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

E. Drigga, E. Koulouridis, E. Pouliasis, Y. Toba, M. Akiyama, C. Vignali, A. Ruiz, I. Georgantopoulos, T. Nagao, S. Paltani, M. Plionis, M. Pierre, B. Vijarnwannaluk

First listed 2025-04-04 | Last updated 2025-05-27

Abstract

There is compelling evidence that AGNs are strongly influenced by their environment. 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. 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. For our purposes, we used 82 X-ray detected AGN found within a $4r_{500}$ radius of 164 X-ray detected and spectroscopically confirmed galaxy clusters of the XXL survey, up to a redshift of $z\sim$1. This field is covered by deep optical observations of the Hyper Suprime-Cam, which allows for a reliable morphological classification of galaxies. Furthermore, using the X-ray hardness ratio, the optical spectra and the SEDs of the X-ray sources, we have studied the obscuration and other AGN properties and the SFR of the hosts as further indicators of interactions. We found a moderately significant 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. Also, we discovered a higher number of X-ray-hard AGN (hence, possibly obscured) in clusters than in the field 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. 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.

Short digest

Using 82 X-ray AGN within 4 r500 of 164 XXL clusters (z ≲ 1) and deep HSC imaging, the authors classify host morphologies (Statmorph + visual) and assess obscuration and SFR via X-ray hardness ratios, optical spectra, and SEDs. They find a moderately significant excess of cluster AGN hosted by merging/disturbed galaxies relative to non-active cluster members and field AGN, with the signal concentrated in the outskirts (≈1–2 r500). Clusters also show a higher share of X-ray-hard (likely obscured) AGN than the field, especially in the outskirts. Together, the results point to mergers/interactions in cluster outskirts as key drivers of both AGN triggering and obscuration.

Key figures to inspect

• Radial distribution of AGN incidence vs r/r500, split by host morphology (disturbed/merger vs undisturbed): verify the peak between 1–2 r500 and quantify the excess in cluster outskirts.
• Hardness-ratio comparisons (cluster vs field and as a function of radius): check where the X-ray-hard/obscured fraction rises and how strongly it differs from the field.
• HSC cutouts with Statmorph outputs (e.g., segmentation maps, Gini–M20/Asymmetry): assess the visual and quantitative disturbance signatures used to flag mergers.
• Host SFR (from SEDs) versus morphology or cluster-centric radius: look for interaction-linked SFR enhancements that accompany the disturbed AGN hosts.