Mass and mass-to-light ratio of galaxy groups from weak lensing

We have measured for the first time the weak lensing signal due to groups of galaxies. The groups are at intermediate redshifts, and have been identified in the CNOC2 field survey, which was kindly made available by Ray Carlberg and Howard Yee. The ensemble averaged group velocity dispersion, based on a preliminary selection of 59 groups, is found to be sigma= 320^{+46}_{-54} km/s, which is in fair agreement with the dynamical estimate from the spectroscopic redshifts. Under the assumption that mass traces light, we find an average mass-to-light ratio in restframe B of (256\pm84)h Msun/Lsun. Using this result we obtain Omega_m=0.22+-0.09 (\Omega_\Lambda=0).