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Clue to Glowing X-Ray Sky Found

University of Arizona News Release

2005 August 3

Why does the sky glow?

Astronomers have found that the sky glows in very energetic X-rays. They think the X-rays are the last gasp of material being swallowed by massive black holes. These objects hide behind thick walls of gas and dust, walls so thick that only radio waves and very high-energy X-rays can escape. Even moderately energetic X-rays are blocked.

When astronomers find massive black holes swallowing their surroundings, they can identify them by their peculiar behavior at optical and ultraviolet wavelengths and they call them active galactic nuclei or quasars. However, the massive black holes that bathe the sky in X-rays are too well hidden to be found this way, even though astronomers believe there are millions of them in the distant universe.

A team at The University of Arizona may now have found several of these elusive black holes. Graduate student Jennifer Donley and her collaborators used the Spitzer Space Telescope to obtain very sensitive infrared - heat radiation - maps of a region that had been observed previously in the radio.

Many galaxies are bright in the infrared because young stars are very efficient at warming interstellar dust, but they are faint in the radio. Some active galactic nuclei, however, are bright in the radio but relatively faint in the infrared. The group surveyed hundreds of radio sources, from which they winnowed 27 that are so bright in the radio and faint in the infrared that they could only be active galactic nuclei. The new method circumvented the dilemma of looking for the hidden sources just at the wavelengths where they are hidden.

All of these 27 objects lie in the region of the deepest X-ray survey ever obtained, a region that the Chandra X-ray Observatory stared at for more than three weeks. Fewer than half of the 27 sources were known sources of X-rays. The group looked for weak hints of X-rays from the rest, but at least five show nothing at all.

It's possible that these five sources differ from other massive black holes and just do not make X-rays, Donley said. However, it's more likely that they are "ordinary" black holes but lie hidden behind walls of gas that are so thick that Chandra cannot see through. "We expect the hidden black holes to behave exactly this way," Donley said.

Various other approaches have also found candidates for the hidden X-ray sources. However, the signature of a hidden black hole is that it should be difficult to detect with Chandra. Although not all the other candidates have been observed in the X-ray, they are usually similar to other sources known to be bright for Chandra. By concentrating on the ultra-deep X-ray field, the Donley group was able to find these five objects that are true prototypes for the many high energy X-ray sources needed to explain the glow in the sky.

"Five hidden black holes out of 27 is about what we should find according to the theoretical predictions," Donley said, "but to know if the predictions are exactly right, we really need more sources. Fortunately, Spitzer is observing other fields with deep X-ray measurements, so we will soon be able to tell if we really understand the glowing X-ray sky."

The work is reported in a paper by Donley, George Rieke, Jane Rigby, and Pablo Peréz-González that was recently accepted by the Astrophysical Journal. A press release on related results from Spitzer can be found at

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA. Spitzer's Multiband Imaging Photometer used for this work was built by The University of Arizona, Tucson, AZ. Its development was led by George Rieke of the UA. For information on the Spitzer Space Telescope visit:

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