Are Rogue Black Holes Wandering the Universe?

Talk about a tough neighborhood! Even black holes aren’t welcome in galaxy CID-42, located about 4 billion light-years away from Earth. Astronomers using the Chandra X-Ray Observatory have found strong evidence that a massive black hole is being ejected from this galaxy, moving out at a speed of several million kilometers per hour. This phenomenon, known as a recoiled black hole, happens due to a gravitational wave “kick” from the merger of two black holes.

While this event is likely to be rare, it could mean that there could be giant black holes roaming undetected out in the vast spaces between galaxies.

“These black holes would be invisible to us,” said Laura Blecha from the Harvard Center for Astrophysics, co-author of a new study, “because they have consumed all of the gas surrounding them after being thrown out of their home galaxy.”

Previous looks at CID-42 detected a bright X-ray source likely caused by super-heated material around one or more supermassive black holes. However, astronomers could not distinguish whether the X-rays came from one or both of the optical sources because Chandra was not pointed directly at CID-42, giving an X-ray source that was less sharp than usual.

The new data help to clarify that X-rays were coming only from one of the sources. The team thinks that when two galaxies collided, the supermassive black holes in the center of each galaxy also collided. The two black holes then merged to form a single black hole that recoiled from gravitational waves produced by the collision, which gave the newly merged black hole a sufficiently large kick for it to eventually escape from the galaxy.

“It’s hard to believe that a supermassive black hole weighing millions of times the mass of the Sun could be moved at all, let alone kicked out of a galaxy at enormous speed,” said Francesca Civano of the Harvard-Smithsonian Center for Astrophysics (CfA), who led the new study. “But these new data support the idea that gravitational waves — ripples in the fabric of space first predicted by Albert Einstein but never detected directly — can exert an extremely powerful force.”

Almost a century ago, Albert Einstein formulated the modern set of equations — general relativity — that are used to describe gravity.

“Einstein’s equations are so complicated that we were able to solve them accurately only a few years ago for a relatively simple system of two black holes in a bound orbit,” said co-author Avi Loeb from the Harvard Center for Astrophysics. “Such a system forms naturally as a result of a merger between two galaxies, each hosting a single black hole at its center. The exact solution to Einstein’s equations, obtained with sophisticated computer algorithms, shows that the two black holes merge into a single black hole which is kicked in a preferred direction like a rocket due to the directional emission of gravitational waves which serve as the substance coming out from the rocket exhaust.”

If CID-42 was indeed produced by this mechanism, it provides the first observational validation of Einstein’s equations in the unexplored regime of dynamical strong gravity, which is responsible for gravitational wave kicks.

Additional data from the ground-based Magellan and Very Large Telescopes in Chile supplied a spectrum that suggested the two sources in CID-42 are moving apart at a speed of at least 5 million kilometers per hour (3 million miles per hour).

There are two other possible explanations for what is happening in CID-42. One would involve an encounter between three supermassive black holes, resulting in the lightest one being ejected. Another idea is that CID-42 contains two supermassive black holes spiraling toward one another, rather than one moving quickly away.

Both of these alternate explanations would require at least one of the supermassive black holes to be very obscured, since only one bright X-ray source is observed. Thus the Chandra data support the idea of a black hole recoiling because of gravitational waves.

These results will appear in the June 10 issue of The Astrophysical Journal.

Read the team’s papers:
Chandra High resolution Observations of CID-42, a candidate recoiling SMBH
Constraints on the Nature of CID-42: Recoil Kick or Supermassive Black Hole Pair?

Source: NASA, Chandra