Veiled black holes

11:17am, August 8, 2007
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Lurking at the centers of many galaxies, supermassive black holes make their presence known by gobbling gas, which heats up to fuel quasars and other fireworks. These so-called active galactic nuclei (AGN) are among the most luminous objects in the universe. Now astronomers say that they've found a new, relatively common class of AGN, so heavily smothered by gas and dust that virtually none of the visible and ultraviolet light generated within them can get out.

These hidden AGN came to light over the past 2 years, when the Earth-orbiting Swift spacecraft detected high-energy X rays coming from the cores of several hundred otherwise unremarkable galaxies. The X rays can pass unimpeded through thick blankets of gas and dust that block lower-energy radiation. Jack Tueller of NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleagues then studied two of these AGN with Suzaku, a Japanese–U.S. X-ray mission.

Although it records X rays over a much broader range of energies than Swift can, Suzaku found little medium and low-energy X-ray emission, which may explain why previous searches hadn't found these AGN. In the leading model of AGN, a supermassive black hole is encircled by a doughnut of gas and dust. An observer looking directly down the hole has a clearer view of the activity within than an observer who looks through the doughnut. But in the case of the hidden AGN, the astronomers propose, the entire doughnut is further hidden by a giant cloud of gas and dust.

This new type of AGN could account for up to 20 percent of the X-ray point sources in the sky, Tueller's team estimates in the Aug. 1 Astrophysical Journal Letters.

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