CHANDRA X-RAY OSSERVATORY CENTER, Cambridge, Mass. – Astronomers have shed new light on dark matter, the invisible and unknown material that comprises most of the Universe.
Using NASA’s Chandra X-ray Observatory, scientists have
precisely determined the distribution of dark matter in a
distant galaxy cluster. These new measurements serve to
narrow the field of candidates that explain this puzzling
element.
John Arabadjis and Mark Bautz of the Massachusetts Institute
of Technology (MIT) in Cambridge, Mass., and Gordon Garmire
of Pennsylvania State University (Penn State) in University
Park, announced their results today at the “Two Years of
Science with Chandra” symposium in Washington. Their
observations enabled them to trace the distribution of dark
matter in 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 hot gas that emits X-rays.
Astronomers can then determine how much dark matter would be
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 four 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 the concept of “self-
interacting dark matter.” By comparing the Chandra data with
theoretical simulations, scientists can place strict
constraints on these 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 be 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.
Chandra observed EMSS 1358+6245, about 4 billion light years
away in the constellation Draco, for more than 15 hours on
Sept. 3-4, 2000, using the Advanced CCD Imaging Spectrometer.
Penn State and MIT developed the instrument for NASA. 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.
