2013MNRAS.431.1638H

We present an analysis of the 2-10keV X-ray emission associated with the active galactic nuclei (AGN) in brightest cluster galaxies (BCGs). Our sample consists of 32 BCGs that lie in highly X-ray luminous cluster of galaxies (L_X-ray(0.1-2.4 keV)>3×10^44 erg s^-1 ) in which AGN-jetted outflows are creating and sustaining clear X-ray cavities. Our sample covers the redshift range 0 < z < 0.6 and reveals strong evolution in the nuclear X-ray luminosities, such that the black holes in these systems have become on average at least 10 times fainter over the last 5 Gyr. Mindful of potential selection effects, we propose two possible scenarios to explain our results: (1) either that the AGN in BCGs with X-ray cavities are steadily becoming fainter, or more likely, (2) that the fraction of these BCGs with radiatively efficient nuclei is decreasing with time from roughly 60 per cent at z ~ 0.6 to 30 per cent at z ~ 0.1. Based on this strong evolution, we predict that a significant fraction of BCGs in z ~ 1 clusters may host quasars at their centres, potentially complicating the search for such clusters at high redshift. In analogy with black hole binaries and based on the observed Eddington ratios of our sources, we further propose that the evolving AGN population in BCGs with X-ray cavities may be transiting from a canonical low/hard state, analogous to that of X-ray binaries, to a quiescent state over the last 5 Gyr.