Hidden Population of Powerful Black Holes Revealed in Large Sky Survey
These pictures show galaxies that host three of the hidden quasars found in the Sloan Digital Sky Survey (SDSS-II) sample. In every image, the quasar is at the center of the galaxy, but our view to it is obscured by dust. In the central image, the blue patches of light serve as an indirect indicator of the hidden quasar in the center of the galaxy. The same signature, although fainter, can be seen in the image on the left. The unusual shape of the right-most galaxy indicates that this object is undergoing interactions with a smaller galaxy that is being ripped apart. These images were obtained using the Advanced Camera for Surveys onboard the Hubble Space Telescope. CREDIT: SDSS collaboration, Nadia Zakamska
A team of Sloan Digital Sky Survey (SDSS-II) scientists, led by Princeton University's Reinabelle Reyes and including astronomers at Penn State, has identified a large number of "hidden quasars" -- supermassive black holes in the centers of galaxies that are shrouded in light-absorbing dust and gas. According to Donald Schneider, coauthor of the paper and Professor of Astronomy and Astrophysics at Penn State, "If one examines a photograph of one of the hidden quasars we discovered, it appears to be just an ordinary galaxy, although quasars are typically are 10 to 100 times more luminous than the Milky Way Galaxy." Schneider is the chair of the SDSS-II science group that studies quasars, which are powered by glowing, super-heated gas as it swirls into black holes a billion times more massive than the Sun.
The research team, which will present its discovery on 9 January 2008 at the annual meeting of the American Astronomical Society in Austin, Texas, has submitted a paper describing the research for publication in the Astronomical Journal. Using the distinctive light-spectrum signature that even highly obscured quasars show as a marker, the SDSS-II team sifted through more than a million spectra to discover 887 hidden quasars, by far the largest sample of these objects ever found.
"A large survey like SDSS-II is important because quasars are about 10,000 times rarer than are normal galaxies," explains Reyes. "We determined how common hidden quasars are, especially the most luminous ones. Perhaps more interestingly, we determined how common they are relative to normal quasars," said team member Nadia Zakamska, a NASA Spitzer Fellow at the Institute for Advanced Studyin Princeton. "We found that hidden quasars make up at least half of the quasars in the relatively recent Universe, implying that most of the powerful black holes in our neighborhood had previously been unrecognized."
Michael Strauss of Princeton University explains that powerful black holes are more common in the last eight billion years of cosmic history than had previously been thought. "Moreover, because the light from these hidden quasars previously had been unaccounted for, black holes turn out to be more efficient in converting the energy of in-falling matter into light than we had thought."
This result also has implications for theoretical models of quasars. "The relative numbers of hidden versus normal quasars tell us something about how dust and gas typically are distributed around these objects," explains Julian Krolik, a collaborator from Johns Hopkins University. "If the dust covers a large fraction of the area around a black hole, this object would more likely appear as a hidden quasar. So the large number of hidden quasars discovered by the SDSS team implies that most of the light emitted by quasars is actually obscured."
CONTACTS
Donald Schneider: 814-863-9554, dps@astro.psu.edu
Barbara Kennedy (PIO): 814-863-4682, science@psu.edu
MORE INFORMATION
More information, including a complete list of the study's authors, is on the Web at www.sdss.org.
The Sloan Digital Sky Survey is the most ambitious survey of the sky ever undertaken, involving more than 300 astronomers and engineers at 25 institutions around the world. SDSS-II, which runs from 2005-2008, is comprised of three complementary projects. The Legacy Survey is completing the original SDSS map of half the northern sky, determining the positions, brightness, and colors of hundreds of millions of celestial objects and measuring distances to more than a million galaxies and quasars. SEGUE (Sloan Extension for Galactic Understanding and Exploration) is mapping the structure and stellar makeup of the Milky Way Galaxy. The Supernova Survey repeatedly scans a stripe along the celestial equator to discover and measure supernovae and other variable objects, probing the accelerating expansion of the cosmos. All three surveys are carried out with special purpose instruments on the 2.5-meter telescope at Apache Point Observatory in New Mexico.
Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, The U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England.
The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory and the University of Washington.