THE ERA OF STAR FORMATION IN GALAXY CLUSTERS

Abstract

We analyze the star formation properties of 16 infrared-selected,\nspectroscopically confirmed galaxy clusters at $1 < z < 1.5$ from the\nSpitzer/IRAC Shallow Cluster Survey (ISCS). We present new spectroscopic\nconfirmation for six of these high-redshift clusters, five of which are at\n$z>1.35$. Using infrared luminosities measured with deep Spitzer/MIPS\nobservations at 24 $\\mu$m, along with robust optical+IRAC photometric redshifts\nand SED-fitted stellar masses, we present the dust-obscured star-forming\nfractions, star formation rates and specific star formation rates in these\nclusters as functions of redshift and projected clustercentric radius. We find\nthat $z\\sim 1.4$ represents a transition redshift for the ISCS sample, with\nclear evidence of an unquenched era of cluster star formation at earlier times.\nBeyond this redshift the fraction of star-forming cluster members increases\nmonotonically toward the cluster centers. Indeed, the specific star formation\nrate in the cores of these distant clusters is consistent with field values at\nsimilar redshifts, indicating that at $z>1.4$ environment-dependent quenching\nhad not yet been established in ISCS clusters. Combining these observations\nwith complementary studies showing a rapid increase in the AGN fraction, a\nstochastic star formation history, and a major merging episode at the same\nepoch in this cluster sample, we suggest that the starburst activity is likely\nmerger-driven and that the subsequent quenching is due to feedback from\nmerger-fueled AGN. The totality of the evidence suggests we are witnessing the\nfinal quenching period that brings an end to the era of star formation in\ngalaxy clusters and initiates the era of passive evolution.\n