Hubble tracks down a galaxy cluster's dark matter (Forwarded)

European Space Agency
Science News Release No. 10-2003
Paris, France 17 July 2003

Hubble tracks down a galaxy cluster's dark matter

Using the powerful trick of gravitational lensing, a European and American team
of astronomers have constructed an extensive 'mass map' of one of the most
massive structures in our Universe. They believe that it will lead to a better
understanding of how such systems assembled and the key role of dark matter.

Clusters of galaxies are the largest stable systems in the Universe. They are
like laboratories for studying the relationship between the distributions of
dark and visible matter. In 1937, Fritz Zwicky realised that the visible
component of a cluster (the thousands of millions of stars in each of the
thousands of galaxies) represents only a tiny fraction of the total mass. About
80-85% of the matter is invisible, the so-called 'dark matter'. Although
astronomers have known about the presence of dark matter for many decades,
finding a technique to view its distribution is a much more recent development.

Led by Drs Jean-Paul Kneib (from the Observatoire Midi-Pyrénées, France/Caltech,
United States), Richard Ellis and Tommaso Treu (both Caltech, United States),
the team used the NASA/ESA Hubble Space Telescope to reconstruct a unique 'mass
map' of the galaxy cluster CL0024+1654. It enabled them to see for the first
time on such large scales how mysterious dark matter is distributed with respect
to galaxies. This comparison gives new clues on how such large clusters assemble
and which role dark matter plays in cosmic evolution.

Tracing dark matter is not an easy task because it does not shine. To make a
map, astronomers must focus on much fainter, more distant galaxies behind the
cluster. The shapes of these distant systems are distorted by the gravity of the
foreground cluster. This distortion provides a measure of the cluster mass, a
phenomenon known as 'weak gravitational lensing'.

To map the dark matter of CL0024+1654, more than 120 hours observing time was
dedicated to the team. This is the largest amount of Hubble time ever devoted to
studying a galaxy cluster. Despite its distance of 4.5 thousand million
light-years (about one third of the look-back time to the Big Bang) from Earth,
this massive cluster is wide enough to equal the angular size of the full Moon.
To make a mass map that covers the entire cluster required observations that
probed 39 regions of the galaxy cluster.

The investigation has resulted in the most comprehensive study of the
distribution of dark matter in a galaxy cluster so far and extends more than 20
million light-years from its centre, much further than previous investigations.
Many groups of researchers have tried to perform these types of measurements
with ground-based telescopes. However, the technique relies heavily on finding
the exact shapes of distant galaxies behind the cluster. The sharp vision of a
space telescope such as NASA-ESA's Hubble is superior.

The study reveals that the density of dark matter on large scales drops sharply
with distance from the cluster centre. This confirms a picture that has emerged
from recent detailed computer simulations.

As Richard Ellis says: "Although theorists have predicted the form of dark
matter in galaxy clusters from numerical simulations based on the effects of
gravity alone, this is the first time we have convincing observations to back
them up. Some astronomers had speculated clusters might contain large reservoirs
of dark matter in their outermost regions. Assuming our cluster is
representative, this is not the case."

The team noticed that dark matter appears to clump together in their map. For
example, they found concentrations of dark matter associated with galaxies known
to be slowly falling into the system. Generally, the researchers found that the
dark matter traces the cluster galaxies remarkably well and over an
unprecedented range of physical scales.

"When a cluster is being assembled, the dark matter will be smeared out between
the galaxies where it acts like a glue," says Jean-Paul Kneib."The overall
association of dark matter and 'glowing matter' is very convincing evidence that
structures like CL0024+1654 grow by merging of smaller groups of galaxies that
were already bound by their own dark matter components."

Future investigations using Hubble's new camera, the Advanced Camera for Surveys
(ACS), will extend this work when Hubble is trained on a second galaxy cluster
later this year. ACS is 10 times more efficient than the Wide Field and
Planetary Camera 2 used for this investigation, making it possible to study
finer mass clumps in galaxy clusters and help work out how the clusters are
assembled.

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IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/export/esaCP/SEME...reWeek_1.html]
This is a mass map of galaxy cluster Cl0024+1654 derived from an extensive
Hubble Space Telescope campaign. The colour image is made from two images: a
dark-matter map (the blue part of the image) and a 'luminous-matter' map
determined from the galaxies in the cluster (the red part of the image). They
were constructed by feeding Hubble and ground-based observations into advanced
mathematical mass-mapping models.

The map shows that dark matter is present where the galaxies clump together. The
mass of the galaxies is shown in red, the mass of the dark matter in blue. The
dark matter behaves like a 'glue', holding the cluster together.

The dark-matter distribution in the cluster is not spherical. A secondary
concentration of dark-matter mass is shown in blue to the upper right of the
main concentration.

Credits: European Space Agency, NASA and Jean-Paul Kneib (Observatoire
Midi-Pyrénées, France/Caltech, USA)

[Image 2:
http://www.esa.int/export/esaCP/SEME...html#subhead1]
This is a 2.5-degree field around galaxy cluster Cl0024+1654. The cluster
galaxies are visible in the centre of the image in yellow. The image is a colour
composite constructed from three Digitized Sky Survey 2 images: Blue (shown in
blue), Red (shown in green), and Infrared (shown in red).

Credits: European Space Agency, NASA and Jean-Paul Kneib (Observatoire
Midi-Pyrénées, France/Caltech, USA)

[Image 3:
http://www.esa.int/export/esaCP/SEME...html#subhead2]
Five days of observations produced the altogether 39 Hubble Wide Field and
Planetary Camera 2 (WFPC2) images required to map the mass of the galaxy cluster
Cl0024+1654. Each WFPC2 image has a size of about 1/150 the diameter of the full
Moon. In total, the image measures 27 arc-minutes across, slightly smaller than
the diameter of the Moon.

The observed warped shapes of more than 7000 faint background galaxies have been
converted into a unique map of the dark matter in the cluster.

The images were taken through a red filter and have been reduced a factor of two
in size.

Credits: European Space Agency, NASA and Jean-Paul Kneib (Observatoire
Midi-Pyrénées, France/Caltech, USA)

[Image 4:
http://www.esa.int/export/esaCP/SEME...html#subhead3]
This is a colour image of the galaxy cluster C10024+1654 obtained with the
CFHT12k camera at the Canada France Hawaii Telescope on Mauna Kea (Hawaii). The
cluster clearly appears as a concentration of yellow galaxies in the centre of
this image although cluster galaxies actually extend at least to the edge of
this image. This image measures 21 x 21 arc-minutes.

Credits: European Space Agency, NASA and Jean-Paul Kneib (Observatoire
Midi-Pyrénées, France/Caltech, USA)