The XXL Survey

Abstract

Context. We present the results of a study of the active galactic nucleus (AGN) density in a homogeneous and well-studied sample of 167 bona fide X-ray galaxy clusters (0.1 < z < 0.5) from the XXL Survey, from the cluster core to the outskirts (up to 6r500). The results can provide evidence of the physical mechanisms that drive AGN and galaxy evolution within clusters, testing the efficiency of ram pressure gas stripping and galaxy merging in dense environments. Aims. The XXL cluster sample mostly comprises poor and moderately rich structures (M = 1013–4 × 1014 M⊙), a poorly studied population that bridges the gap between optically selected groups and massive X-ray selected clusters. Our aim is to statistically study the demographics of cluster AGNs as a function of cluster mass and host galaxy position. Methods. To investigate the effect of the environment on AGN activity, we computed the fraction of spectroscopically confirmed X-ray AGNs (LX [0.5-10 keV] > 1042 erg cm−1) in bright cluster galaxies with Mi* − 2 < M < Mi* + 1, up to 6r500 radius. The corresponding field fraction was computed from 200 mock cluster catalogues with reshuffled positions within the XXL fields. To study the mass dependence and the evolution of the AGN population, we further divided the sample into low- and high-mass clusters (below and above 1014M⊙, respectively) and two redshift bins (0.1–0.28 and 0.28–0.5). Results. We detect a significant excess of X-ray AGNs, at the 95% confidence level, in low-mass clusters between 0.5r500 and 2r500, which drops to the field value within the cluster cores (r < 0.5r500). In contrast, high-mass clusters present a decreasing AGN fraction towards the cluster centres, in agreement with previous studies. The high AGN fraction in the outskirts is caused by low-luminosity AGNs, up to LX [0.5-10 keV] = 1043 erg cm−1. It can be explained by a higher galaxy merging rate in low-mass clusters, where velocity dispersions are not high enough to prevent galaxy interactions and merging. Ram pressure stripping is possible in the cores of all our clusters, but probably stronger in deeper gravitational potentials. Compared with previous studies of massive or high-redshift clusters, we conclude that the AGN fraction in cluster galaxies anti-correlates strongly with cluster mass. The AGN fraction also increases with redshift, but at the same rate with the respective fraction in field galaxies.

Lower AGN Abundance in Galaxy Clusters at z < 0.5

“…In the left panel of Figure 3, we see that AGN fraction is lower in cluster galaxies than that of field galaxies for each redshift bin. This result agrees with other studies in the literature that look at the environmental dependence of AGNs in clusters and fields (e.g., Lopes et al 2017;Koulouridis et al 2018). We infer that at low redshifts, z ≤ 0.5, relatively more AGNs are found in low density field environment surrounding galaxy clusters compared to the regions closer to the core of the clusters.…”

“…Outer regions of clusters and low density fields in the immediate vicinity of the clusters are seen to have higher fraction of AGNs at low redshifts, attributed to more efficient mergers while AGN activity is strongly suppressed in the inner regions of rich clusters (e.g. Koulouridis, & Plionis 2010;Koulouridis et al 2018). We find a significant difference between cluster and field AGN fractions for both the cases, when we divide our clusters based on mass and when we do not, which is clearly indicative of an environmental dependence for these intermediate-mass clusters.…”

Large-scale environment of FR 0 radio galaxies

“…Using optical observations, the local density of galaxies is not the only parameter which defines the environmental properties as it is also important to establish in which location within a, e.g., cluster of galaxies a given source is located. The location of an AGN within the group/cluster of galaxies might have a profound effect on the level of accretion and, consequently, on its nuclear power (see, e.g., Koulouridis et al 2018). For example, by exploring the properties of early-type galaxies in the Virgo cluster, Vattakunnel et al (2010) found a suggestive trend be-tween jet power and location within the cluster.…”

High density of active galactic nuclei in the outskirts of distant galaxy clusters

“…Previous studies have shown that AGN in clusters of galaxies are strongly affected by their environment, but in a complicated way. Their ability to accrete was found to depend on both their distance from the cluster centre and the mass of the cluster (Koulouridis et al 2018;Ehlert et al 2015). In more detail, the hot intra-cluster medium (ICM) is probably able to strip or evaporate the cold gas reservoir of galaxies (e.g.…”