The rapid evolution of AGN feedback in brightest cluster galaxies:
  switching from quasar-mode to radio-mode feedback
  We present an analysis of the 2-10 keV X-ray emission associated with the
active galactic nuclei (AGNs) 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) in which AGN-jetted outflows are
creating and sustaining clear Xray 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 Gyrs. Mindful of potential selection
effects, we propose two possible scenarios to explain our results: 1) either
that the AGNs 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 Gyrs.
