The XXL Survey

XXI. The environment and clustering of X-ray AGN in the XXL-South field

¹ Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 1000 Zagreb, Croatia
e-mail: melnykol@gmail.com
² Main Astronomical Observatory, Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St., 03680 Kyiv, Ukraine
³ Physics Dept., Aristotle Univ. of Thessaloniki, Thessaloniki 54124, Greece
⁴ Instituto Nacional de Astrófisica, Óptica y Electrónica, 72000 Puebla, Mexico
⁵ IAASARS, National Observatory of Athens, 15236 Penteli, Greece
⁶ DSM/Irfu/SAp, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
⁷ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁸ INAF-IASF Milano, via Bassini 15, 20133 Milano, Italy
⁹ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹⁰ International Centre for Radio Astronomy Research, M468, University of Western Australia, Crawley, WA 6009, Australia
¹¹ Subaru Telescope, National Astronomical Observatory of Japan, 650 North Aohoku Place, Hilo, HI 96720, USA
¹² Australian Astronomical Observatory, North Ryde, NSW 2113, Australia
¹³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, 19001 Casilla, Santiago 19, Chile
¹⁴ Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹⁵ INAF – Osservatorio Astronomico di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁶ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19c, 4000 Liège, Belgium

Received: 21 January 2017
Accepted: 22 November 2017

Abstract

Context. This work is part of a series of studies focusing on the environment and the properties of the X-ray selected active galactic nuclei (AGN) population from the XXL survey. The present survey, given its large area, continuity, extensive multiwavelength coverage, and large-scale structure information, is ideal for this kind of study. Here, we focus on the XXL-South (XXL-S) field.

Aims. Our main aim is to study the environment of the various types of X-ray selected AGN and investigate its possible role in AGN triggering and evolution.

Methods. We studied the large-scale (>1 Mpc) environment up to redshift z = 1 using the nearest neighbour distance method to compare various pairs of AGN types. We also investigated the small-scale environment (<0.4 Mpc) by calculating the local overdensities of optical galaxies. In addition, we built a catalogue of AGN concentrations with two or more members using the hierarchical clustering method and we correlated them with the X-ray galaxy clusters detected in the XXL survey.

Results. It is found that radio detected X-ray sources are more obscured than non-radio ones, though not all radio sources are obscured AGN. We did not find any significant differences in the large-scale clustering between luminous and faint X-ray AGN, or between obscured and unobscured ones, or between radio and non-radio sources. At local scales (<0.4 Mpc), AGN typically reside in overdense regions, compared to non-AGN; however, no differences were found between the various types of AGN. A majority of AGN concentrations with two or more members are found in the neighbourhood of X-ray galaxy clusters within <25–45 Mpc. Our results suggest that X-ray AGN are typically located in supercluster filaments, but they are also found in over- and underdense regions.