Weak lensing mass reconstruction of MS1008.1-1224 with the VLT

We present an in-depth weak lensing analysis of the cluster MS1008.1-1224 based on deep multicolor imaging obtained during the Science Verification of FORS1 at the VLT. The image quality (half arcsec seeing) and depth of the VLT images allow the shear signal to be mapped with high signal-to-noise and to be traced out to 1.2 h50-1 Mpc, near the edge of the 8' 8' field of view. Using BVRI color information, as well as 81 redshifts in the field from the CNOC survey, background galaxies can be effectively separated from cluster and foreground objects. PSF distorsions are found to be moderate across the FORS images and thus easily removed. Due to the small statistical errors in the mass reconstruction, this dataset provides a testing ground where several systematic effects (e.g. mass-sheet degeneracy, redshift distribution of the background sources, cluster galaxy contamination), which are involved in the weak lensing analysis, can be quantified. Several methods are used to remove the mass-sheet degeneracy which is found to dominate the systematic error budget. We measure a lower limit to the mass of 2.6 x 1014 h50-1 Msun within 1 h50-1 Mpc and a ``total'' mass of 5.3x 1014 h50-1 Msun by fitting a softened isothermal sphere. We find the mass distribution fairly uniform, with no significant substructures, in agreement with the virial analysis. The availability of the CNOC redshift data and X-ray observations on this cluster allow a comparison of different determinations of the mass radial profile. We find the lensing and X-ray measurements in excellent agreement, while the mass derived from the virial analysis is marginally (1-2 sigma ) in agreement at radii where both methods are reliable. This analysis underscores the importance of systematics in the mass determination of clusters, particularly when such a high quality dataset is not available or in similar studies at higher redshifts. Based on observations performed at the European Southern Observatory, Paranal, Chile