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Researchers Uncover Hidden Active Galaxies

In extremely obscured active galactic nuclei, the disk and torus surrounding the black hole are so deeply obscured by gas and dust that no visible light and very little X-ray light escapes, making them very difficult to detect. (Image: NASA/Aurore Simonnet, Sonoma State University.)

Researchers from the Kavli Institute for Particle Astrophysics and Cosmology at SLAC and the Max Planck Institute in Germany have confirmed that a high number of X-ray sources, thought to be massive black holes, are deeply hidden in our local universe. Obscured by dust and gas, the objects are difficult to spot from Earth.

"People expected that more of these [hidden black holes] should exist, and this confirms it," said KIPAC physicist Marco Ajello.

To human eyes, the night sky looks like a dark canvas, punctured with many pinpoints of light. But seen in X-rays, the sky fills with a permeating glow. That glow is known as the cosmic X-ray background, and astronomers are working hard to find its source.

The new analysis by Ajello and Davide Burlon, a Ph.D. student at the Max Planck Institute, will significantly impact the study of this universal X-ray background. Using data from the Swift-BAT telescope, a NASA mission built to detect very bright sources of X-rays, the researchers examined a key source of the cosmic X-ray background: galaxies with massive black holes at their centers. Gas and dust spiral around the black hole, radiating high levels of X-ray light. These bright X-ray centers are known as active galactic nuclei, or AGNs.

"These AGNs are normally powered by black holes that are maybe a hundred million times more massive than our sun," Ajello said. "Some people think some of these AGNs might form because of the merging of two galaxies, which would bring a lot of gas around them."

X-rays from AGNs most likely make up most of the cosmic X-ray background, but many AGNs are partially obscured from view by gas and dust that lie near them. These obscured AGNs are also called absorbed AGNs. Scientists hope to detect some high-energy X-rays from absorbed AGNs, but they remain very difficult to locate. Ajello and Burlon wanted to look for those that are most hidden from our line of sight: extremely absorbed AGNs.

Only a small portion of very high-energy X-rays from extremely absorbed AGNs reach Earth, so these objects would not be immediately visible in the telescope data from Swift. Ajello and Burlon studied a sample of X-ray sources in the local universe (an area out to about 326 million light years from Earth) in an effort to identify previously unseen extremely absorbed AGNs. The pair combined archival data with three years of data from Swift-BAT. Together they confirmed experimentally that between 20 and 30 percent of all AGNs in the local universe are extremely absorbed. Ajello said the results line up with theories on this subject.

"People have tried to look for these objects for a very long time," Ajello said. "We found more than anyone has found in the past."

Ajello said the results will contribute to the overall study of the cosmic X-ray background, and more directly to future missions that aim to seek out these extremely hidden X-ray sources.

In 2011 NASA plans to launch a satellite mission called NuSTAR (Nuclear Spectroscopic Telescope Array) into orbit, where it will detect "hard" or high-energy X-rays, with the expressed goal of identifying black holes and AGNs. SLAC is collaborating on NuSTAR plans for data interpretation and analysis.

For more information on this work, see the NASA Web feature Swift Survey Finds 'Missing' Active Galaxies.

—Calla Cofield
SLAC Today, January 21, 2011