Scientists have precisely determined the distribution of dark matter
in a distant galaxy cluster with NASA’s Chandra X-ray Observatory. These new
measurements serve to narrow the field of candidates that explain “dark
matter,” the invisible and unknown material that comprises most the

John Arabadjis and Mark Bautz of the Massachusetts Institute of
Technology in Cambridge, Mass., and Gordon Garmire of Penn State University
in University Park, announced their results today at the “Two Years of
Science with Chandra” symposium in Washington, D.C. Their observations
enabled them to trace the distribution of dark matter the galaxy cluster
EMSS 1358+6245.

Previous evidence from radio, optical and X-ray observations
convinced astronomers that most of the matter in the universe is in some
dark, as yet undetected, form that makes its presence felt only through
gravity. “The new Chandra observations are providing new clues about the
nature of this mysterious stuff,” said Bautz.

“When combined with data from the Hubble Space Telescope, we are
able to place restrictions on the cross section, or size, of the dark matter
particles,” said Arabadjis. “The larger the particles, the more strongly
they interact, and the more they alter the dark matter distribution.”

In galaxy clusters, the amount of dark matter can be inferred by
measuring the pressure in the X-ray emitting hot gas and determining how
much dark matter is required to provide the gravity necessary to keep the
gas from escaping the cluster. In the cluster EMSS 1358+6245, the mass of
the dark matter is found to be about 4 times that of the “normal” matter
(matter not comprised of exotic particles), typical of large galaxy
clusters. The distribution of dark matter holds the key to understanding
its composition.

The most popular model for dark matter invokes slowly moving
particles called cold dark matter, which interact with “normal” matter only
through gravity. Recent optical observations of galaxies and galaxy clusters
have suggested that dark matter particles may interact more vigorously than
simple cold dark matter. The problem is that galaxies composed primarily of
cold dark matter should have a greater central concentration of dark matter
than the optical data suggest.

One solution has been to introduce self-interacting dark matter, or
SIDM. By comparing the Chandra data with theoretical simulations, scientists
can place strict constraints on the SIDM particles. Chandra observations
show there is no evidence for an excessively spread-out dark matter
distribution at distances larger than 150,000 light years from the cluster’s
center. Inside that distance the dark matter may rather uniformly
distributed, so some collisions between dark matter particles may still be
needed. These results over a range of distances from the cluster center
place the strongest observational limits yet on the dark matter interaction
rate in galaxy cluster cores.

EMSS 1358+6245, about 4 billion light years away in the
constellation Draco, was observed by Chandra for 15.3 hours on Sept. 3-4,
2000 using the ACIS detector.

The ACIS instrument was developed for NASA by Penn State and MIT.
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra
program, and TRW, Inc., Redondo Beach, Calif., is the prime contractor for
the spacecraft. The Smithsonian’s Chandra X-ray Center controls science and
flight operations from Cambridge, Mass.

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