NASA's Spitzer Finds Hidden, Hungry Black Holes

With its sharp infrared eyes, NASA's Spitzer Space Telescope peeredthrough walls of galactic dust to uncover what may be the long-soughtmissing population of hungry black holes known as quasars.

"From past studies using X-rays, we expected there were a lot ofhidden quasars, but we couldn't find them," said AlejoMartínez-Sansigre of the University of Oxford, England. He is leadauthor of a paper about the research in this week's Nature. "We had towait for Spitzer to find an entire population of these dust-obscuredobjects."

Quasars are super-massive black holes that are circled by a giantring of gas and dust. They live at the heart of distant galaxies andcan consume up to the equivalent mass of one thousand stars in a singleyear. As their black holes suck in material from their dusty rings, thematerial lights up brilliantly, making quasars the brightest objects inthe universe. This bright light comes in many forms, including X-rays,visible and infrared light.

Astronomers have puzzled for years over the question of how many ofthese cosmic behemoths are out there. One standard method forestimating the number is to measure the cosmic X-ray background.Quasars outshine everything else in the universe in X-rays. By countingthe background buzz of X-rays, it is possible to predict theapproximate total number of quasars.

But this estimate has not matched previous X-ray and visible-lightobservations of actual quasars, which number far fewer than expected.Astronomers thought this might be because most quasars are blocked fromour view by gas and dust. They proposed that some quasars arepositioned in such a way that their dusty rings hide their light, whileothers are buried in dust-drenched galaxies.

Spitzer appears to have found both types of missing quasars bylooking in infrared light. Unlike X-rays and visible light, infraredlight can travel through gas and dust.

Researchers found 21 examples of these quasars in a small patch ofsky. All the objects were confirmed as quasars by the National RadioAstronomy Observatory's Very Large Array radio telescope in New Mexicoand by the Particle Physics and Astronomy Research Council's WilliamHerschel Telescope in Spain.

"If you extrapolate our 21 quasars out to the rest of the sky, youget a whole lot of quasars," said Dr. Mark Lacy of the Spitzer ScienceCenter, California Institute of Technology, Pasadena, Calif., aco-author of the Nature paper. "This means that, as suspected, mostsuper-massive black hole growth is hidden by dust."

The discovery will allow astronomers to put together a more completepicture of how and where quasars form in our universe. Of the 21quasars uncovered by Spitzer, 10 are believed to be inside fairlymature, giant, elliptical galaxies. The rest are thought to be encasedin thick, dusty galaxies that are still forming stars.

A team of researchers based at the University of Arizona, Tucson,found similar quasars using Spitzer. Their research is described at http://uanews.org/science.

Other authors of the Nature paper include Drs. Steve Rawlings andMatt Jarvis, University of Oxford; Drs. Dario Fadda and FrancineMarleau, Spitzer Science Center; Dr. Chris Simpson, University ofDurham, England; and Dr. Chris Willott, National Research CouncilCanada, Victoria.

The Jet Propulsion Laboratory, Pasadena, Calif., a division ofCaltech, manages the Spitzer Space Telescope mission for NASA's ScienceMission Directorate, Washington. Science operations are conducted atthe Spitzer Science Center at Caltech. Spitzer's multiband imagingphotometer, which observed the quasars, was built by Ball AerospaceCorporation, Boulder, Colo.; the University of Arizona; and BoeingNorth America, Canoga Park, Calif. Spitzer's infrared array camera,which also observed the quasars, was built by NASA Goddard Space FlightCenter, Greenbelt, Md.

A Spitzer false-colored picture of one of the newfound quasars is available at http://www.spitzer.caltech.edu/Media/index.shtml.

For information about NASA and agency programs visit http://www.nasa.gov/home/.