Researchers discover two black holes in the center of a gala

by Dr. Stefanie Komossa of the Max Planck Institute for Extraterrestrial Physics

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.

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).

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.

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/

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