Groups and clusters of galaxies are the largest gravitationally bound, observable structures, and much can be understood about the global cosmological properties of the universe by studying their properties - such as their dynamical status and evolution, their morphological content and interactions with the environment. To this aim, it is of fundamental importance to gather cluster samples representative of different dynamical structures - from groups to rich clusters - in a wide range of redshift and covering large areas of the sky.
Existing wide-area cluster samples based on visual inspection of optical plates (Abell et al. 1989) or obtained through objective algorithms (EDCC, Lumsden et al. 1992; APM, Dalton et al. 1994) are limited to redshift less than 0.2, and suffer from the possibility of misclassification due to projection effects along the line of sight. Even more difficult is the detection of groups of galaxies, due to their low density contrast with respect to field galaxy distribution. In the optical band, cluster samples at higher z have been built over selected areas of few square degrees (Postman et al. 1996; Scodeggio et al. 1999). Alternatively, the X-ray emission of the hot intracluster medium has been widely used to build distant cluster samples, but this technique suffers from the limited sensitivity of wide-area X-ray surveys and from the possibility of evolutionary effects (Gioia et al. 1990; Henry et al. 1992; RDCS, Rosati et al. 1998).
A different approach - complementary to purely optical or X-ray cluster selection methods - is the use of radiogalaxies as suitable tracers of dense environments. Faranoff-Riley I and II radio sources have been shown to inhabit different environments at different epochs and proved to be efficient tracers of galaxy groups and clusters (Prestage & Peacock 1988; Hill & Lilly 1991; Allington-Smith et al. 1993; Zirbel 1997). FRI sources are found on average in rich groups or clusters at any redshift, and are associated with elliptical galaxies, with the most powerful FRI often hosted by a cD or double nucleus galaxy. FRII radio sources are typically associated with disturbed ellipticals and they avoid rich clusters at low z (Zirbel 1996).
Since there is no significant correlation between the radio properties of galaxies within a cluster with the cluster X-ray luminosity (Feigelson et al. 1982; Burns et al. 1994), or richness (Zhao et al. 1989; Ledlow & Owen 1996), as well as between the properties of group members and the radio characteristics of the radiogalaxies (Zirbel 1997), radio selection should not impact on the X-ray or optical properties of the clusters found in this way.
Radiogalaxies can thus be used to study the global properties of galaxy groups and clusters, such as their morphological content, dynamical status and number density, as well as the effect of the environment on the radio emission phenomena.
We used the NRAO VLA Sky Survey (NVSS, Condon et al. 1998) publicly available data to build a sample of radio-optically selected clusters through optical identifications of radio sources and search of excesses in the surface density of galaxies around these radiogalaxies. The NVSS survey offers indeed an unprecedented possibility to study a wide-area, homogeneous sample of radio sources down to relatively low flux levels, together with a positional accuracy suitable for optical identifications.
In a previous paper (Zanichelli et al. 2001, hereafter Paper I) we described how we extracted a radio source catalogue from the NVSS maps and the optical identification procedure that led to the compilation of a radiogalaxy sample. In this paper we discuss the cluster finding method used for the compilation of a new sample of candidate groups and clusters of galaxies, and present the first observational results that spectroscopically confirmed the presence of a group or cluster for 9 out of the 11 successfully observed candidates.
This paper is structured as follows: in Sect. 2 we give a summary of the properties of the radio and optical data samples used for the cluster search. In Sect. 3 we describe the cluster finding method. The new sample of candidate clusters is presented in Sect. 4. Spectroscopic observations of a subsample of candidate clusters, aimed to obtain an observational confirmation of the presence of a cluster are presented and discussed in Sects. 5 and 6.
Copyright ESO 2001