A Census of Mid-Infrared Selected Active Galactic Nuclei in Massive
  Galaxy Clusters at 0 < z < 1.3
  We conduct a deep mid-infrared census of nine massive galaxy clusters at
(0<z<1.3) with a total of ~1500 spectroscopically confirmed member galaxies
using Spitzer/IRAC photometry and established mid-infrared color selection
techniques. Of the 949 cluster galaxies that are detected in at least three of
the four IRAC channels at the >3 sigma level, we identify 12 that host
mid-infrared selected active galactic nuclei (IR-AGN). To compare the IR-AGN
across our redshift range, we define two complete samples of cluster galaxies:
(1) optically-selected members with rest-frame V(AB) magnitude <-21.5 and (2)
mid-IR selected members brighter than (M*+0.5), i.e. essentially a stellar mass
cut. In both samples, we measure \agnfrac ~1% with a strong upper limit of ~3%
at z<1. This uniformly low IR-AGN fraction at z<1 is surprising given the
fraction of 24 micron sources in the same galaxy clusters is observed to
increase by about a factor of four from z~0 to z~1; this indicates that most of
the detected 24 micron flux is due to star formation. Only in our single galaxy
cluster at z=1.24 is the IR-AGN fraction measurably higher at ~15% (all
members; ~70% for late-types only). In agreement with recent studies, we find
the cluster IR-AGN are predominantly hosted by late-type galaxies with blue
optical colors, i.e. members with recent/ongoing star formation. The four
brightest IR-AGN are also X-ray sources; these IR+X-ray AGN all lie outside the
cluster core (Rproj>0.5 Mpc) and are hosted by highly morphologically disturbed
members. Although our sample is limited, our results suggest that \agnfrac in
massive galaxy clusters is not strongly correlated with star formation at z<1,
and that IR-AGN have a more prominent role at z>1.
