Radio – Optically Selected Clusters of Galaxies II . The Cluster Sample

We present a sample of 171 candidate groups and clusters of galaxies at intermediate redshift over an area of ≈ 550 sq. degrees at the South Galactic Pole, selected by using optically identified radio sources from the NRAO VLA Sky Survey as tracers of dense environments. Out of these 171 candidates, 76 have a counterpart in the literature while 95 of them are previously unknown clusters. This paper presents the cluster selection technique, based on the search of excesses in the optical surface density of galaxies near identified radiogalaxies, and the first spectroscopic results aimed to confirm the presence of a cluster. Spectroscopy for 11 candidates led to the detection of 9 clusters at redshift in the range 0.13 ÷ 0.3, with estimated velocity dispersions ranging from values typical of clusters to those of galaxy groups. These results show that this technique represents a powerful tool for the selection of homogeneous samples of intermediate redshift clusters over a wide range of richness.