Euclid Quick Data Release (Q1) Photometric redshifts and physical properties of galaxies through the PHZ processing function

The ESA Euclid mission will measure the photometric redshift of billions of galaxies in order to provide an accurate 3D view of the Universe atoptical and near-infrared wavelengths. Photometric redshifts are determined by the PHZ processing function on the basis of the multi-wavelengthphotometry of Euclid and ground-based observations. In this paper, we describe in detail the PHZ pipeline used for the first ‘quick’ (Q1) Eucliddata release, the output products and their validation with respect to the Euclid requirements. The PHZ pipeline is responsible of the followingmain tasks: i) source classification into star, galaxy, and QSO classes based on photometric colours, using the probabilistic random forest algorithm; ii) determination of photometric redshifts for the core science, based on a new custom-made template fitting algorithm (Phosphoros); iii)determination of physical properties of galaxies for legacy science, using a machine learning approach based on the nearest neighbour algorithm(NNPZ). The classification is able to provide a star (galaxy) sample with high level of purity (completeness), and reliable probabilities for thoseclasses. The identification of QSOs is instead more problematic: photometric information alone seems to be not sufficient to accurately separateQSOs from galaxies, whose colour spaces overlap with the Euclid photometry. The performance of Phosphoros in the determination of photometric redshifts has been tested using the COSMOS2020 catalogue and a large sample of spectroscopic redshifts. The results in both cases arein line with expectations: the precision of the estimates are compatible with Euclid requirements, while, as expected, a bias correction is neededto achieve the accuracy level required for the cosmological probes. Finally, the pipeline provides reliable estimates of the physical properties, ingood agreement with findings from the COSMOS2020 catalogue, when an appropriate prior on the galaxy age is applied. Without such a prior,a unrealistic large fraction of young galaxies with very high specific star-formation rates is observed. The upcoming refinement of the referencesample for NNPZ will improve the physical properties results.Key words. Methods: data analysis, Catalogs, Galaxies: evolution