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
