Mass-to-Light Ratios of Galaxy Groups from Weak Lensing1

We present the findings of our weak-lensing study of a sample of 116 CNOC2 galaxy groups. The lensing signal is used to estimate the mass-tolight ratio of these galaxy groups. The best-fit isothermal sphere model to our lensing data has an Einstein radius of 0"88 ± 0"12, which corresponds to a shear-weighted velocity dispersion of 245 ± 18 km s-1. The mean mass-tolight ratio within 1 h-1 Mpc is 185 ± 28 h M(circled dot operator) L^-1(B(circled dot operator) and is independent of radius from the group center. The signal-to-noise ratio of the shear measurement is sufficient to split the sample into subsets of "poor" and "rich" galaxy groups. The poor galaxy groups were found to have an average velocity dispersion of 193 ± 38 km s-1 and a mass-to-light ratio of 134 ± 26 h M(circled dot operator) L^-1, while the rich galaxy groups have a velocity dispersion of 270 ± 39 km s-1 and a mass-to-light ratio of 278 ± 42 h M(circled dot operator) L^-1(B(circled dot operator), similar to the mass-to-light ratio of clusters. This steep increase in the mass-to-light ratio as a function of mass suggests that the mass scale of ~10^13 M(circled dot operator) is where the transition between the actively star-forming field environment and the passively evolving cluster environment occurs. This is the first such detection from weak lensing.