Euclid: Early Release Observations – A preview of the Euclid era through a galaxy cluster magnifying lens⋆

We present the first analysis of the Euclid Early Release Observations (ERO) program that targets fields around two lensing clusters, Abell 2390 and Abell 2764. We use VIS and NISP imaging to produce photometric catalogs for a total of ∼ 500 000 objects. The imaging data reach a 5σ typical depth in the range 25.1–25.4 AB in the NISP bands, and 27.1–27.3 AB in the VIS band. Using the Lyman-break method in combination with photometric redshifts, we search for high-redshift galaxies. We identify 30 Lyman-break galaxy (LBG) candidates at z > 6 and 139 extremely red sources (ERSs), most likely at lower redshift. The deeper VIS imaging compared to NISP means we can routinely identify high-redshift Lyman breaks of the order of 3 magnitudes, which reduces contamination by brown dwarf stars and low-redshift galaxies. The difficulty of spatially resolving most of these sources in 0′.′3 pix−1 imaging makes the distinction between galaxies and quasars more challenging. Spectroscopic followup campaigns of such bright sources will help constrain both the bright end of the ultraviolet galaxy luminosity function and the quasar luminosity function at z > 6, and constrain the physical nature of these objects. Additionally, we have performed a combined strong lensing and weak lensing analysis of A2390, and demonstrate how Euclid will contribute to better constraining the virial mass of galaxy clusters. From these data, we also identify optical and near-infrared counterparts of known z > 0.6 clusters, which exhibit strong lensing features, establishing the ability of Euclid to characterize high-redshift clusters. Finally, we provide a glimpse of Euclid’s ability to map the intracluster light out to larger radii than current facilities, enabling a better understanding of the cluster assembly history and mapping of the dark matter distribution. This initial dataset illustrates the diverse spectrum of legacy science that will be enabled by the Euclid survey.