For the first time, scientists have proof two
supermassive black holes exist together in the same galaxy,
thanks to data from NASA’s Chandra X-ray Observatory. These
black holes are orbiting each other and will merge several
hundred million years from now, to create an even larger
black hole resulting in a catastrophic event that will
unleash intense radiation and gravitational waves.
The Chandra image reveals that the nucleus of an
extraordinarily bright galaxy, known as NGC 6240, contains
not one, but two giant black holes, actively accreting
material from their surroundings. This discovery shows that
massive black holes can grow through mergers in the centers
of galaxies, and that these enigmatic events will be
detectable with future space-borne gravitational wave
observatories.
“The breakthrough came with Chandra’s ability to clearly
distinguish the two nuclei, and measure the details of the X-
radiation from each nucleus,” said Guenther Hasinger, of the
Max Planck Institute for Extraterrestrial Physics in Germany,
a coauthor of an upcoming Astrophysical Journal Letters paper
describing the research. “These cosmic fingerprints revealed
features characteristic of supermassive black holes — an
excess of high-energy photons from gas swirling around a
black hole, and X-rays from fluorescing iron atoms in gas
near black holes,” he said.
Previous X-ray observatories had shown that the central
region produces X-rays, while radio, infrared and optical
observations had detected two bright nuclei, but the nature
of this region remained a mystery. Astronomers did not know
the location of the X-ray source, or the nature of the two
bright nuclei.
“With Chandra, we hoped to determine which one, if either, of
the nuclei was an active supermassive black hole,” said
Stefanie Komossa, also of the Max Planck Institute, lead
author of the paper on NGC 6240. “Much to our surprise, we
found that both were active black holes!”
At a distance of about 400 million light-years, NGC 6240 is a
prime example of a massive galaxy in which stars are forming
at an exceptionally rapid rate due to a recent collision and
subsequent merger of two smaller galaxies. Because of the
large amount of dust and gas in such galaxies, it is
difficult to peer deep into their central regions with
optical telescopes. However, X-rays emanating from the
galactic core can penetrate the veil of gas and dust.
“The detection of a binary black hole supports the idea that
black holes can grow to enormous masses in the centers of
galaxies by merging with other black holes,” said Komossa.
“This is important for understanding how galaxies form and
evolve,” she said.
Over the course of the next few hundred million years, the
two black holes in NGC 6240, which are about 3000 light-years
apart, will drift toward one another and merge to form an
even larger supermassive black hole. Toward the end of this
process an enormous burst of gravitational waves will be
produced several hundred million years from now.
These gravitational waves will spread through the universe
and produce ripples in the fabric of space, which would
appear as minute changes in the distance between any two
points. NASA’s planned space-based detector, LISA (Laser
Interferometer Space Antenna), will search for gravitational
waves from massive black-hole mergers. These events are
estimated to occur several times each year in the observable
universe.
“This is the first time we see a binary black hole in action,
the smoking gun for something that will become a major
gravitational wave burst in the future,” said Hasinger.
Chandra observed NGC 6240 for 10.3 hours with the Advanced
CCD Imaging Spectrometer (ACIS). Other members of the team
are Vadim Burwitz and Peter Predehl of the Max Planck
Institute, Jelle Kaastra of the Space Research Organization
Netherlands and Yasushi Ikebe of the University of Maryland
in Baltimore.
NASA’s Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program for the Office of Space Science,
Washington, and TRW, Inc., Redondo Beach, Calif., is the
prime contractor for the spacecraft. The Smithsonian’s
Chandra X-ray Center controls science and flight operations
from Cambridge, Mass.
Images and additional information about this result are
available at:
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