The XXL Survey^⋆,⋆⋆

XII. Optical spectroscopy of X-ray-selected clusters and the frequency of AGN in superclusters

¹ Service d’Astrophysique AIM, DSM/IRFU/SAp, CEA Saclay, 91191 Gif-sur-Yvette, France
e-mail: elias.koulouridis@cea.fr
² Institute for Astronomy & Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
³ INAF–Astronomical Observatory of Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
⁴ European Space Astronomy Centre (ESA/ESAC), Operations Department, Villanueva de la Canãda, Madrid, Spain
⁵ Department of Astronomy, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁶ LAM, OAMP, Université Aix-Marseille, CNRS, Pôle de l’Étoile, Site de Chãteau Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille 13 Cedex, France
⁷ Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁸ Main Astronomical Observatory, Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St., 03680 Kyiv, Ukraine
⁹ Department of Physics, University of Zagreb, Bijenicka cesta 32, 10000 Zagreb, Croatia
¹⁰ Astronomical Observatory, Taras Shevchenko National University of Kyiv, 3 Observatorna St., 04053 Kyiv, Ukraine
¹¹ Department of Astronomy, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
¹² INAF–IASF–Milano, via Bassini 15, 20133 Milano, Italy
¹³ Dept. of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁴ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁵ Physics Department of Aristotle University of Thessaloniki, University Campus, 54124 Thessaloniki, Greece
¹⁶ Instituto Nacional de Astrofísica Optica y Electrónica, CP 72840 Puebla, México
¹⁷ Institut d’Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium

Received: 16 July 2015
Accepted: 26 November 2015

Abstract

Context. This article belongs to the first series of XXL publications. It presents multifibre spectroscopic observations of three 0.55 deg² fields in the XXL Survey, which were selected on the basis of their high density of X-ray-detected clusters. The observations were obtained with the AutoFib2+WYFFOS (AF2) wide-field fibre spectrograph mounted on the 4.2 m William Herschel Telescope.

Aims. The paper first describes the scientific rationale, the preparation, the data reduction, and the results of the observations, and then presents a study of active galactic nuclei (AGN) within three superclusters.

Methods. To determine the redshift of galaxy clusters and AGN, we assign high priority to a) the brightest cluster galaxies (BCGs), b) the most probable cluster galaxy candidates, and c) the optical counterparts of X-ray point-like sources. We use the outcome of the observations to study the projected (2D) and the spatial (3D) overdensity of AGN in three superclusters.

Results. We obtained redshifts for 455 galaxies in total, 56 of which are counterparts of X-ray point-like sources. We were able to determine the redshift of the merging supercluster XLSSC-e, which consists of six individual clusters at z ~ 0.43, and we confirmed the redshift of supercluster XLSSC-d at z ~ 0.3. More importantly, we discovered a new supercluster, XLSSC-f, that comprises three galaxy clusters also at z ~ 0.3. We find a significant 2D overdensity of X-ray point-like sources only around the supercluster XLSSC-f. This result is also supported by the spatial (3D) analysis of XLSSC-f, where we find four AGN with compatible spectroscopic redshifts and possibly one more with compatible photometric redshift. In addition, we find two AGN (3D analysis) at the redshift of XLSSC-e, but no AGN in XLSSC-d. Comparing these findings with the optical galaxy overdensity we conclude that the total number of AGN in the area of the three superclusters significantly exceeds the field expectations. All of the AGN found have luminosities below 7 × 10⁴² erg s^-1.

Conclusions. The difference in the AGN frequency between the three superclusters cannot be explained by the present study because of small number statistics. Further analysis of a larger number of superclusters within the 50 deg² of the XXL is needed before any conclusions on the effect of the supercluster environment on AGN can be reached.