The XXL Survey IX. Optical overdensity and radio continuum analysis of a supercluster at z=0.43

Review article, 2016

We present observations with the Karl G. Jansky Very Large Array (VLA) at 3 GHz (10 cm) toward a sub-field of the XXL-North 25 deg(2) field targeting the first supercluster discovered in the XXL Survey. The structure has been found at a spectroscopic redshift of 0.43 and extending over 0.degrees 35x0.degrees 1 on the sky. The aim of this paper is twofold. First, we present the 3 GHz VLA radio continuum observations, the final radio mosaic and radio source catalogue, and, second, we perform a detailed analysis of the supercluster in the optical and radio regimes using photometric redshifts from the CFHTLS survey and our new VLA-XXL data. Our final 3 GHz radio mosaic has a resolution of 3 ''.2 x 1 ''.9, and encompasses an area of 41'x41' with rms noise level lower than similar to 20 mu Jy beam(-1). The noise in the central 15'x15' region is approximate to 11 mu Jy beam(-1). From the mosaic we extract a catalogue of 155 radio sources with signal-to-noise ratio (S/N) >= 6, eight of which are large, multicomponent sources, and 123 (79%) of which can be associated with optical sources in the CFHTLS W1 catalogue. Applying Voronoi tessellation analysis (VTA) in the area around the X-ray identified supercluster using photometric redshifts from the CFHTLS survey we identify a total of seventeen overdensities at z(phot) = 0.35-0.50, 7 of which are associated with clusters detected in the XMM-Newton XXL data. We find a mean photometric redshift of 0.43 for our overdensities, consistent with the spectroscopic redshifts of the brightest cluster galaxies of seven X-ray detected clusters. The full VTA-identified structure extends over similar to 0.degrees 6x0.degrees 2on the sky, which corresponds to a physical size of similar to 12x4 Mpc(2) at z = 0.43. No large radio galaxies are present within the overdensities, and we associate eight (S/N > 7) radio sources with potential group/cluster member galaxies. The spatial distribution of the red and blue VTA-identified potential group member galaxies, selected by their observed g -r colours, suggests that the clusters are not virialised yet, but are dynamically young, as expected for hierarchical structure growth in a Lambda CDM universe. Further spectroscopic data are required to analyse the dynamical state of the groups.