Galaxy Clusters around radio-loud AGN at 1.3 < z < 3.2 as seen by
  Spitzer
  We report the first results from the Clusters Around Radio-Loud AGN (CARLA)
program, a Cycle 7 and 8 Spitzer Space Telescope snapshot program to
investigate the environments of a large sample of obscured and unobscured
luminous radio-loud AGN at 1.2 < z < 3.2. These data, obtained for 387 fields,
reach 3.6 and 4.5 um depths of [3.6] (AB) = 22.6 and [4.5] (AB) = 22.9 at the
95% completeness level, which is two to three times fainter than L* in this
redshift range. By using the color cut [3.6]-[4.5] > -0.1 (AB), which
efficiently selects high-redshift (z > 1.3) galaxies of all types, we identify
galaxy cluster member candidates in the fields of the radio-loud AGN. The local
density of these IRAC-selected sources is compared to the density of similarly
selected sources in blank fields. We find that 92% of the radio-loud AGN reside
in environments richer than average. The majority (55%) of the radio-loud AGN
fields are found to be overdense at a > 2 {\sigma} level; 10% are overdense at
a > 5 {\sigma} level. A clear rise in surface density of IRAC-selected sources
towards the position of the radio-loud AGN strongly supports an association of
the majority of the IRAC-selected sources with the radio-loud AGN. Our results
provide solid statistical evidence that radio-loud AGN are likely beacons for
finding high-redshift galaxy (proto-)clusters. We investigate how environment
depends on AGN type (unobscured radio-loud quasars vs. obscured radio
galaxies), radio luminosity and redshift, finding no correlation with either
AGN type or radio luminosity. We find a decrease in density with redshift,
consistent with galaxy evolution for this uniform, flux-limited survey. These
results are consistent with expectations from the orientation-driven AGN
unification model, at least for the high radio luminosity regimes considered in
this sample.
