Radio-optically selected clusters of galaxies - I. The radiogalaxy sample

In order to study the status and the possible evolution of clusters of gala xies at intermediate redshifts (z similar to 0.1-0.3), as well as their spa tial correlation and relationship with the local environment, we built a sa mple of candidate groups and clusters of galaxies using radiogalaxies as tr acers of dense environments. This technique - complementary to purely optic al or X-ray cluster selection methods - represents an interesting tool for the selection of clusters in a wide range of richness, so to make it possib le to study the global properties of groups and clusters of galaxies, such as their morphological content, dynamical status and number density, as wel l as the effect of the environment on the radio emission phenomena. In this paper we describe the compilation of a catalogue of similar to 16 000 radi o sources in the region of the South Galactic Pole extracted from the publi cly available NRAO VLA Sky Survey maps, and the optical identification proc edure with galaxies brighter than b(J) = 20.0 in the EDSGC Catalogue. The r adiogalaxy sample, valuable for the study of radio source populations down to low flux levels, consists of 1288 identifications and has been used to d etect candidate groups and clusters associated to NVSS radio sources. In a companion paper we will discuss the cluster detection method, the cluster s ample as well as first spectroscopic results.