Andrew Yee
August 4th 05, 03:19 PM
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/UANews.woa/8/wa/SciDetails?ArticleID=11458
]
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/UANews.woa/8/wa/SciDetails?ArticleID=11458
]