The XXL Survey

VII. A supercluster of galaxies at z = 0.43^⋆,⋆⋆

¹ ESO-Chile, Alonso de Cordova 3107, Vitacura, Chile
e-mail: epompei@eso.org
² Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
³ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁴ INAF–IASF-Milano, via Bassini 15, 20133 Milano, Italy Department of Physics and Astronomy, University of California at Irvine, USA
⁵ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, Canada
⁶ Osservatorio astronomico di Padova, INAF, 35141 Padova, Italy
⁷ European Space Astronomy Centre, European Space Agency, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁸ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia Antipolis, CNRS, Observatoire de la Cote d’Azur, 06304 Nice, France
⁹ Department of Physics, University of Zagreb, Bijenička cesta 32, 45 10000 Zagreb, Croatia
¹⁰ Department of Astronomy, Universidad de Concepción, Casilla 160-C, Concepción, Chile
¹¹ IAASARS, National Observatory of Athens, 15236 Penteli, Greece
¹² Argelander Institut für Astronomie, Universitat Bonn, 53121 Bonn, Germany
¹³ Service d’Astrophysique AIM, CEA/DSM/IRFU/SAp, CEA Saclay, 91191 Gif-sur-Yvette, France
¹⁴ Department of Astronomy and Space Sciences, Faculty of Science, Istanbul University, 34119 Istanbul, Turkey

Received: 7 August 2015
Accepted: 27 October 2015

Abstract

Context. The XXL Survey is the largest homogeneous and contiguous survey carried out with XMM-Newton. Covering an area of 50 deg² distributed over two fields, it primarily investigates the large-scale structures of the Universe using the distribution of galaxy clusters and active galactic nuclei as tracers of the matter distribution.

Aims. Given its depth and sky coverage, XXL is particularly suited to systematically unveiling the clustering of X-ray clusters and to identifying superstructures in a homogeneous X-ray sample down to the typical mass scale of a local massive cluster.

Methods. A friends-of-friends algorithm in three-dimensional physical space was run to identify large-scale structures. In this paper we report the discovery of the highest redshift supercluster of galaxies found in the XXL Survey. We describe the X-ray properties of the clusters members of the structure and the optical follow-up.

Results. The newly discovered supercluster is composed of six clusters of galaxies at a median redshift z ~ 0.43 and distributed across ~30′× 15′ (10 × 5 Mpc) on the sky. This structure is very compact with all the clusters residing in one XMM pointing; for this reason this is the first supercluster discovered with the XXL Survey. Photometric redshifts from the CFHTLS (Canada-France-Hawaii Telescope Legacy Survey) data release T0007 placed the supercluster at an approximate redshift of z_phot ~ 0.45; subsequent spectroscopic follow-up with WHT (William Herschel Telescope) and NTT (New Technology Telescope) confirmed a median redshift of z ~ 0.43. An estimate of the X-ray mass and luminosity of this supercluster returns values of 1.7 × 10¹⁵ M_⊙ and of 1.68 × 10⁴⁴ erg s^-1, respectively, and a total gas mass of M_gas = 9.3 × 10¹³ M_⊙. These values put XLSSC-e at the average mass range of superclusters; its appearance, with two members of equal size, is quite unusual with respect to other superclusters and provides a unique view of the formation process of a massive structure.