NASA’s Chandra X-ray Observatory has identified a massive black hole being ejected from its host galaxy at a staggering speed of several million miles per hour.
Astronomers hypothesize the black hole collided and merged with another black hole, receiving a powerful recoil kick from gravitational wave radiation.
“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).
“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.”
Although the ejection of a supermassive black hole from a galaxy by recoil is somewhat rare, it nevertheless could mean that there are many giant black holes roaming undetected out in the vast spaces between galaxies.
“These black holes would be invisible to us,” explained Laura Blecha, also of CfA, “because they have consumed all of the gas surrounding them after being thrown out of their home galaxy.”
Civano and her group have been studying a system known as CID-42, located in the middle of a galaxy approximately 4 billion light years away. They had previously spotted two distinct, compact sources of optical light in CID-42, using NASA’s Hubble Space Telescope.
More optical data from the ground-based Magellan and Very Large Telescopes in Chile supplied a spectrum suggesting the two sources in CID-42 are moving apart at a speed of at least 3 million miles per hour.
Previous Chandra observations detected a bright X-ray source likely caused by super-heated material around one or more supermassive black holes. However, they 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.
However, when Chandra’s sharp High Resolution Camera was pointed directly at CID-42, the resulting data showed that X-rays were coming only from one of the sources. As such, the team believes that when two galaxies collided, the supermassive black holes in the center of each galaxy also collided.
The two black holes subsequently merged to form a single black hole that recoiled from gravitational waves produced by the collision, which provided the newly merged black hole with a sufficiently large kick for it to eventually escape from the galaxy.
The other optical source is thought to be the bright star cluster that was left behind. This picture is consistent with recent computer simulations of merging black holes, which show that merged black holes can receive powerful kicks from the emission of gravitational waves.
Nevertheless, 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.
Of course, 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 supports the idea of a black hole recoiling because of gravitational waves.