Radio-optically selected clusters of galaxies - II. The cluster sample

We present a sample of 171 candidate groups and clusters of galaxies at int ermediate redshift over an area of approximate to 550 sq degrees at the Sou th 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 p reviously unknown clusters. This paper presents the cluster selection techn ique, based on the search of excesses in the optical surface density of gal axies near identified radiogalaxies, and the first spectroscopic results ai med to confirm the presence of a cluster. Spectroscopy for 11 candidates le d to the detection of 9 clusters at a redshift in the range 0.13 divided by 0.3, with estimated velocity dispersions ranging from values typical of cl usters to those of galaxy groups. These results show that this technique re presents a powerful tool for the selection of homogeneous samples of interm ediate redshift clusters over a wide range of richness.