The Cluster M/L and the Value of Omega_m

From an up-to-date compilation of ACO cluster redshifts and velocity dispersions we extract a homogeneous sample of 459 clusters with robust velocity dispersion. Using the virial theorem to estimate cluster masses, and a correlation between Abell galaxy counts and R-band luminosity, we find a median M/L_R of 174 h_50 M_sun/L_sun. However, if we correct the virial masses according to the X-ray vs. virial mass correlation, as derived from our analysis, we obtain M/L_R ~100 h_50 M_sun/L_sun. These two values of M/L imply a mass content of the Universe of Omega_m ~0.26 and 0.15, respectively.