The environmental dependence of X-ray AGN activity at $z\sim 0.4$

Abstract

We present an analysis of the X-ray Active Galactic Nucleus (AGN) population in a sample of seven massive galaxy clusters in the redshift range $0.35<z<0.45$. We utilize high-quality Chandra X-ray imaging to robustly identify AGN and precisely determine cluster masses and centroids. Follow-up VIMOS optical spectroscopy allows us to determine which AGN are cluster members. Studying the subset of AGN with 0.5-8 keV luminosities $>6.8\times 10^{42}\mathrm{erg}{\mathrm{s}}^{-1}$, within $r\le 2r_{500}$ (approximately the virial radius), we find that the cluster AGN space density scales with cluster mass as $\sim M^{-2.0_{-0.9}^{+0.8}}$. This result rules out zero mass dependence of the cluster X-ray AGN space density at the 2.5$\ddot{I}ƒ$ level. We compare our cluster X-ray AGN sample to a control field with identical selection and find that the cluster AGN fraction is significantly suppressed relative to the field when considering the brightest galaxies with $V<21.5$. For fainter galaxies, this difference is not present. Comparing the X-ray hardness ratios of cluster member AGN to those in the control field, we find no evidence for enhanced X-ray obscuration of cluster member AGN. Lastly, we see tentative evidence that disturbed cluster environments may contribute to enhanced AGN activity.

13 pages, 11 figures, accepted for publication in MNRAS