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

We present an in-depth weak lensing analysis of the cluster MS 1008.1-1224 based on deep multicolor imaging obtained during the Science Verification o f 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 a nd to be traced out to 1.2 h(50)(-1). Mpc, near the edge of the 6.'8 x 6.'8 field of view. Using BVRI. color information, as well as 81 redshifts in t he field from the CNOC survey, background galaxies can be effectively separ ated from cluster and foreground objects. PSF distorsions are found to be m oderate across the FORS images and thus easily removed. Due to the small st atistical errors in the mass reconstruction, this dataset provides a testin g ground where several systematic effects (e.g. mass-sheet degeneracy, reds hift distribution of the background sources, duster 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 dom inate the systematic error budget. We measure a lower limit to the mass of 2.6 x 10(14) h(50)(-1) M. within 1 h(50)(-1) Mpc and a "total" mass of 5.3 x 10(14) h(50)(-1) M. by fitting a softened isothermal sphere. We fmd the m ass distribution fairly uniform, with no significant substructures, in agre ement with the virial analysis. The availability of the CNOC redshift data and X-ray observations on this cluster allow a comparison of different dete rminations of the mass radial profile. We find the lensing and X-ray measur ements in excellent agreement, while the mass derived from the virial analy sis is marginally (1-2 sigma) in agreement at radii where both methods are reliable. This analysis underscores the importance of systematics in the ma ss determination of clusters, particularly when such a high quality dataset is not available or in similar studies at higher redshifts.