Luminous X-ray AGN in Clusters of Galaxies

TL;DR

This study investigates the prevalence of luminous X-ray AGN in galaxy clusters and finds that their occurrence is significantly suppressed compared to optical galaxy overdensities, suggesting environmental effects on AGN triggering.

Contribution

It provides the first detailed comparison of X-ray AGN and optical galaxy overdensities in galaxy clusters, revealing suppression of luminous X-ray AGN in these environments.

Findings

X-ray AGN overdensity is about 4 times lower than optical galaxy overdensity.

Most X-ray sources are background QSOs or AGN, not cluster members.

Luminous X-ray AGN triggering is strongly suppressed in rich clusters.

Abstract

We present a study of X-ray AGN overdensities in 16 Abell clusters, within the redshift range 0.073<z<0.279, in order to investigate the effect of the hot inter-cluster environment on the triggering of the AGN phenomenon. The X-ray AGN overdensities, with respect to the field expectations, were estimated for sources with L_x>= 10^{42} erg s^{-1} (at the redshift of the clusters) and within an area of 1 h^{-1}_{72} Mpc radius (excluding the core). To investigate the presence or not of a true enhancement of luminous X-ray AGN in the cluster area, we also derived the corresponding optical galaxy overdensities, using a suitable range of $r$-band magnitudes. We always find the latter to be significantly higher (and only in two cases roughly equal) with respect to the corresponding X-ray overdensities. Over the whole cluster sample, the mean X-ray point-source overdensity is a factor of ~4 less than that corresponding to bright optical galaxies, a difference which is significant at a >0.995 level, as indicated by an appropriate t-student test. We conclude that the triggering of luminous X-ray AGN in rich clusters is strongly suppressed. Furthermore, searching for optical SDSS counterparts of all the X-ray sources, associated with our clusters, we found that about half appear to be background QSOs, while others are background and foreground AGN or stars. The true overdensity of X-ray point sources, associated to the clusters, is therefore even smaller than what our statistical approach revealed.