Observational mass-to-light ratio of galaxy systems from poor groups to rich clusters

We study the mass-to-light ratio of galaxy systems from poor groups to rich clusters and present for the first time a large database for useful comparisons with theoretical predictions. We extend a previous work, where B-j band luminosities and optical virial masses were analyzed for a sample of 89 clusters. Here we also consider a sample of 52 more clusters, 36 poor clusters, seven rich groups, and two catalogs, of similar to500 groups each, recently identified in the Nearby Optical Galaxy sample by using two different algorithms. We obtain the blue luminosity and virial mass for all systems considered. We devote a large effort to establishing the homogeneity of the resulting values, as well as to considering comparable physical regions, i.e., those included within the virial radius. By analyzing a fiducial, combined sample of 294 systems we find that the mass increases faster than the luminosity: the linear fit gives M proportional to L-B(1.34+/-0.03), B, with a tendency for a steeper increase in the low-mass range. In agreement with the previous work, our present results are superior owing to the much higher statistical significance and the wider dynamical range covered (similar to10(12)-10(15) M.). We present a comparison between our results and the theoretical predictions on the relation between M/L-B and halo mass, obtained by combining cosmological numerical simulations and semianalytic modeling of galaxy formation.