked to the curb very often, Miller says. But because supermassive black holes at the centers of galaxies produce winds and jets of material that regulate star formation, the rare expulsion of the gravitational guzzlers could alter the evolution of the galaxies they once called home, Blecha says.