The Velocity Dispersions Distribution of Galaxy Clusters

We compute the distribution of internal velocity dispersions, a, for a sample of 153 nearby Abell-ACO galaxy clusters (z less than or equal to 0.15), each of which has at least 30 available galaxy redshifts. Throughout our work we applied homogeneous procedures to the analysis of the redshifts of the clusters selected. Previous sigma-distributions, based on smaller cluster samples, are complete for the Abell richness class R greater than or equal to 1. In order to improve sigma completeness, we enlarge our sample by including also poorer clusters. By resampling our 153 clusters, according to the richness class frequencies of the Edinburgh-Durham Cluster Catalog (EDCC), we obtain a cluster sample which can be taken as representative of the nearby Universe. Our cumulative sigma-distribution agrees with previous distributions within their sigma completeness limit (sigma greater than or equal to 800 km s(-1)). We estimate that our distribution is complete for at least sigma greater than or equal to 650 km s(-1). In this completeness range, a fit of the form dN alpha sigma(alpha)d sigma gives alpha = -(7.4(-0.8)(+0.7)), in fair agreement with results coming from the X-ray temperature distributions of nearby clusters.