We evaluate in a homogeneous way the optical masses of 170 nearby clusters (z< 0.15). The sample includes both data from the literature and the new ENACS data (Katgert et al. 1996, 1998). On the assumption that mass follows the galaxy distribution, we compute the masses of each cluster by applying the virial theorem to the member galaxies. We constrain the masses of very substructured clusters (about 10% of our clusters) between two limiting values. After appropriate rescaling to the X-ray radii, we compare our optical mass estimates to those derived from X-ray analyses, which we compiled from the literature (for 66 clusters). We find a good overall agreement. This agreement is expected in the framework of two common assumptions: that mass follows the galaxy distribution, and that clusters are not far from a situation of dynamical equilibrium with both gas and galaxies reflecting the same underlying mass distribution. We stress that our study strongly supports the reliability of present cluster mass estimates derived from X-ray analyses and/or (appropriate) optical analyses.Comment: 13 pages, 7 eps figures, tables are not included, USE LaTeX2e !!, accepted by Ap

We analyze the internal velocity dispersion p of a sample of 172 nearby galaxy clusters (z ¹ 0.15), each of which has at least 30 available galaxy redshifts and spans a large richness range. Cluster membership selection is based on nonparametric methods. In the estimate of galaxy velocity dispersion, we consider the e †ects of possible velocity anisotropies in galaxy orbits, the infall of late-type galaxies, and velocity gradients. The dynamical uncertainties due to the presence of substructures are also taken into account. Previous p-distributions, based on smaller cluster samples, are complete for the Abell richness class R º 1. In order to improve p completeness, we enlarge our sample by also including poorer clusters. By resampling 153 AbellÈAbell-Corwin-Olowin clusters, according to the richness class frequencies of the Edinburgh-Durham Cluster Catalog, we obtain a cluster sample which can be taken as representative of the nearby universe. Our cumulative p-distribution agrees with previous distributions within their p completeness limit km s~1). We estimate that our distribution is complete for at least p º 650 (p Z 800 km s~1. In this completeness range, a Ðt of the form dN P pa dp gives in fair agreement a \ [(7.4~0 .8 0.7), with results coming from the X-ray temperature distributions of nearby clusters. We brieÑy discuss our results with respect to p-distributions for galaxy groups and to theories of large-scale structure formation.

Using a large and well-controlled sample of clusters of galaxies, we investigate the relation between cluster velocity dispersions and X-ray temperatures of intracluster gas.\ud \ud The cluster selection is based on nonparametric methods. In particular, we present the two-dimensional optical maps of our sample clusters, obtained via the kernel adaptive technique, using an optimized smoothing parameter.\ud \ud In order to obtain a reliable estimate of the total velocity dispersion of a cluster, independent of the level of anisotropies in galaxy orbits, we analyze the integrated velocity dispersion profiles over increasing distances from the cluster centers. Both increasing and decreasing integrated profiles are found, but the general trend is a flattening of the integrated velocity dispersion profile at the largest radii, thus enabling us to take the asymptotic value of the integrated profile as an estimate of the total velocity dispersion, which is independent of possible anisotropies.\ud \ud Distortions in the velocity fields, the effect of close clusters, the presence of substructures, and the presence of a population of (spiral) galaxies not in virial equilibrium with the cluster potential are taken into account for reducing the errors in the estimate of the cluster velocity dispersions.\ud \ud Using our final sample of 37 clusters for which a reliable estimate of velocity dispersion could be obtained, we derive a relation between the velocity dispersions and the X-ray temperatures, with a scatter reduced by more than 30% with respect to previous works.\ud \ud A χ2 fit to the temperature-velocity dispersion relation does not exclude the hypothesis that the ratio between galaxy and gas energy density (the so-called βspec) is a constant for all clusters. In particular, the value of βspec = 1, corresponding to energy equipartition, is acceptable.\ud \ud However, the large data scatter in the σ-T relation may suggest the presence of intrinsic dispersion. This intrinsic dispersion may be due to spurious effects (we consider the effect of cluster ellipticity) as well as to physical reasons, different values of βspec pertaining to clusters with different properties