A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant Cosmic Structures Formed

Caltech News Release

Embargoed for Release at 12 a.m. PDT, Thursday, July 17, 2003


A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant
Cosmic Structures Formed

PASADENA, Calif.--Astrophysicists have had an exceedingly difficult
time charting the mysterious stuff called dark matter that permeates
the universe because it's--well--dark. Now, a unique "mass map" of a
cluster of galaxies shows in unprecedented detail how dark
matter is distributed with respect to the shining galaxies. The new
comparison gives a convincing indication of how dark matter figures
into the grand scheme of the cosmos.

Using a technique based on Einstein's theory of general relativity,
an international group of astronomers led by Jean-Paul Kneib, Richard
Ellis, and Tommaso Treu of the California Institute of Technology
mapped the mass distribution of a gigantic cluster of galaxies about
4.5 billion light-years from Earth. They did this by studying the way
the cluster bends the light from other galaxies behind it. This
technique, known as gravitational lensing, allowed the researchers to
infer the mass contribution of the dark matter, even though it is
otherwise invisible.

Clusters of galaxies are the largest stable systems in the universe
and ideal "laboratories" for studying the relationship between the
distributions of dark and visible matter. Caltech's Fritz Zwicky
realized in 1937 from studies of the motions of galaxies in the
nearby Coma cluster that the visible component of a cluster--the
stars in galaxies--represents only a tiny fraction of the total mass.
About 80 to 85 percent of the matter is invisible.

In a campaign of over 120 hours of observations using the Hubble
Space Telescope, the researchers surveyed a patch of sky almost as
large as the full moon, which contained the cluster and thousands of
more distant galaxies behind it. The distorted shapes of these
distant systems were used to map the dark matter in the foreground
cluster. The study achieved a new level of precision, not only for
the center of the cluster, as has been done before for many systems,
but also for the previously uncharted outlying regions.

The result is the most comprehensive study to date of the
distribution of dark matter and its relationship to the shining
galaxies. Signals were traced as far out as 15 million light-years
from the cluster center, a much larger range than in previous
investigations.

Many researchers have tried to perform these types of measurements
with ground-based telescopes, but the technique relies heavily on
measuring the exact shapes of distant galaxies behind the cluster,
and for this the "surgeon's eye" of the Hubble Space Telescope is far
superior.

The study, to be published soon in the Astrophysical Journal, reveals
that the density of dark matter falls fairly sharply with distance
from the cluster center, defining a limit to its distribution and
hence the total mass of the cluster. The falloff in density with
radius confirms a picture that has emerged from detailed computer
simulations over the past years.

Team member Richard Ellis said, "Although theorists have predicted
the distribution of dark matter in clusters from numerical
simulations based on the effects of gravity, this is the first time
we have convincing observations on large scales to back them up.

"Some astronomers had speculated clusters might contain large
reservoirs of dark matter in their outermost regions," Ellis added.
"Assuming our cluster is representative, this is not the case."

In finer detail, the team noticed that some structure emerged from
their map of the dark matter. For example they found localized
concentrations of dark matter associated with galaxies known to be
slowly falling into the system. Overall there is a striking
correspondence between features in the dark matter map and that
delineated by the cluster galaxies, which is an important result in
the new study.

"The close association of dark matter with structure in the galaxy
distribution is convincing evidence that clusters like the one
studied built up from the merging of smaller groups of galaxies,
which were prevented from flying away by the gravitational pull of
their dark matter," says Jean-Paul Kneib, who is the lead author in
the publication.

Future investigations will extend this work using Hubble's new
camera, the Advanced Camera for Surveys (ACS), which will be trained
on a second cluster later this year. ACS is 10 times more efficient
than the Wide Field and Planetary Camera 2, which was used for this
investigation. With the new instrument, it will be possible to study
clumps of finer mass in galaxy clusters in order to investigate how
the clusters originally were assembled.

By tracing the distribution of dark matter in the most massive
structure in the universe using the powerful trick of gravitational
lensing, astronomers are making great progress towards a better
understanding of how such systems were assembled, as well as toward
defining the key role of dark matter.

In addition to Kneib, Ellis, and Treu, the other team members are
Patrick Hudelot of the Observatoire Midi-Pyrénées in France, Graham
P. Smith of Caltech, Phil Marshall of the Mullard Radio Observatory
in England, Oliver Czoske of the Institut für Astrophysik und
Extraterrestrische Forschung in Germany, Ian Smail of the University
of Durham in England, and Priya Natarajan of Yale University.



For more information, please contact:

Jean-Paul Kneib
Caltech/Observatoire Midi-Pyr»n»es
(currently in Hawaii)
Phone: (808) 881-3865
E-mail:

Richard Ellis
Caltech
Phone: (626) 395-4970 (secretary)
(Australia: Cellular: 011-44-7768-923277)
E-mail: