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

We present a new determination of the mass function of galaxy clusters, based on optical virial mass estimates for a large sample of 152 nearby (z <= 0.15) Abell-ACO clusters, as provided by Girardi et al. This sample includes both data from the literature and the new ENACS data. The resulting mass function is reliably estimated for masses larger than M_lim ~= 4 x 10^14 h^-1 M_sun, while it is affected by sample incompleteness at smaller masses. We find N(>M_lim) = (6.3 +/- 1.2) x 10^-6 (h^-1 Mpc)^-3 for cluster masses estimated within a 1.5 h^-1 Mpc radius. Our mass function is intermediate between the two previous estimates of Bahcall & Cen and Biviano et al. Based on the Press-Schechter (PS) approach, we use this mass function to constrain the amplitude of the fluctuation power spectrum at the cluster scale. After suitably convolving the PS predictions with observational errors on cluster masses and COBE-normalizing the fluctuation power spectrum, we find sigma_8=(0.60+/-0.04)Omega^-0.46+0.09Omega_0_0 for flat low-density models and sigma_8=(0.60+/-0.04)Omega^-0.48+0.17Omega_0_0 for open models (at the 90% confidence level)

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We analyze the presence of substructures in a set of 48 galaxy clusters by using galaxy positions and redshifts. The data are taken from literature sources, with the addition of some new data provided by recent observations of galaxy clusters. We use a multiscale analysis that couples kinematical estimators with the wavelet transform. With respect to previous works, we introduce three new kinematical estimators. These estimators parameterize the departures of the local means and/or local dispersions of the measured radial velocities with respect to their global values for the environment. We classify the analyzed clusters as unimodal, bimodal, and complex systems. We find that about 14% of our clusters are strongly substructured (i.e., they are bimodal or complex) and that about 24% of the remaining unimodal clusters contain substructures at small scales. Thus, in substantial agreement with previous studies, about one-third of clusters show substructures. We find that the presence of substructures in unimodal clusters does not affect the estimates of both velocity dispersions and virial masses. Moreover, the galaxy velocity dispersion is generally in good agreement with the X-ray temperature, according to the expectations of the standard isothermal model for galaxy clusters. These facts suggest that unimodal clusters, which are the most frequent cases in the nearby universe, are not too far from a status of dynamical equilibrium. In contrast, the estimates of velocity dispersions and masses for some bimodal or complex clusters strongly depend on whether they are treated as single systems or as sums of different clumps. In these cases the X-ray temperature and the velocity dispersion may be very different

We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well-deÐned high degree of completeness. In particular, we consider a subsample of 89 clusters with galaxy magnitudes taken from the COSMOS/UKST B j -band Southern Sky Object Catalog. After suitable magnitude corrections and uniform conversions to band, B j we compute cluster luminosities within several clustercentric distances, 0.5, 1.0, 1.5 h~1 Mpc and L Bj within the virialization radiusIn particular, we use the luminosity function and background counts R vir . estimated by Lumsden et al. on the Edinburgh/Durham Southern Galaxy Catalog, which is the wellcalibrated part of the COSMOS catalog. We analyze the e †ect 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 Ðnd a very good correlation between cluster luminosity, and galaxy velocity dispersion, with Our estimate of . the typical value for the mass-to-light ratio isWe do not Ðnd any correlation ofwith cluster morphologies, i.e., Rood-Sastry and Bautz-Morgan types, and only a weak signiÐcant M/L Bj correlation with cluster richness. We Ðnd that mass has a slight, but signiÐcant, tendency to increase faster than the luminosity does,We verify the robustness of this relation against a number M P L Bj 1.2h1.3. 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.

The velocity field of galaxies in clusters is investigated on the basis of a sample consisting of 68 clusters with at least 30 galaxies for which redshifts are available in the literature; for 61 of these clusters, most of the galaxy magnitudes were collected. Galaxies brighter than the magnitude of the third-ranked object, m3, have velocities lower than average. These galaxies are preferentially located in the central regions. The effect is not induced by morphological segregation, it is not restricted to cD clusters, and it does not depend on the presence of substructures. The energy equipartition status seems to be achieved by these low-velocity galaxies. This evidence of segregation stresses the importance of m3 as a boundary between different dynamical statuses, and constrains the theory of structure and evolution of clusters of galaxies. A possible interpretation is given in terms of dynamical friction and merging