File: space/space3 - item number: 0
Galaxy Cluster X-ray Revealed
posted: 29 May 2009
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.
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