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 µm, along with robust optical+IRAC photometric redshifts and SED-fitted stellar masses, we present the dust-obscured starforming fractions, star formation rates and specific star formation rates in these clusters as functions of redshift and projected clustercentric radius. We find that z ∼ 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 environmentdependent 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.

We present composite 3.6 and 4.5µm luminosity functions for cluster galaxies measured from the Spitzer Deep, Wide-Field Survey (SDWFS) for 0.3 < z < 2. We compare the evolution of m * for these luminosity functions to models for passively evolving stellar populations to constrain the primary epoch of star formation in massive cluster galaxies. At low redshifts (z 1.3) our results agree well with models with no mass assembly and passively evolving stellar populations with a luminosity-weighted mean formation redshift z f = 2.4 assuming a Kroupa initial mass function (IMF). We conduct a thorough investigation of systematic biases that might influence our results, and estimate systematic uncertainites of ∆z f = +0.16 −0.18 (model normalization), ∆z f = +0.40 −0.05 (α), and ∆z f = +0.30 −0.45 (choice of stellar population model). For a Salpeter type IMF, the typical formation epoch is thus strongly constrained to be z ∼ 2 − 3. Higher formation redshifts can only be made consistent with the data if one permits an evolving IMF that is bottom-light at high redshift, as suggested by van Dokkum (2008). At high redshift (z 1.3) we also witness a statistically significant (> 5σ) disagreement between the measured luminosity function and the continuation of the passive evolution model from lower redshifts. After considering potential systematic biases that might influence our highest redshift data points, we interpret the observed deviation as potential evidence for ongoing mass assembly at this epoch.

We report the discovery of an IR-selected massive galaxy cluster in the IRAC Distant Cluster Survey (IDCS). We present new data from the Hubble Space Telescope and the W. M. Keck Observatory that spectroscopically confirm IDCS J1426+3508 at z = 1.75. Moreover, the cluster is detected in archival Chandra data as an extended X-ray source, comprising 54 counts after the removal of point sources. We calculate an X-ray luminosity of L 0.5−2keV = (5.5 ± 1.2) × 10 44 ergs s −1 within r = 60 arcsec (∼ 1 Mpc diameter), which implies M 200,Lx = (5.6 ± 1.6) × 10 14 M ⊙ . IDCS J1426+3508 appears to be an exceptionally massive cluster for its redshift.

We present a new catalog of star cluster candidates in the nearby spiral galaxy M33. It is based on eight existing catalogs wherein we have cross-referenced identifications and endeavored to resolve inconsistencies between them. Our catalog contains 451 candidates, of which 255 are confirmed clusters based on Hubble Space Telescope and highresolution ground-based imaging. The catalog contains precise cluster positions (right ascension and declination), magnitudes and colors in the UBVRIJHK S filters, metallicities, radial velocities, masses and ages, where available, and galactocentric distances for each cluster. The color distribution of the M33 clusters appears to be similar to those in the Large Magellanic Cloud, with major peaks at (B À V ) 0 $ 0:15 and (B À V ) 0 $ 0:65. The intrinsic colors are correlated with cluster ages, which range from 10 7:5 to 10 10:3 yr. The age distribution of the star clusters supports the notion of rapid cluster disruption with a slope of ¼ À1:09 AE 0:07 in the dN clus /d / relation. In addition, comparison to theoretical single stellar population models suggests the presence of an age-metallicity relation among these clusters, with younger clusters being more metal-rich. Analysis of the radial distribution of the clusters yields some evidence that younger clusters (age P1 Gyr) may be more concentrated toward the center of M33 than older ones. A similar comparison with the radial profile of the M33 field stars shows the clusters to be more centrally concentrated at the greater than 99.9% confidence level. Possible reasons for this are presented and discussed; however, the overwhelming conclusion seems to be that a more complete and thorough cluster search is needed, covering at least 4 deg 2 centered on M33.

We compare the star formation (SF) activity in cluster galaxies to the field from z = 0.3 − 1.5 using Herschel Spectral and Photometric Imaging REceiver (SPIRE) 250µm imaging. We utilize 274 clusters from the IRAC Shallow Cluster Survey (ISCS) selected as rest-frame near-infrared overdensities over the 9 square degree Boötes field . This analysis allows us to quantify the evolution of SF in clusters over a long redshift baseline without bias against active cluster systems. Using a stacking analysis, we determine the average star formation rates (SFRs) and specific-SFRs (SSFR=SFR/M ) of stellar mass-limited (M 1.3 × 10 10 M ), statistical samples of cluster and field galaxies, probing both the star forming and quiescent populations. We find a clear indication that the average SF in cluster galaxies is evolving more rapidly than in the field, with field SF levels at z ∼ > 1.2 in the cluster cores (r < 0.5 Mpc), in good agreement with previous ISCS studies. By quantifying the SF in cluster and field galaxies as an exponential function of cosmic time, we determine that cluster galaxies are evolving ∼ 2 times faster than the field. Additionally, we see enhanced SF above the field level at z ∼ 1.4 in the cluster outskirts (r > 0.5 Mpc). These general trends in the cluster cores and outskirts are driven by the lower mass galaxies in our sample. Blue cluster galaxies have systematically lower SSFRs than blue field galaxies, but otherwise show no strong differential evolution with respect to the field over our redshift range. This suggests that the cluster environment is both suppressing the star formation in blue galaxies on long time-scales and rapidly transitioning some fraction of blue galaxies to the quiescent galaxy population on short time-scales. We argue that our results are consistent with both strangulation and ram pressure stripping acting in these clusters, with merger activity occurring in the cluster outskirts.