Astronomers have found strong evidence that a massive
black hole is being ejected from its host galaxy at a speed of
several million miles per hour. New observations from NASA's Chandra
X-ray Observatory suggest that the black hole collided and merged
with another black hole and received 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), 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
Although the ejection of a supermassive black hole from a galaxy by
recoil because more gravitational waves are being emitted in one
direction than another is likely to be 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," said co-author 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 about 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 (that is, the distribution of
optical light with energy) that suggested 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.
"The previous data told us that there was something special going on,
but we couldn't tell if there were two black holes or just one," said
another co-author Martin Elvis, also of CfA. "We needed new X-ray
data to separate the sources."
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. 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.
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
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
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra Program for the agency's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Mass., controls Chandra's science and flight operations.