Carnegie Institution

Carnegie Institution

News Release

FOR IMMEDIATE RELEASE

Pasadena, California, September 13, 2002

For more information contact Paul Martini by email at martini@ociw.edu; John Mulchaey at 626-304-0257 e-mail mulchaey@ociw.edu;  Dan Kelson at 626-304-0285  e-mail kelson@ociw.edu; or Nancy Davis, Observatories External Affairs, 626-304-0270 e-mail ndavis@ociw.edu

Active black holes discovered in galaxy cluster question story of cluster evolution

Pasadena, California.

Until now astronomers thought that old, red cluster galaxies were past their prime and subdued. Only about one percent were supposed to have Active Galactic Nuclei (AGN)--violent centers where supermassive black holes gobble up surrounding material and emit it as X-rays.  A surprising find by a team of astronomers at the Carnegie Observatories in Pasadena, California, has altered this belief. Using a combination of space-based X-ray and Earth-based optical instrumentation, the scientists found that six times the expected number of galaxies in a nearby cluster have active centers. "This changes our view of galaxy clusters as the retirement homes for old and quiet black holes," said team member, Dr. Dan Kelson. "The question now is: How do these black holes turn themselves on again?" The discovery has also brought into question how galaxies evolve and how stars form in these environments.

The Carnegie group, headed by Dr. Paul Martini,  published their results in the September 10, 2002, Astrophysical Journal Letters. They took an unusual approach to their study by using NASA's X-ray Chandra satellite in concert with Carnegie's new 6.5-meter Walter Baade optical telescope at the Las Campanas observatory in Chile. Using the Chandra data, they discovered six X-ray sources in galaxy cluster Abell 2104, about 700 million light-years from Earth. They then used the Carnegie telescope to confirm that all of the galaxies are in the cluster and not in the foreground or background. "If we used optical data alone, we would have missed these hidden monsters," said  Dr. John Mulchaey. "If we'd used only X-ray data, we would not have been sure that all the AGNs were in the cluster," he concluded. 

Galaxy clusters typically have hundreds to thousands of galaxy members. The Carnegie researchers surveyed the 100 brightest galaxies in Abell 2104.  It is believed that old, red galaxies generally populate clusters because during cluster formation the raw material for making stars and feeding black holes--gas--is burned off and nothing is left to fuel these systems.  "The presence of these AGNs indicate that supermassive black holes have somehow retained a fuel source," said  Martini.  "Despite the harsh treatment these galaxies suffered as a cluster, they seem to be having the black hole equivalent of a mid-life crisis. They aren't over the hill after all." The Carnegie group has already started studying other clusters to see if similar activity is present elsewhere.

Image available at  http://www.carnegieinstitution.org/carnegieobservatories

Figure caption: An X-ray picture (false color) of the Abell 2104 cluster of galaxies taken with NASA's Chandra X-ray Observatory overlaid on an optical image taken with Carnegie Observatories' 6.5-meter Walter Baade telescope in Las Campanas, Chile. The Chandra image reveals X-ray emissions produced by both hot gas (the blue area near the center of the image) and by accretion of dust and gas onto supermassive black holes (the smaller blue patches on the outskirts of the image). The number of active, supermassive black holes found in this cluster is six times the amount found using other techniques.The finding suggests that active black holes are much more common in clusters of galaxies than previously believed. (Image courtesy NASA and the Carnegie Observatories.) ___________________________________________________________________________

The Carnegie Institution (www.CarnegieInstitution.org) has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments in the U.S.: Plant Biology, Global Ecology,  Embryology,  Geophysical Laboratory,  the  Department of Terrestrial Magnetism, and The  Observatories.