U.Arizona Astronomers Find Clue to Glowing X-ray Sky (Forwarded)

University Communications
University of Arizona
Tucson, Arizona

Contact Information:

Jennifer Donley, 520-621-6535
George Rieke, 520-621-2832

August 03, 2005

UA Astronomers Find Clue to Glowing X-ray Sky
By Lori Stiles

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 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," she 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
http://www.spitzer.caltech.edu/Media/index.shtml

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 UA's George Rieke.

For information on the Spitzer Space Telescope visit:
http://www.spitzer.caltech.edu/spitzer

Related Web sites:

* Spitzer Space Telescope
http://www.spitzer.caltech.edu/spitzer
* Spitzer's Multiband Imaging Photometer
http://mips.as.arizona.edu/mipspage/

[NOTE: Images supporting this release are available at
http://uanews.org/cgi-bin/WebObjects...rticleID=11458
]