Optical Luminosities and Mass-to-Light Ratios of Nearby Galaxy Clusters

We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well-defined high degree of completeness. In particular, we consider a subsample of 89 clusters with Bj-band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalog. After suitable magnitude corrections and uniform conversions to Bj band, we compute cluster luminosities LBj within several clustercentric distances, 0.5, 1.0, 1.5 h-1 Mpc and within the virialization radius Rvir. In particular, we use the luminosity function and background counts estimated by Lumsden et al. on the Edinburgh/Durham Southern Galaxy Catalog, which is the well-calibrated part of the COSMOS catalog. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, LBj, and galaxy velocity dispersion, σv, with LBj~σ2.1-2.3v. Our estimate of the typical value for the mass-to-light ratio is M/LBj~250 h Msolar/Lsolar. We do not find any correlation of M/LBj with cluster morphologies, i.e., Rood-Sastry and Bautz-Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M~L1.2-1.3Bj. We verify the robustness of this relation against a number of possible systematics. We verify that this increasing trend of M/L with cluster mass cannot be entirely due to a higher spiral fraction in poorer clusters, thus suggesting that a similar result would also be found by using R-band galaxy magnitudes.