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 the 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. The scientists 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. It is invisible, yet undoubtedly there
— scientists can measure its effects — but 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, an
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 "collisionless," Jee said. Unlike
normal matter particles, physicists believe, they do not
collide and scatter like billiard balls but rather simply
pass through each other.
"Collisionless 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 Johns Hopkins and the Space Telescope Science Institute,
which is on the university's Homewood campus. It also
includes scientists from other major universities in the
United States and Europe. ACS was developed by the team
under a NASA contract, and this research was supported by a
NASA grant.
To see graphics that illustrate this research, go to
www.jhu.edu/news/home05/dec05/darkpix.html.
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