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Steve Roy
Marshall Space Flight Center, Huntsville, AL
steve.roy{at}msfc.nasa.gov
Phone: 256-544-6535

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, MA
cxcpress{at}cfa.harvard.edu
Phone: 617-496-7998

Science Contacts:
Aaron Lewis, lewisa{at}uci.edu, 949-824-2650
David Buote, buote{at}uci.edu, 949-824-6280

For Release: June 11, 2003

CXC Press Release: 06-01

Hot News for Cold Dark Matter

Astronomers have used NASA's Chandra X-ray Observatory to make the
most detailed probe yet of the distribution of dark matter in a
massive cluster of galaxies. Their results indicate that about 80
percent of the matter in the universe consists of cold dark matter --
mysterious subatomic particles left over from the dense early
universe.
 
Chandra observed a cluster of galaxies called Abell 2029 located
about a billion light years from Earth. The cluster is composed of
thousands of galaxies enveloped in a gigantic cloud of hot gas, and
an amount of dark matter equivalent to more than a hundred trillion
Suns. At the center of this cluster is an enormous, elliptically
shaped galaxy that is thought to have been formed from the mergers of
many smaller galaxies. The X-ray data show that the density of dark
matter increases smoothly all the way into the central galaxy of the
cluster. This discovery agrees with the predictions of cold dark
matter models, and is contrary to other dark matter models that
predict a leveling off of the amount of dark matter in the center of
the cluster. 

"I was really surprised at how well we could measure the dark matter
so deep into the core of a rich cluster," said Aaron Lewis of the
University of California, Irvine, lead author of a paper describing
the results in a recent issue of The Astrophysical Journal. "We still
have very little idea as to the exact nature of these particles, but
our results show that they must behave like cold dark matter."

Cold dark matter gets its name from the assumption that the dark
matter particles were moving slowly when galaxies and galaxy clusters
began to form. Dark matter particles interact with each other and
"normal" matter only through gravity.

The astronomers' success in placing such tight constraints on the
dark matter distribution was partly due to Chandra's ability to make
a high resolution intensity and temperature map, and partly due to
their choice of a target. The cluster and central galaxy are
unusually regular, with little or no sign of disturbance.

The hot gas in a cluster is held in the cluster primarily by the
gravity of the dark matter, so the distribution of the hot gas is
determined by that of the dark matter. By precisely measuring the
distribution of X-rays from the hot gas, the astronomers were able to
make the best measurement yet of the distribution of dark matter in
the inner region of a galaxy cluster. 

"While Abell 2029 might be boring for the average person to look at,"
said David Buote, a coauthor of the paper, "it is a pure delight for
astrophysicists to study, because it allows for a very
straightforward and accurate comparison of theory and observation."

As a case in point, earlier observations of the Hydra A galaxy
cluster by Larry David of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass. and colleagues found a similar
result but the evidence of explosive activity in the central galaxy
made it difficult to draw definite conclusions about the nature of
the dark matter. The dark matter profile deduced for Abell 2029
provides evidence that the Hydra results are reliable and is an
important independent confirmation of cold dark matter predictions.

John Stocke of the University of Colorado, Boulder was also involved
in this research. Chandra observed Abell 2029 with the ACIS detector
for 5.6 hours on April 12, 2000. NASA's Marshall Space Flight Center,
Huntsville, Ala., manages the Chandra program for the Office of Space
Science, NASA Headquarters, Washington. Northrop Grumman of Redondo
Beach, Calif., formerly TRW, Inc., was the prime development
contractor for the observatory. The Smithsonian Astrophysical
Observatory controls science and flight operations from the Chandra
X-ray Center in Cambridge, Mass.

The image and additional information are available at:

      http://chandra.harvard.edu
and
      http://chandra.nasa.gov

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