LoCuSS: A Dynamical Analysis of X-ray AGN in Local Clusters
  We present a study of the distribution of X-ray AGN in a representative
sample of 26 massive clusters at 0.15<z<0.30, combining Chandra observations
with highly complete spectroscopy of cluster members down to M_K*+2. In total
we identify 48 X-ray AGN among the cluster members, with luminosities
2x10^41-1x10^44erg/s. In the stacked caustic diagram, the X-ray AGN appear to
preferentially lie along the caustics, suggestive of an infalling population.
They also appear to avoid the region with lowest cluster-centric radii and
relative velocities (r_proj<0.4 r_500; |v-<v>|/sigma_v<0.8), which is dominated
by the virialized population of galaxies accreted earliest into the clusters.
Moreover the velocity dispersion of the 48 X-ray AGN is 1.51x that of the
overall cluster population, which is consistent with the sqrt(2) ratio expected
by simple energetic arguments when comparing infalling versus virialized
populations. This kinematic segregation is significant at the 4.66-sigma level.
When splitting the X-ray AGN sample into two according to X-ray or infrared
(IR) luminosity, both X-ray bright and IR-bright sub-samples show higher
velocity dispersions than their X-ray dim and IR-dim counterparts at >2sigma
significance. This is consistent with the nuclear activity responsible for the
X-ray and IR emission being slowly shut down as the host galaxies are accreted
into the cluster. Overall our results provide the strongest observational
evidence to date that X-ray AGN found in massive clusters are an infalling
population, and that the cluster environment very effectively suppresses
radiatively-efficient nuclear activity in its member galaxies. These results
are consistent with the view that for galaxies to host an X-ray AGN they should
be the central galaxy within their dark matter halo and have a ready supply of
cold gas.
