Euclid Data Release 1 (DR1). Multi-band photometry validation on COSMOS extragalactic sources

The Euclid Data Release 1 (DR1) contains catalogues of sources covering 1900 deg2, including deep coverage of legacy fields that enable an extensive comparison between Euclid imaging and multiwavelength observations from other instruments. The Cosmic Evolution Survey (COSMOS) field, observed by Euclid and included in DR1, has been observed with the James Webb Space Telescope, resulting in the COSMOS2025 catalogue. We compare the Euclid DR1 photometry with the multiwavelength COSMOS2025 data to assess the accuracy of the photometric measurements produced by the different methods implemented in the OU-MER pipeline. We use both the Euclid Wide and Deep Surveys, focussing exclusively on extragalactic sources. We use the fluxes of COSMOS2025 to predict the expected fluxes in the Euclid bands via spectral energy distribution fitting, while the redshifts are either spectroscopic, for a sample of brighter sources, or photometric, to include fainter sources. Both the fluxes in the common Hyper Suprime-Cam bands and the SEDs from COSMOS2025 are used to validate the analysis and to characterise any biases or limitations of the method. The predicted fluxes and the OU-MER measurements are in good agreement, especially for bright and isolated sources. Median offsets depend on band, photometric method, survey depth, and redshift sample, most well within 10%, the largest offset seen in the NIR bands for the deep photo-z model-fitting measurements. However, complementary measurements on Gaia stars show an opposite trend, highlighting that photometric validation approaches may suffer from intrinsic uncertainties that prevent a thorough assessment of systematic effects. Among the three methods, template-fitting photometry generally shows the smallest offset from the SED-based predictions in the NISP bands and toward fainter magnitudes, while aperture photometry and model-fitting are comparable in IE.