JHU-STScI Team Maps Dark Matter in Startling Detail (Forwarded)

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FOR IMMEDIATE RELEASE: December 9, 2005

JHU-STScI Team Maps Dark Matter in Startling Detail

Clues revealed by the recently sharpened view of the Hubble Space
Telescope have allowed astronomers to map the location of invisible
"dark matter" in unprecedented detail in two very young galaxy clusters.

A Johns Hopkins University-Space Telescope Science Institute team
reports its findings in the December issue of Astrophysical Journal.
(Other, less-detailed observations appeared in the January 2005 issue of
that publication.)

The team's results lend credence to the theory that the galaxies we can
see form at the densest regions of "cosmic webs" of invisible dark
matter, just as froth gathers on top of ocean waves, said study
co-author Myungkook James Jee, assistant research scientist in the Henry
A. Rowland Department of Physics and Astronomy in Johns Hopkins' Krieger
School of Arts and Sciences.

"Advances in computer technology now allow us to simulate the entire
universe and to follow the coalescence of matter into stars, galaxies,
clusters of galaxies and enormously long filaments of matter from the
first hundred thousand years to the present," Jee said. "However, it is
very challenging to verify the simulation results observationally,
because dark matter does not emit light."

Jee said the team measured the subtle gravitational "lensing" apparent
in Hubble images -- that is, the small distortions of galaxies' shapes
caused by gravity from unseen dark matter -- to produce its detailed
dark matter maps. They conducted their observations in two clusters of
galaxies that were forming when the universe was about half its present age.

"The images we took show clearly that the cluster galaxies are located
at the densest regions of the dark matter haloes, which are rendered in
purple in our images," Jee said.

The work buttresses the theory that dark matter -- which constitutes 90
percent of matter in the universe -- and visible matter should coalesce
at the same places because gravity pulls them together, Jee said.
Concentrations of dark matter should attract visible matter, and as a
result, assist in the formation of luminous stars, galaxies and galaxy
clusters.

Dark matter presents one of the most puzzling problems in modern
cosmology. Invisible, yet undoubtedly there -- scientists can measure
its effects -- its exact characteristics remain elusive. Previous
attempts to map dark matter in detail with ground-based telescopes were
handicapped by turbulence in the Earth's atmosphere, which blurred the
resulting images.

"Observing through the atmosphere is like trying to see the details of a
picture at the bottom of a swimming pool full of waves," said Holland
Ford, one of the paper's co-authors and a professor of physics and
astronomy at Johns Hopkins.

The Johns Hopkins-STScI team was able to overcome the atmospheric
obstacle through the use of the space-based Hubble telescope. The
installation of the Advanced Camera for Surveys in the Hubble three
years ago was an additional boon, increasing the discovery efficiency of
the previous HST by a factor of 10.

The team concentrated on two galaxy clusters (each containing more than
400 galaxies) in the southern sky.

"These images were actually intended mainly to study the galaxies in the
clusters, and not the lensing of the background galaxies," said
co-author Richard White, a STScI astronomer who also is head of the
Hubble data archive for STScI. "But the sharpness and sensitivity of the
images made them ideal for this project. That's the real beauty of
Hubble images: they will be used for years for new scientific
investigations."

The result of the team's analysis is a series of vividly detailed,
computer-simulated images illustrating the dark matter's location.
According to Jee, these images provide researchers with an unprecedented
opportunity to infer dark matter's properties.

The clumped structure of dark matter around the cluster galaxies is
consistent with the current belief that dark matter particles are
"collision-less," Jee said. Unlike normal matter particles, physicists
believe, they do not collide and scatter like billiard balls but rather
simply pass through each other.

"Collision-less particles do not bombard one another, the way two
hydrogen atoms do. If dark matter particles were collisional, we would
observe a much smoother distribution of dark matter, without any
small-scale clumpy structures," Jee said.

Ford said this study demonstrates that the ACS is uniquely advantageous
for gravitational lensing studies and will, over time, substantially
enhance understanding of the formation and evolution of the cosmic
structure, as well as of dark matter.

"I am enormously gratified that the seven years of hard work by so many
talented scientists and engineers to make the Advanced Camera for
Surveys is providing all of humanity with deeper images and
understandings of the origins of our marvelous universe," said Ford, who
is principal investigator for ACS and a leader of the science team.

The ACS science and engineering team is concentrated at the Johns
Hopkins University and the Space Telescope Science Institute on the
university's Homewood campus in Baltimore. It also includes scientists
from other major universities in the United States and Europe. ACS was
developed by the team under NASA contract NAS5-32865 and this research
was supported by NASA grant NAG5-7697.

High resolution images also are available, as are digital photos of Jee,
Ford and White. Contact Lisa De Nike at Lde @ jhu.edu .

[NOTE: Images supporting this release are available at
http://www.jhu.edu/news/home05/dec05/darkpix.html ]