The Era of Star Formation in Galaxy Clusters
  We analyze the star formation properties of 16 infrared-selected,
spectroscopically confirmed galaxy clusters at $1 < z < 1.5$ from the
Spitzer/IRAC Shallow Cluster Survey (ISCS). We present new spectroscopic
confirmation for six of these high-redshift clusters, five of which are at
$z>1.35$. Using infrared luminosities measured with deep Spitzer/MIPS
observations at 24 $\mu$m, along with robust optical+IRAC photometric redshifts
and SED-fitted stellar masses, we present the dust-obscured star-forming
fractions, star formation rates and specific star formation rates in these
clusters as functions of redshift and projected clustercentric radius. We find
that $z\sim 1.4$ represents a transition redshift for the ISCS sample, with
clear evidence of an unquenched era of cluster star formation at earlier times.
Beyond this redshift the fraction of star-forming cluster members increases
monotonically toward the cluster centers. Indeed, the specific star formation
rate in the cores of these distant clusters is consistent with field values at
similar redshifts, indicating that at $z>1.4$ environment-dependent quenching
had not yet been established in ISCS clusters. Combining these observations
with complementary studies showing a rapid increase in the AGN fraction, a
stochastic star formation history, and a major merging episode at the same
epoch in this cluster sample, we suggest that the starburst activity is likely
merger-driven and that the subsequent quenching is due to feedback from
merger-fueled AGN. The totality of the evidence suggests we are witnessing the
final quenching period that brings an end to the era of star formation in
galaxy clusters and initiates the era of passive evolution.
