Large surveys of galaxy clusters with the Hubble and Spitzer Space Telescopes, including CLASH and the Frontier Fields, have demonstrated the power of strong gravitational lensing to efficiently deliver large samples of high-redshift galaxies. We extend this strategy through a wider, shallower survey named RELICS, the Reionization Lensing Cluster Survey. This survey, described here, was designed primarily to deliver the best and brightest high-redshift candidates from the first billion years after the Big Bang. RELICS observed 41 massive galaxy clusters with Hubble and Spitzer at 0.4-1.7µm and 3.0-5.0µm, respectively. We selected 21 clusters based on Planck PSZ2 mass estimates and the other 20 based on observed or inferred lensing strength. Our 188-orbit Hubble Treasury Program obtained the first high-resolution near-infrared images of these clusters to efficiently search for lensed highredshift galaxies. We observed 46 WFC3/IR pointings (∼200 arcmin 2 ) with two orbits divided among four filters (F105W, F125W, F140W, and F160W) and ACS imaging as needed to achieve single-orbit depth in each of three filters (F435W, F606W, and F814W). As previously reported by Salmon et al., we discovered 322 z ∼ 6 − 10 candidates, including the brightest known at z ∼ 6, and the most spatially-resolved distant lensed arc known at z ∼ 10. Spitzer IRAC imaging (945 hours awarded, plus 100 archival) has crucially enabled us to distinguish z ∼ 10 candidates from z ∼ 2 interlopers. For each cluster, two HST observing epochs were staggered by about a month, enabling us to discover 11 supernovae, including 3 lensed supernovae, which we followed up with 20 orbits from our program. We delivered reduced HST images and catalogs of all clusters to the public via MAST and reduced Spitzer images via IRSA. We have also begun delivering lens models of all clusters, to be completed before the JWST GO Cycle 1 call for proposals.

The most distant galaxies known are at z ∼ 10 − 11, observed 400 − 500 Myr after the Big Bang. The few z ∼ 10−11 candidates discovered to date have been exceptionally small-barely resolved, if at all, by the Hubble Space Telescope. Here we present the discovery of SPT0615-JD, a fortuitous z ∼ 10 (z phot =9.9 ± 0.6) galaxy candidate stretched into an arc over ∼ 2.5 by the effects of strong gravitational lensing. Discovered in the Reionization Lensing Cluster Survey (RELICS) Hubble Treasury program and companion S-RELICS Spitzer program, this candidate has a lensed H-band magnitude of 25.7±0.1 AB mag. With a magnification of µ ∼ 4−7 estimated from our lens models, the de-lensed intrinsic magnitude is 27.6 ± 0.3 AB mag, and the half-light radius is r e < 0.8 kpc, both consistent with other z > 9 candidates. The inferred stellar mass (log[M /M ] = 9.7 +0.7 −0.5 ) and star formation rate (log[SFR/M yr −1 ] = 1.3 +0.2 −0.3 ) indicate that this candidate is a typical star-forming galaxy on the z > 6 SFR-M relation. We note that three independent lens models predict two counterimages, at least one of which should be of a similar magnitude to the arc, but these counterimages are not yet detected. Counterimages would not be expected if the arc were at lower redshift. However, the only spectral energy distributions capable of fitting the Hubble and Spitzer photometry well at lower redshifts require unphysical combinations of z ∼ 2 galaxy properties. The unprecedented lensed size of this z ∼ 10 candidate offers the potential for the James Webb Space Telescope to study the geometric and kinematic properties of a galaxy observed 500 Myr after the Big Bang.

We have imaged the Sunyaev-Zel'dovich (SZ) effect signals at 140 and 270 GHz towards ten galaxy clusters with Bolocam and AzTEC/ASTE. We also used Planck data to constrain the signal at large angular scales, Herschel-SPIRE images to subtract the brightest galaxies that comprise the cosmic infrared background (CIB), Chandra imaging to map the electron temperature T e of the intra-cluster medium (ICM), and HST imaging to derive models of each galaxy cluster's mass density. The galaxy clusters gravitationally lens the background CIB, which produced an on-average reduction in brightness towards the galaxy clusters' centers after the brightest galaxies were subtracted. We corrected for this deficit, which was between 5-25% of the 270 GHz SZ effect signal within R 2500 . Using the SZ effect measurements, along with the X-ray constraint on T e , we measured each galaxy cluster's average line of sight (LOS) velocity v z within R 2500 , with a median per-cluster uncertainty of ±700 km s −1 . We found an ensemble-mean v z of 430 ± 210 km s −1 , and an intrinsic cluster-to-cluster scatter σ int of 470 ± 340 km s −1 . We also obtained maps of v z over each galaxy cluster's face with an angular resolution of 70 . All four galaxy clusters previously identified as having a merger oriented along the LOS showed an excess variance in these maps at a significance of 2-4σ, indicating an internal v z rms of 1000 km s −1 . None of the six galaxy clusters previously identified as relaxed or plane of sky mergers showed any such excess variance.

We present a stacked weak lensing analysis of 27 richness selected galaxy clusters at 0.40 z 0.62 in the CODEX survey. The fields were observed in 5 bands with the CFHT. We measure the stacked surface mass density profile with a 14σ significance in the radial range 0.1 < R M pc h −1 < 2.5. The profile is well described by the halo model, with the main halo term following an NFW profile and including the off-centring effect. We select the background sample using a conservative colourmagnitude method to reduce the potential systematic errors and contamination by cluster member galaxies. We perform a Bayesian analysis for the stacked profile and constrain the best-fit NFW parameters M 200c = 6.6 +1.0 −0.8 × 10 14 h −1 M and c 200c = 3.7 +0.7 −0.6 . The off-centring effect was modelled based on previous observational results found for redMaPPer SDSS clusters. Our constraints on M 200c and c 200c allow us to investigate the consistency with numerical predictions and select a concentrationmass relation to describe the high richness CODEX sample. Comparing our best-fit values for M 200c and c 200c with other observational surveys at different redshifts, we find no evidence for evolution in the concentration-mass relation, though it could be mitigated by particular selection functions. Similar to previous studies investigating the X-ray luminosity-mass relation, our data suggests a lower evolution than expected from self-similarity.