Astronomers took a peek at the first X-ray image of an
entire galaxy cluster, courtesy of a Japan-U.S. space observatory.
The target cluster known as PKS 0745-191 lies 1.3 billion
light-years away in the southern constellation Puppis. The Suzaku observatory
("red bird of the south") took five images of the cluster's million-degree
gas during last May.
"These Suzaku observations are exciting because we
can finally see how these structures, the largest bound objects in the
universe, grow even more massive," said Matt George, an astronomer at the
University of California-Berkeley.
By looking at a cluster in X-rays, astronomers can
measure the temperature and density of the gas, which provides clues about the
gas pressure and total mass of the cluster. Astronomers expect that the gas in
the inner part of a galaxy cluster has settled into a "relaxed" state
in equilibrium with the cluster's gravity. This means that the hottest, densest
gas lies near the cluster's
center, and temperatures and densities steadily decline at greater
distances.
In the cluster's outer regions, though, the gas is no
longer in an orderly state because matter is still falling inward.
"Clusters are the most massive, relaxed objects in
the universe, and they are continuing to form now,"
said team member Andy Fabian at the Cambridge Institute of Astronomy in the UK.
The distance where order turns to chaos is referred to as the cluster's
"virial radius."
For the first time, this study shows the X-ray emission
and gas density and temperature out to — and even beyond — the virial radius,
where the cluster continues to form. "It gives us the first complete X-ray
view of a cluster
of galaxies," Fabian said.
In PKS 0745-191, the gas temperature peaks at 164 million
degrees Fahrenheit (91 million C) about 1.1 million light-years from the
cluster's center. Then, the temperature declines smoothly with distance,
dropping to 45 million F (25 million C) more than 5.6 million light-years from
the center. The findings appear in the May 11 issue of the journal Monthly
Notices of the Royal Astronomical Society.
To discern the cluster's outermost X-ray emission
requires detectors with exceptionally low background noise. Suzaku's advanced
X-ray detectors, coupled with a low-altitude orbit, give the observatory much
lower background noise than other X-ray
satellites. The low orbit means that Suzaku is largely protected by Earth's
magnetic field, which deflects energetic particles from the sun and beyond.
"With more Suzaku observations in the outskirts of
other galaxy clusters, we'll get a better picture of how these massive
structures evolve," George added.
The Suzaku observatory mission launched July 10, 2005.
advertisement
