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.

We present a new deep determination of the spectroscopic LF within the virial radius of the nearby and massive Abell 85 (A85) cluster down to the dwarf regime (M* + 6) using VLT/VIMOS spectra for ∼ 2000 galaxies with m r 21 mag and µ e,r 24 mag arcsec −2 . The resulting LF from 438 cluster members is best modelled by a double Schechter function due to the presence of a statistically significant upturn at the faintend. The amplitude of this upturn (α f = −1.58 +0.19 −0.15 ), however, is much smaller than that of the SDSS composite photometric cluster LF by Popesso et al. (2006), α f ∼ -2. The faint-end slope of the LF in A85 is consistent, within the uncertainties, with that of the field. The red galaxy population dominates the LF at low luminosities, and is the main responsible for the upturn. The fact that the slopes of the spectroscopic LFs in the field and in a cluster as massive as A85 are similar suggests that the cluster environment does not play a major role in determining the abundance of low-mass galaxies.

We present a new deep spectroscopic catalogue for Abell 85, within 3.0 × 2.6 Mpc 2 and down to M r ∼ M * r +6. Using the Visible Multi-Object Spectrograph at the Very Large Telescope and the AutoFiber 2 at the William Herschel Telescope, we obtained almost 1430 new redshifts for galaxies with m r ≤ 21 mag and µ e,r ≤ 24 mag arcsec −2 . These redshifts, together with SDSS-DR6 and NED spectroscopic information, result in 460 confirmed cluster members. This data set allows the study of the luminosity function (LF) of the cluster galaxies covering three orders of magnitudes in luminosities. The total and radial LFs are best modelled by a double Schechter function. The normalized LFs show that their bright (M r ≤ −21.5) and faint (M r ≥ −18.0) ends are independent of clustercentric distance and similar to the field LFs unlike the intermediate luminosity range (−21.5 ≤ M r ≤ −18.0). Similar results are found for the LFs of the dominant types of galaxies: red, passive, virialized and early-infall members. On the contrary, the LFs of blue, star-forming, non-virialized and recent-infall galaxies are well described by a single Schechter function. These populations contribute to a small fraction of the galaxy density in the innermost cluster region. However, in the outskirts of the cluster, they have similar densities to red, passive, virialized and early-infall members at the LF faint end. These results confirm a clear dependence of the colour and star formation of Abell 85 members in the cluster centric distance.

Galaxies in clusters are strongly affected by their environment. They evolve according to several physical mechanisms that are active in clusters. Their efficiency can strongly depend on the orbital configuration of the galaxies. Our aim is to analyse the orbits of the galaxies in the cluster Abell 85, based on the study of the galaxy velocity anisotropy parameter. We have solved the Jeans equation under the assumption that the galaxies in A 85 are collisionless objects, within the spherically symmetric gravitational potential of the virialized cluster. The mass of the cluster was estimated with X-ray and caustic analyses. We find that the anisotropy profile of the full galaxy population in A 85 is an increasing monotonic function of the distance from the cluster centre: on average, galaxies in the central region (r/r 200 < 0.3) are on isotropic orbits, while galaxies in the outer regions are on radial orbits. We also find that the orbital properties of the galaxies strongly depend on their stellar colour. In particular, blue galaxies are on less radial orbits than red galaxies. The different families of cluster galaxies considered here have the pseudo phase-space density profiles Q(r) and Q r (r) consistent with the profiles expected in virialized dark matter halos in N-body simulations. This result suggests that the galaxies in A 85 have reached dynamical equilibrium within the cluster potential. Our results indicate that the origin of the blue and red colour of the different galaxy populations is the different orbital shape rather than the accretion time.

We carried out the deep spectroscopic observations of the nearby cluster A 2151 with AF2/WYFFOS@WHT. The caustic technique enables us to identify 360 members brighter than M r = −16 and within 1.3R 200 . We separated the members into subsamples according to photometrical and dynamical properties such as colour, local environment and infall time. The completeness of the catalogue and our large sample allow us to analyse the velocity dispersion and the luminosity functions of the identified populations. We found evidence of a cluster still in its collapsing phase. The LF of the red population of A 2151 shows a deficit of dwarf red galaxies. Moreover, the normalized LFs of the red and blue populations of A 2151 are comparable to the red and blue LFs of the field, even if the blue galaxies start dominating one magnitude fainter and the red LF is well represented by a single Schechter function rather than a double Schechter function. We discuss how the evolution of cluster galaxies depends on their mass: bright and intermediate galaxies are mainly affected by dynamical friction and internal/mass quenching, while the evolution of dwarfs is driven by environmental processes which need time and a hostile cluster environment to remove the gas reservoirs and halt the star formation.

