We present the statistically complete and cosmologically most relevant subset of the twelve most distant galaxy clusters detected at z > 0.5 by the MAssive Cluster Survey (MACS). Ten of these systems are new discoveries; only two (MACSJ0018.5+1626 aka CL 0016+1609, and MACSJ0454.1−0300 aka MS 0451.6−0305) were previously known. We provide fundamental cluster properties derived from our optical and X-ray follow-up observations as well as the selection function in tabulated form to facilitate cosmological studies using this sample.

We present high-resolution (0.3 ′′ ) ALMA 870 µm imaging of 52 sub-millimeter galaxies (SMGs) in the Ultra Deep Survey (UDS) field and investigate the size and morphology of the sub-millimeter (submm) emission on 2-10 kpc scales. We derive a median intrinsic angular size of FWHM = 0.30 ± 0.04 ′′ for the 23 SMGs in the sample detected at a signal-to-noise ratio (SNR) > 10. Using the photometric redshifts of the SMGs we show that this corresponds to a median physical half-light diameter of 2.4 ± 0.2 kpc. A stacking analysis of the SMGs detected at an SNR < 10 shows they have sizes consistent with the 870 µm-bright SMGs in the sample. We compare our results to the sizes of SMGs derived from other multi-wavelength studies, and show that the rest-frame ∼ 250 µm sizes of SMGs are consistent with studies of resolved 12 CO (J = 3-2 to 7-6) emission lines, but that sizes derived from 1.4 GHz imaging appear to be approximately two times larger on average, which we attribute to cosmic ray diffusion. The rest-frame optical sizes of SMGs are around four times larger than the sub-millimeter sizes, indicating that the star formation in these galaxies is compact relative to the pre-existing stellar distribution. The size of the starburst region in SMGs is consistent with the majority of the star formation occurring in a central region, a few kpc in extent, with a median star formation rate surface density of 90 ± 30 M ⊙ yr −1 kpc −2 , which may suggest that we are witnessing an intense period of bulge growth in these galaxies .

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 distribution 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 −1 , 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 Advanced Camera for Surveys observations of MACS J1149.5+2223, an X-ray luminous galaxy cluster at z=0.544 discovered by the Massive Cluster Survey. The data reveal at least seven multiply-imaged galaxies, three of which we have confirmed spectroscopically. One of these is a spectacular face-on spiral galaxy at z = 1.491, the four images of which are gravitationally magnified by 8 ∼ < µ ∼ < 23. We identify this as an L ⋆ (M B ≃ −20.7), disk-dominated (B/T ∼ < 0.5) galaxy, forming stars at ∼ 6 M ⊙ yr −1 . We use a robust sample of multiply-imaged galaxies to constrain a parameterized model of the cluster mass distribution. In addition to the main cluster dark matter halo and the bright cluster galaxies, our best model includes three galaxygroup-sized halos. The relative probability of this model is P(N halo = 4)/P(N halo < 4) ≥ 10 12 where N halo is the number of cluster/group-scale halos. In terms of sheer number of merging cluster/group-scale components, this is the most complex strong-lensing cluster core studied to date. The total cluster mass and fraction of that mass associated with substructures within R ≤ 500 kpc, are measured to be M tot = (6.7 ± 0.4) × 10 14 M ⊙ and f sub = 0.25 ± 0.12 respectively. Our model also rules out recent claims of a flat density profile at ∼ > 7σ confidence, thus highlighting the critical importance of spectroscopic redshifts of multiply-imaged galaxies when modeling strong lensing clusters. Overall our results attest to the efficiency of X-ray selection in finding the most powerful cluster lenses, including complicated merging systems.

We report the first weak lensing detection of a large-scale filament funnelling matter on to the core of the massive galaxy cluster MACS J0717.5+3745.Our analysis is based on a mosaic of 18 multipassband images obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope, covering an area of ∼10 × 20 arcmin 2 . We use a weak lensing pipeline developed for the Cosmic Evolution Survey, modified for the analysis of galaxy clusters, to produce a weak lensing catalogue. A mass map is then computed by applying a weak gravitational lensing multiscale reconstruction technique designed to describe irregular mass distributions such as the one investigated here. We test the resulting mass map by comparing the mass distribution inferred for the cluster core with the one derived from strong lensing constraints and find excellent agreement.Our analysis detects the MACS J0717.5+3745 filament within the 3σ detection contour of the lensing mass reconstruction, and underlines the importance of filaments for theoretical and numerical models of the mass distribution in the cosmic web. We measure the filament's projected length as ∼4.5 h −1 74 Mpc, and its mean density as (2.92 ± 0.66) × 10 8 h 74 M kpc −2 . Combined with the redshift distribution of galaxies obtained after an extensive spectroscopic follow-up in the area, we can rule out any projection effect resulting from the chance alignment on the sky of unrelated galaxy group-scale structures. Assuming plausible constraints concerning the structure's geometry based on its galaxy velocity field, we construct a threedimensional (3D) model of the large-scale filament. Within this framework, we derive the 3D length of the filament to be 18 h −1 74 Mpc. The filament's deprojected density in terms of the critical density of the Universe is measured as (206 ± 46) ρ crit , a value that lies at the very high end of the range predicted by numerical simulations. Finally, we study the distribution of stellar mass in the field of MACS J0717.5+3749 and, adopting a mean mass-to-light ratio M * /L K of 0.73 ± 0.22 and assuming a Chabrier initial mass function, measure a stellar mass fraction along the filament of (0.9 ± 0.2) per cent, consistent with previous measurements in the vicinity of massive clusters.