Euclid Quick Data Release (Q1) From Images to Multiwavelength Catalogues: the Euclid MERge Processing Function

Context. The Euclid satellite is an ESA mission that was launched in July 2023. Euclid is working in its regular observing mode with the target ofobserving an area of 15, 000 deg2 with two instruments, the Visible Imaging Channel (VIS) and the Near IR Spectrometer and imaging Photometer(NISP) down to VIS = 24.5 mag (10 sigma) in the Wide Survey. Ground-based imaging data in griz from surveys such as the Dark Energy Survey(DES) and Pan-STARRS complement the Euclid data to enable photo-z determination. Euclid investigates the distance-redshift relationship andthe evolution of cosmic structures by measuring the shapes and redshifts of galaxies and clusters of galaxies out to redshifts ∼ 2.Aims. Generating the multi-wavelength catalogs of Euclid and ground-based data is an essential part of the Euclid data processing system. In theframework of the Euclid Science Ground Segment (SGS), the aim of the MER Processing Function (PF) pipeline is to detect objects in the Euclidimaging data, measure their properties, and MERge them into a single multiwavelength catalogue.Methods. The MER PF pipeline performs source detection on both visible (VIS) and near-infrared (NIR) images and offers four different photometric measurements: Kron total flux, aperture photometry on PSF-matched images, template fitting photometry and Sersic fitting photometry.Furthermore, the MER PF pipeline measures a set of ancillary quantities, spanning from morphology to quality flags, to better characterize alldetected sources.Results. In this paper, we show how the MER PF pipeline is designed, detailing its main steps, and we show that the pipeline products meetthe tight requirements that Euclid aims to achieve on photometric accuracy. We also present the other measurements (e.g. morphology) that areincluded in the OU-MER output catalogues and we list all output products coming out of the MER PF pipeline.Key words. photometry, morphology, catalogues, multi-band, pipeline