We report the results of a deep SCUBA-2 850- and 450-$mu$m survey for dust-obscured ultra-luminous infrared galaxies (U/LIRGs) in the field of the z=1.46 cluster XCS J2215.9-1738. We detect a striking overdensity of sub-millimeter sources coincident
with the core of this cluster: $sim 3-4 times$ higher than expected in a blank field. We use the likely radio and mid-infrared counterparts to show that the bulk of these sub-millimeter sources have spectroscopic or photometric redshifts which place them in the cluster and that their multi-wavelength properties are consistent with this association. The average far-infrared luminosities of these galaxies are $(1.0pm0.1) times 10^{12} L_{odot}$, placing them on the U/LIRG boundary. Using the total star formation occurring in the obscured U/LIRG population within the cluster we show that the resulting mass-normalized star-formation rate for this system supports previous claims of a rapid increase in star-formation activity in cluster cores out to $zsim1.5$, which must be associated with the on-going formation of the early-type galaxies which reside in massive clusters today.
We present an analysis of the structures and dynamics of the merging cluster Abell~1201, which has two sloshing cold fronts around a cooling core, and an offset gas core approximately 500kpc northwest of the center. New Chandra and XMM-Newton data re
veal a region of enhanced brightness east of the offset core, with breaks in surface brightness along its boundary to the north and east. This is interpreted as a tail of gas stripped from the offset core. Gas in the offset core and the tail is distinguished from other gas at the same distance from the cluster center chiefly by having higher density, hence lower entropy. In addition, the offset core shows marginally lower temperature and metallicity than the surrounding area. The metallicity in the cool core is high and there is an abrupt drop in metallicity across the southern cold front. We interpret the observed properties of the system, including the placement of the cold fronts, the offset core and its tail in terms of a simple merger scenario. The offset core is the remnant of a merging subcluster, which first passed pericenter southeast of the center of the primary cluster and is now close to its second pericenter passage, moving at ~1000 km/s. Sloshing excited by the merger gave rise to the two cold fronts and the disposition of the cold fronts reveals that we view the merger from close to the plane of the orbit of the offset core.
Using CHANDRA, we investigate the spatial temperature distribution of the intracluster medium (ICM) within 700 kpc of the center of the massive merging cluster MACSJ0717.5+3745 at z=0.55. Combining the X-ray evidence with information about the distri
bution and velocities of the cluster galaxies near the core provides us with a snapshot of the three-dimensional geometry and dynamics of one of the most complex cluster studied to date. We find MACSJ0717.5+3745 to be an active triple merger with ICM temperatures exceeding 20 keV. Although radial velocity information and X-ray/optical offsets indicate that all three mergers proceed along distinctly different directions, the partial alignment of the merger axes points to a common origin in the large-scale filament south-east of the cluster core. Clear decrements in the ICM temperature observed near two of these subclusters identify the respective X-ray surface brightness peaks as remnants of cool cores; the compactness and low temperature of 5.7 keV of one of these features suggest that the respective merger, a high-velocity collision at 3,000 km/s, is still in its very early stages. Looking beyond the triple merger, we find the large-scale filament to not only provide a spatial as well as temporal arrow for the interpretation of the dynamics of the merger events near the cluster core; we also find tantalizing, if circumstantial, evidence for direct, large-scale heating of the ICM by contiguous infall of low-density gas from the filament.
We present the results of a wide-field spectroscopic analysis of the galaxy population of the massive cluster MACSJ0717.5+3745 and the surrounding filamentary structure (z=0.55), as part of our systematic study of the 12 most distant clusters in the
MACS sample. Of 1368 galaxies spectroscopically observed in this field, 563 are identified as cluster members; of those, 203 are classified as emission-line galaxies, 260 as absorption-line galaxies, and 17 as E+A galaxies (defined by $frac{H_{delta}+H_{gamma}}{2}>6$AA and no detection of [OII] and $H_{beta}$ in emission). The variation of the fraction of emission- and absorption-line galaxies as a function of local projected galaxy density confirms the well-known morphology-density relation, and becomes flat at projected galaxy densities less than $sim 20Mpc^{-2}. Interestingly, 16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping radius around the cluster core, which we take to be direct evidence of ram-pressure stripping being the primary mechanism that terminates star-formation in the E+A population of galaxy clusters. This conclusion is supported by the rarity of E+A galaxies in the filament which rules out galaxy mergers as the dominant driver of evolution for E+A galaxies in clusters. In addition, we find the 42 e(a) and 27 e(b) member galaxies, i.e., the dusty-starburst and starburst galaxies respectively, to be spread out across almost the entire study area. Their spatial distribution, which shows a strong preference for the filament region, suggests that starbursts are triggered in relatively low-density environments as galaxies are accreted from the field population.
