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J. Frieman (Fermilab/Univ. of Chicago), M. Joffre (Univ. of Chicago), P. Fischer (Univ. of Michigan), B. Nichol (Carnegie Mellon Univ.), D. Johnston (Univ. of Chicago), T. McKay (Univ. of Michigan)
The study of weak gravitational lensing has grown into a mature science as the image distortion present in several galactic clusters has been detected. This very weak distortion has been extracted to give two-dimensional mass-maps of the clusters and allow determination of their total masses. To detect this very weak tidal shearing of the light from background galaxies requires averaging over many such galaxies to reduce the noise introduced by the intrinsic ellipticity of these background galaxies. In the past, distant clusters with a small angular size on the sky were observed very deeply to gain the requisite number of galaxies. However, with the advent of mosaic CCD cameras, we have been able to observe very nearby clusters (z < 0.1) with large angular size for shorter durations, thus obtaining a large sample of galaxies by going very wide (44' \times 44') rather than very deep. This method provides a unique opportunity to directly study the mass concentrations in nearby clusters. It is these same clusters on which there exists large amounts of ancillary data on the light emitted by the clusters, from which we gain most of our understanding of cluster properties and dynamics. We are part of a project to find the weak lensing maps and masses for a complete, X-ray luminosity-limited sample of 24 nearby southern clusters scheduled for Sunyaev--Zel'dovich (SZ) observations by the Viper telescope at the South Pole. We have collected data from 1/4 of the sample and have shown that the systematic errors can be reduced to a level where lensing signal can be seen, proving the feasibility of this study. Combined with the extensive X-ray data, optical red shifts and high quality SZ observations available on these clusters, we will be able form robust measurements of their total mass. This is a first step towards a robust determination of the local cluster mass function: a fundamental goal of cosmology, used to constrain the underlying cosmological model and \Omega0.
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