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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. Go back to start page |