The Cluster M/L Ratio and the Value of Ωm

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 ≃ 174 h₅₀ M_⊙/L_⊙. 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 h₅₀ M_⊙/L_⊙. These two values of M/L imply a mass content of the Universe of Ω_m ≃ 0.26 and 0.15, respectively.