Monday, July 30, 2007

New Type of Active Galaxy

Image credit: Aurore Simonnet, Sonoma State University.

This illustration shows the different features of an active galactic nucleus (AGN), and how our viewing angle determines what type of AGN we observe. The extreme luminosity of an AGN is powered by a supermassive black hole at the center. Some AGN have jets, while others do not.
[+/-] Click here to expand

Japanese and NASA Satellites Unveil New Type of Active Galaxy

Active Galactic Nuclei AGN, the extraordinarily energetic cores of galaxies such as Quasars, Blazars, and Seyfert galaxies, powered by accreting supermassive black holes, are among the most luminous objects in our Universe, often pouring out the energy of billions of stars from a region no larger than our solar system.

By using Swift and Suzaku, a team of astronomers has discovered that a relatively common class of AGN escaped detection…until now. These objects are so heavily shrouded in gas and dust that virtually no light gets out.

Evidence for this new type of AGN began surfacing over the past two years. Using Swift’s Burst Alert Telescope (BAT), a team led by Tueller has found several hundred relatively nearby AGNs that were previously missed because their visible and ultraviolet light was smothered by gas and dust. The BAT was able to detect high-energy X-rays from these heavily blanketed AGNs because, unlike visible light, high-energy X-rays can punch through thick gas and dust.

According to popular models, AGNs are surrounded by a donut-shaped ring of material, which partially obscures our view of the black hole. Our viewing angle with respect to the donut determines what type of object we see.
But team member Richard Mushotzky, also at NASA Goddard, thinks these newly discovered AGN are completely surrounded by a shell of obscuring material.

Another possibility is that these AGN have little gas in their vicinity. In other AGN, the gas scatters light at other wavelengths, which makes the AGN visible even if they are shrouded in obscuring material. The results imply that there must be a large number of yet unrecognized obscured AGNs in the local universe.

In fact, these objects might comprise about 20 percent of point sources comprising the X-ray background, a glow of X-ray radiation that pervades our Universe. NASA’s Chandra X-ray Observatory has found that this background is actually produced by huge numbers of AGNs, but was unable to identify the nature of all the sources.

By missing this new class, previous AGN surveys were heavily biased, and thus gave an incomplete picture of how supermassive black holes and their host galaxies have evolved over cosmic history.

"We think these black holes have played a crucial role in controlling the formation of galaxies, and they control the flow of matter into clusters," says Tueller. "You can’t understand the universe without understanding giant black holes and what they’re doing."

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