XXL-HSC: Host properties of X-ray detected AGNs in XXL clusters

¹ Institute for Astronomy & Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, GR-15236 Palaia Penteli, Greece
² Sector of Astrophysics, Astronomy & Mechanics, Department of Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Academia Sinica Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU, No.1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
⁶ Research Centre for Space and Cosmic Evolution, Ehime University, 2–5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
⁷ Astronomical Institute, Tohoku University, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁸ Università di Bologna, Dip. di Fisica e Astronomia “A. Righi”, Via P. Gobetti 93/2, 40129 Bologna, Italy
⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁰ Department of Astronomy, University of Geneva, ch. d’Écogia 16, CH-1290 Versoix, Switzerland
¹¹ National Observatory of Athens, 18100 Thessio, Athens, Greece
¹² CERIDES, Centre of Excellence in Risk & Decision Sciences, European University of Cyprus, Egkomi 2404, Cyprus

^⋆ Corresponding author; edrigga@noa.gr

Received: 12 November 2024
Accepted: 23 March 2025

Abstract

Context. There is compelling evidence that AGNs are strongly influenced by their environment, from their host galaxies to immense structures such as galaxy clusters. Therefore, studying the AGN population of clusters is essential, as both large-scale structures and AGN play key roles in galaxy evolution, though the interactions between these elements are still not well understood.

Aims. The primary objective of this study is to unravel the different factors that may significantly affect the triggering of AGN activity in cluster galaxies, including galaxy merging and interactions with other galaxies, and ram pressure from the hot intracluster medium.

Methods. For our purposes, we used 82 X-ray detected AGN found within a 4r₅₀₀ radius of 164 X-ray detected and spectroscopically confirmed galaxy clusters in the northern 25 deg² field of the XXL survey, up to a redshift of z ∼ 1. This field is also covered by deep optical observations of the Hyper Suprime-Cam, mounted on the 8 m Subaru Telescope, which allows for a reliable morphological classification of galaxies. We thoroughly investigated the morphology of X-ray AGN host galaxies, using both Statmorph software and visual inspection, in an attempt to discover disturbances as indications of interactions that could lead to AGN triggering. Furthermore, using the X-ray hardness ratio, the optical spectra and the spectral energy distributions of the X-ray sources, we have studied the obscuration and other AGN properties, as well as the star formation rate of the hosts as further indicators of interactions.

Results. We found a moderately significant, at the 2σ confidence level, higher fraction of X-ray AGN in galaxy clusters hosted by merging or disturbed galaxies, compared to non-active cluster galaxies or X-ray AGN in the field. This excess is primarily localised in the cluster outskirts (between 1 and 2r₅₀₀). Also, we discovered a higher number of X-ray-hard (hence, possibly obscured) AGN in clusters than in the field, at the 2σ confidence level, particularly in the outskirts. These findings further support the idea that galaxy mergers and interactions may serve as mechanisms for the triggering and obscuration of AGN activity.

Conclusions. The relatively high number of disturbed, merging, and possibly obscured AGN hosts in cluster outskirts suggests that galaxy merging and interactions are key drivers in triggering AGN activity in these outer regions of clusters.