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Cosmic Shear in Sky Field with Galaxy Cluster
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Cosmic Shear in Sky Field with Galaxy Cluster

May 16, 2005
These photos show an example of the mapping of the dark mass distribution in one of the 50 sky fields observed with the VLT and FORS1. To the left is the original image, a 36-min exposure in a near-infrared wavelength band. To the right is the reconstructed map of the mass (a "mass photo") in this direction, based on an analysis of the weak shear effect seen in the field; that is, on the measured elongations and directions of the axes of the galaxy images in this field. The brighter areas indicate the directions in which there is most mass along the line of sight. The circle in the left photo surrounds the images of a distant cluster (or group) of galaxies, seen in this direction. Note that there is a corresponding concentration of mass in the "mass photo"; this is obviously the mass of that cluster. The mass reconstruction map shows the (mostly) dark matter responsible for the cosmic shear found on the small scales, now measured with the VLT.