May 31, 2009 - 1:34:31 PM
The joint Japan-U.S. Suzaku mission is
providing new insight into how assemblages of thousands of galaxies
pull themselves together. For the first time, Suzaku has detected
X-ray-emitting gas at a cluster's outskirts, where a billion-year
plunge to the center begins.
"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, the study's lead author at the University of California,
Berkeley.
The team trained Suzaku's X-ray telescopes on the cluster PKS 0745-191,
which lies 1.3 billion light-years away in the southern constellation
Puppis. Between May 11 and 14, 2007, Suzaku acquired five images of the
million-degree gas that permeates the cluster.
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
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," added George.
