Hubble detects ring of dark matter!!

Astronomers using the Hubble Space Telescope have discovered a ghostlyring of dark matter that formed during a titanic collision between twomassive galaxy clusters. Because ordinary matter in the cluster showsno evidence of such a ring, this discovery is among the strongestevidence yet for non-baryonic dark matter. Clusters of galaxies are the largest gravitationally bound structuresin the universe. They typically contain hundreds or thousands ofgalaxies, forming at the knots of the filamentary sponge-likedistribution of matter on very large scales. Numerical simulationsshow how the accretion of matter from the filaments to the knots makegalaxy clusters grow in size. This one-dimensional accretion (along afilament) results in frequent, near head-on collisions among clustersor groups of galaxies, whereas interactions between individualgalaxies usually occur only when there is significant rotation.

The galaxy cluster Cl 0024+17 – some 5 x 109 light-years away (z =0.4) – is supposed to have experienced exactly such a head-oncollision 1 or 2 thousand million years ago. The first evidence ofthis was obtained in 2002 by Oliver Czoske, from the University ofBonn, and collaborators. By studying the velocity distribution of thegalaxies in the cluster, they found two distinct groups with oppositevelocity, suggesting that there are two sub-clusters moving away fromeach other along the line-of-sight. Their numerical simulationsconfirm the collision scenario and suggest a sub-cluster mass ratio of2:1. In 2004, when the dark-matter distribution in Cl 0024+117 was studied there was no such a peculiar cluster of galaxies. Ring-like distribution was never seen in other clusters.It is indeed tricky to derive the dark-matter distribution in acluster from the distortion it causes on the shape of backgroundgalaxies, but the analysis of this weak gravitational lensing nowseems to be well under control, with the release of the first 3D mapof the dark matter distribution (CERN Courier January/February 2007)