Black Holes in a Double Pack

Researchers discover two black holes in the center of a galaxy – Candidates for gravitational waves

For the first time ever, scientists have discovered a pair
of supermassive black holes in the same galaxy. The team of Professor
Guenther Hasinger and Dr. Stefanie Komossa from the Max Planck Institute
for Extraterrestrial Physics in Garching near Munich made the discovery
using NASA’s Chandra X-ray Observatory. The black holes in the nucleus
of the galaxy NGC 6240 will merge in several hundred million
years from now creating an even larger black hole. A burst of gravitational
waves will accompany this merging.

The extraordinarily bright galaxy NGC 6240
is roughly 400 million light years away from earth and is a prime example
of the collision and subsequent merging of two galaxies causing "fireworks"
as new stars are created. The center of this galaxy is hidden behind
innumerable dusty gas clouds and is therefore not visible with optical
telescopes. X-rays can, however, penetrate the veil of
gas and dust.

Previous observations have shown that NGC 6240
produces high energy X-radiation. Using radio, infrared, and optical
observations, astronomers detected two bright nuclei in this system,
whose nature remained a mystery. "With Chandra, we hoped to determine
which of the two nuclei, if any, contains an active super massive black
hole," says Stefanie Komossa from the Max Planck Institute for
Extraterrestrial Physics and lead author of the paper on NGC 6240
that is soon to be published in the Astrophysical
Journal Letters. NASA’s Chandra observed NGC 6240
with the Advanced CCD Imaging Spectrometer
(ACIS) for a total of 10.3 hours.

Much to our surprise, we found that both nuclei harbor
active black holes", explains Komossa. The detection of a binary
black hole supports the idea that black holes can grow to become enormously
massive in the centers of galaxies by merging with other black holes.
"This is important for our understanding of how galaxies form and
evolve."

The breakthrough came with Chandra’s ability to clearly
distinguish the two nuclei and measure the details of the X-radiation
from each nucleus," says Guenther Hasinger, director at the Max
Planck Institute for Extraterrestrial Physics and co-author of the paper.
According to Hasinger, both active cosmic monsters leave fingerprints.
"We have observed an excess of high energy photons from hot gas
swirling around a black hole and X-rays from fluorescing
iron atoms in gas near the black hole."

Figure 1: X-ray zoom into
the nucleus of the nearby, ultra luminous galaxy NGC 6240
using X-rays reveals two black holes. The conspicuous of galaxy NGC
6240 as seen on the left from a telescope on earth are remnants
of a collision between two galaxies that are now gradually merging with
each other. Thick dust and gas clouds hide the galaxy’s nucleus, which
cannot be observed in visible light. X-rays can, however, penetrate
this curtain. On the right, Chandra imaged two extremely massive black
holes that betray themselves by high energy radiation (black rings).
The figures are color-coded. Regions with low radiation are red, and
those with high energy radiation are blue.

Images: Max Planck Institute for Extraterrestrial Physics
(optical image from W. Keel)

Over the next few hundred million years, the two black holes
in NGC 6240, which are roughly 3000 light years apart,
will drift toward each other and eventually merge to form an even larger,
supermassive black hole. The process will end several hundred million
years from now with an enormous burst of gravitational waves. These
gravitational waves will spread through the universe and produce ripples
in the fabric of space, which will appear as minute changes in the distance
between any two points.

The merging of two super massive black holes like those
in NGC 6240 will create the most powerful gravitational
waves in the universe. LISA (Laser Interferometer
Space Antenna), the space-based detector planned by NASA
and ESA, will search for gravitational waves from massive
black hole mergers. Such events are estimated to occur several times
each year in the observable universe. "This is the first time that
we see a binary black hole in action, the smoking gun evidence for something
which will become a major gravitational wave burst in the future",
says Hasinger.

Figure 2: High energy
radiation (blue) emanating from the two black holes in the center of
the galaxy NGC 6240 superimposed with an optical image
from the Hubble Space Telescope (yellow).

Image: NASA / Max Planck Institute for Extraterrestrial
Physics

Figure 3:In addition
to the high energy radiation emanating from the center of NGC
6240, there are also regions of lower energy, which are depicted
in red. Superimposed on this is a picture from the Hubble Space Telescope
(yellow and blue). The low-energy X-radiation does not come from the
two black holes but is attributed to the afterglow of earlier supernovae
in the center of the galaxy. They point to a "firework" of
supernovae, which catapulted their outer layers into space. If these
stellar winds collide with the surrounding interstellar medium, the
gas heats up and glows in the X-ray band.

Image: NASA / Max Planck Institute for Extraterrestrial
Physics

Other members of the team include Vadim Burwitz and Peter
Predehl from the Max Planck Institute for Extraterrestrial Physics,
Jelle Kaastra from the Space Research Organization in the Netherlands,
and Yasushi Ikebe from the University of Maryland in Baltimore.

NASA’s Marshall Space Flight Center in Huntsville, Alabama
manages the Chandra program for the Office of Space Science in Washington.
The company TRW in Redondo Beach, California is the prime contractor
for the spacecraft. The Smithsonian’s Chandra X-ray Center controls
science and flight operations from Cambridge, Massachusetts. The Deutsches
Zentrum fuer Luft- und Raumfahrt (DLR) funded Germany’s contribution
to Chandra.

Images and additional information are available online at:

http://chandra.harvard.edu,
http://chandra.nasa.gov,
http://www.mpe.mpg.de

Max Planck Society
for the Advancement of Science
Press and Public Relations Department

Hofgartenstrafle 8
D-80539 Munich

PO Box 10 10 62
D-80084 Munich

Phone: +49-89-2108-1276
Fax: +49-89-2108-1207

E-mail: presse@mpg-gv.mpg.de
Internet: http://www.mpg.de

Responsibility
for content:
Dr. Bernd Wirsing (-1276)

Executive Editor:

Dr. Andreas Trepte (-1238)

Biology, Medicine:

Dr. Christina Beck (-1306)
Walter Frese (-1272)

Chemistry, Physics,
Technology:
Eugen Hintsches (-1257)
Helmut Hornung (-1404)

Humanities:
Susanne Beer (-1342)

Online-Editor:
Michael Frewin (-1273)

ISSN 0170-4656

Contact:

Prof. Guenther Hasinger
Max Planck Institute for Extraterrestrial
Physics
Giessenbachstraße
85748 Garching
Phone: +49-89 – 30000 – 3402
Fax: +49-89 – 30000 – 3569
E-Mail: ghasinger@mpe.mpg.de

Dr. Stefanie Komossa
Max Planck Institute for Extraterrestrial
Physics
Giessenbachstraße
85748 Garching
Phone: +49-89 – 30000 – 3577
Fax: +49-89 – 30000 – 3569
E-Mail: skomossa@mpe.mpg.de