The rapid evolution of AGN feedback in brightest cluster galaxies: switching from quasar-mode to radio-mode feedback. (arXiv:1211.5606v1 [astro-ph.CO])

The rapid evolution of AGN feedback in brightest cluster galaxies: switching from quasar-mode to radio-mode feedback. (arXiv:1211.5606v1 [astro-ph.CO]):
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 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 Gyrs. Mindful of the potential selection effects that
may affect our results, 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.