Cosmic shear in sky field with galaxy cluster

The image shows 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. Technical information about these photos is available below.

Technical information about the photo: The sky photo (left) is reproduced from a 36 min exposure, obtained on 15 June 1999 with VLT ANTU and the multi-mode FORS1 instrument. The optical filter was I (900 nm) and the seeing was 0.53 arcsec. The field measures approx. 6 x 6 arcmin 2. North is up and East is left.