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Published August 2016 | Published + Submitted
Journal Article Open

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


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.̊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 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 ΛCDM universe. Further spectroscopic data are required to analyse the dynamical state of the groups.

Additional Information

© 2016 ESO. Received: 13 July 2015. Accepted: 22 April 2016. XXL is an international project based around an XMM Very Large Programme surveying two 25 deg2 extragalactic fields at a depth of ~ 5 × 10 − 15 erg cm-2 s-1 in the [0.5−2] keV band for point-like sources. The XXL website is http://irfu.cea.fr/xxl. Multiband information and spectroscopic follow-up of the X-ray sources are obtained through a number of survey programmes, summarised at http://xxlmultiwave.pbworks.com/. Funding Saclay group: long-term support by the Centre National d'Études Spatiales (CNES). F.P.: support from BMBF DLR grant 50 OR 1117 and support from the DfG Transregio Programme TR33. Zgal group: European Union's Seventh Framework program under grant agreement 333654 (CIG, "AGN feedback"; N.B., V.S.) and grant agreement 337595 (ERC Starting Grant, "CoSMass"; V.S., J.D., O.Mi., M. N.). Italian and French groups: support from the International Programme for Scientific Cooperation CNRS-INAF PICS 2012. O.Me. acknowledges for the financial support provided by the European Union Seventh Framework Programme (FP7 2007-2013), grant agreement No. 291823 Marie Curie FP7-PEOPLE-2011-COFUND (The new International Fellowship Mobility Programme for Experienced Researchers in Croatia – NEWFELPRO, project "AGN environs in XXL", Grant Agreement #83). We thank the referee for a very useful report. Authors of this paper acknowledge Olivier Ilbert's valuable input. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This work is also based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. CFHTLenS data processing was made possible thanks to significant computing support from the NSERC Research Tools and Instruments grant program.

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