We present a strong-lensing analysis of four massive galaxy clusters imaged with the Hubble Space Telescope in the Reionization Lensing Cluster Survey. We use a Light-Traces-Mass technique to uncover sets of multiply images and constrain the mass distribution of the clusters. These mass models are the first published for Abell S295 and MACS J0159.8-0849, and are improvements over previous models for Abell 697 and MACS J0025.4-1222. Our analysis for MACS J0025.4-1222 and Abell S295 shows a bimodal mass distribution supporting the merger scenarios proposed for these clusters. The updated model for MACS J0025.4-1222 suggests a substantially smaller critical area than previously estimated. For MACS J0159.8-0849 and Abell 697 we find a single peak and relatively regular morphology, revealing fairly relaxed clusters. Despite being less prominent lenses, three of these clusters seem to have lensing strengths, i.e. cumulative area above certain magnification, similar to the Hubble Frontier Fields clusters (e.g., A(µ > 5) ∼ 1 − 3 arcmin 2 , A(µ > 10) ∼ 0.5 − 1.5 arcmin 2 ), which in part can be attributed to their merging configurations. We make our lens models publicly available through the Mikulski Archive for Space Telescopes. Finally, using Gemini-N/GMOS spectroscopic observations we detect a single emission line from a high-redshift J 125 25.7 galaxy candidate lensed by Abell 697. While we cannot rule out a lower-redshift solution, we interpret the line as Lyα at z = 5.800 ± 0.001, in agreement with its photometric redshift and dropout nature. Within this scenario we measure a Lyα rest-frame equivalent width of 52 ± 22Å and an observed Gaussian width of 117 ± 15 km/s.

For the first time, we explore the dynamics of the central region of a galaxy cluster within r 500 ∼600 h −1 kpc from its center by combining optical and X-ray spectroscopy. We use (1) the caustic technique, whichidentifies the cluster substructures and their galaxy members with optical spectroscopic data, and (2) the X-ray redshift fitting procedure, which estimates the redshift distribution of the intracluster medium (ICM). We use the spatial and redshift distributions of the galaxies and of the X-ray-emitting gas to associate the optical substructures to the X-ray regions. When we apply this approach to Abell 85 (A85), a complex dynamic structure of A85 emerges from our analysis: a galaxy group, with redshift z=0.0509±0.0021 is passing through the cluster center along the line of sight dragging part of the ICM present in the cluster core; two additional groups, at redshift z=0.0547±0.0022 and z=0.0570±0.0020, are going through the cluster in opposite directions, almost perpendicularly to the line of sight, and have substantially perturbed the dynamics of the ICM. An additional group in the outskirts of A85, at redshift z=0.0561±0.0023, is associated with a secondary peak of X-ray emission, at redshift = -+ z 0.0583 0.0047 0.0039 . Although our analysis and results on A85 need to be confirmed by high-resolution spectroscopy, they demonstrate how our new approach can be a powerful tool to constrain the formation history of galaxy clusters by unveiling their central and surrounding structures.

Strong gravitational lensing by galaxy clusters has become a powerful tool for probing the highredshift Universe, magnifying distant and faint background galaxies. Reliable strong lensing (SL) models are crucial for determining the intrinsic properties of distant, magnified sources and for constructing their luminosity function. We present here the first SL analysis of MACS J0308.9+2645 and PLCK G171.9-40.7, two massive galaxy clusters imaged with the Hubble Space Telescope in the framework of the Reionization Lensing Cluster Survey (RELICS). We use the Light-Traces-Mass modeling technique to uncover sets of multiply imaged galaxies and constrain the mass distribution of the clusters. Our SL analysis reveals that both clusters have particularly large Einstein radii (θ E > 30 for a source redshift of z s = 2), providing fairly large areas with high magnifications, useful for highredshift galaxy searches (∼ 2 arcmin 2 with µ > 5 to ∼ 1 arcmin 2 with µ > 10, similar to a typical Hubble Frontier Fields cluster). We also find that MACS J0308.9+2645 hosts a promising, apparently bright (J∼ 23.2 − 24.6 AB), multiply imaged high-redshift candidate at z ∼ 6.4. These images are amongst the brightest high-redshift candidates found in RELICS. Our mass models, including magnification maps, are made publicly available for the community through the Mikulski Archive for Space Telescopes.