X-ray AGN in the XMM-LSS galaxy clusters: no evidence of AGN suppression

Abstract

We present a study of the overdensity of X-ray-selected active galactic nuclei (AGN) in 33 galaxy clusters in the XMM-LSS field (The XMM-Newton Large Scale Structure Survey), up to redshift z = 1.05 and further divided into a lower (0.14 ≤ z ≤ 0.35) and a higher redshift (0.43 ≤ z ≤ 1.05) subsample. Previous studies have shown that the presence of X-ray-selected AGN in rich galaxy clusters is suppressed, since their number is significantly lower than what is expected from the high galaxy overdensities in the area. In the current study we have investigated the occurrence of X-ray-selected AGN in low ( L x , bol = 2.7 × 10 43 erg/s) and moderate ( L x , bol = 2.4 × 10 44 erg/s) X-ray luminosity galaxy clusters in an attempt to trace back the relation between high-density environments and nuclear activity. Owing to the wide contiguous XMM-LSS survey area, we were able to extend the study to the cluster outskirts. We therefore determined the projected overdensity of X-ray point-like sources around each cluster out to 6r 500 radius, within δr 500 = 1 annulus, with respect to the field expectations based on the X-ray source log N − log S of the XMM-LSS field. To provide robust statistical results we also conducted a consistent stacking analysis separately for the two z ranges. We investigated whether the observed X-ray overdensities are to be expected thanks to the obvious enhancement of galaxy numbers in the cluster environment by also estimating the corresponding optical galaxy overdensities, and we assessed the possible enhancement or suppression of AGN activity in clusters. We find a positive X-ray projected overdensity in both redshift ranges at the first radial bins, which however has the same amplitude as that of optical galaxies. Therefore, no suppression (or enhancement) of X-ray AGN activity with respect to the field is found, in sharp contrast to previous results based on rich galaxy clusters, implying that the mechanisms responsible for the suppression are not as effective in lower density environments. After a drop to roughly the background level between 2 and 3r 500 , the X-ray overdensity exhibits a rise at larger radii, significantly greater than the corresponding optical overdensity. The radial distance of this overdensity "bump", corresponding to ∼1.5−3 Mpc, depends on the richness of the clusters, as well as on the overall X-ray overdensity profile. Finally, using the redshift information, photometric or spectroscopic, of the optical counterparts, we derive the spatial overdensity profile of the clusters. We find that the agreement between X-ray and optical overdensities in the first radial bins is also suggested in the 3-dimensional analysis. However, we argue that the X-ray overdensity "bump" at larger radial distance is at least partially a result of flux boosting by gravitational lensing of background quasi-stellar objects, confirming previous results. For high-redshift clusters, the enhancement of X-ray AGN activity in their outskirts appears to be intrinsic. We argue that a spatial ana...

The XXL Survey

“…We chose to analyse the 2D case as well since we lacked complete spectroscopy for all the X-ray point sources. However, we had to take into account that the 2D case is hampered by a variety of systematic effects, related for example to flux-boosting due to lensing (see discussion in Koulouridis et al 2014). For the statistical evaluation of our results we used the confidence limits for small numbers of events in astrophysical data, based on Poisson statistics (Gehrels 1986).…”

“…), one would expect the AGN presence to rise in shallower gravitational potentials (see Arnold et al 2009;Gavazzi et al 2011;Bitsakis et al 2015) and be completely nullified within the deepest ones. In Koulouridis et al (2014), we investigated the AGN presence in two samples of poor and moderate clusters and found evidence of this anti-correlation. Interestingly, in merging or actively growing clusters the high incidence of galaxy mergers can potentially enhance the number of AGN, while at the same time, shock waves may also enhance the ram pressure stripping intensity (Vijayaraghavan & Ricker 2013;Jaffé et al, in prep.…”

“…Clusters can be very extended, of the order of a few Mpc, and AGN may preferentially reside even further out in their outskirts (e.g. Fassbender et al 2012;Haines et al 2012;Koulouridis et al 2014). More than half of the detected extended sources are 1−3 keV clusters in the 0.2 < z < 0.5 range (Fig.…”

X-ray-emitting active galactic nuclei fromz= 0.6 to 1.3 in the intermediate- and high-density environments of the ORELSE survey

“…While major galaxy mergers are thought to fuel some of the brightest AGNs, they are not common, and are likely not responsible for all AGNs (See, e.g., Wild et al 2007;Reichard et al 2009;Kormendy & Ho 2013;Heckman & Best 2014). Some evidence has been found of an association between merging galaxies or those with signs of recent mergers or interactions (Kocevski et al 2009b;Koss et al 2010;Treister et al 2012;Ellison et al 2013;Hernández-Ibarra et al 2016), but it is likely this is representative of only a subset of the AGN population, since large numbers of AGNs have been found associated with hosts not likely to have recently undergone major mergers (Georgakakis et al 2009;Cisternas et al 2011;Schawinski et al 2011;Kocevski et al 2012;Rosario et al 2012;Koulouridis et al 2014). While major mergers are thought to be important at the high-luminosity end, there is a dearth of evidence supporting alternative mechanisms for triggering lower luminosity AGNs, and a multitude of options.…”

The XXL Survey

“…For example, the Hard Bright Serendipitous Survey (HBSS) covers 25 deg 2 reaching a flux limit of F 4.5−10 keV = 7 × 10 −14 erg s −1 cm 2 (Della Ceca et al 2004). The XMM-LSS survey ), a survey with XMMNewton covering contiguously 11 deg 2 , is a good compromise between sky area and depth which has provided several results on the environmental properties of AGN (Elyiv et al 2012;Melnyk et al 2013;Koulouridis et al 2014, showing in particular different behaviors between objects having soft and hard X-ray spectra.…”