The XXL Survey

IX. Optical overdensity and radio continuum analysis of a supercluster at z = 0.43^⋆,⋆⋆

¹ University of Zagreb, Physics Department, Bijenička cesta 32, 10000 Zagreb, Croatia
e-mail: nbaran@phy.hr
² INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
³ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ California Institute of Technology, Department of Astronomy, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁵ National Radio Astronomy Observatory, PO Box O, Socorro, New Mexico, NM 87801, USA
⁶ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸ Service d’Astrophysique AIM, DSM/IRFU/SAp, CEA-Saclay, 91191 Gif-sur-Yvette, France
⁹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, 19001 Casilla, Santiago 19, Chile
¹⁰ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Astronomical Observatory, Taras Shevchenko National University of Kyiv, 3 Observatorna St., 04053 Kyiv, Ukraine

Received: 13 July 2015
Accepted: 22 April 2016

Abstract

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 deg² 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.̊35 × 0.̊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 × 1.̋9, and encompasses an area of 41′ × 41′ with rms noise level lower than ~ 20 μJy beam^-1. The noise in the central 15′ × 15′ region is ≈ 11 μJy 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 ~ 0.̊6 × 0.̊2 on the sky, which corresponds to a physical size of ~ 12 × 4 Mpc² 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 ΛCDM universe. Further spectroscopic data are required to analyse the dynamical state of the groups.