Euclid: Galaxy morphology and photometry from bulge-disc decomposition of Early Release Observations

The background galaxies in Euclid Early Release Observation images of the Perseus cluster make up a remarkable sample for the combination of a 0.57 deg(2) area, 25.3 and 23.2 AB mag depth, and angular resolutions in the optical and near-infrared bands of 0.'' 1 and 0.'' 3, respectively. As part of the effort towards characterising the history of the Hubble sequence, we performed a morphological analysis of 2445 and 12 786 galaxies with I-E <= 21 and I-E <= 23, respectively. We used single-Sersic profiles and the sums of a Sersic bulge and an exponential disc to model these galaxies with SourceXtractor++ and analysed their positional, structural, and flux parameters in order to assess their similarities and differences. The fitted galaxies to I-E <= 21 span the various Hubble types with ubiquitous bulge and disc components and a bulge-to-total light ratio (B/T) that takes all values from 0 to 1. The effective radius of the single-Sersic profile is an intermediate estimate of galaxy size (between the bulge and disc effective radii) depending on B/T. The axis ratio of the single-Sersic profile is higher than the disc axis ratio, and this difference increases with B/T. The choice of model impacts the photometry with -0.08 to 0.01 mag median systematic I-E offsets between single-Sersic and bulge-disc total magnitudes and a 0.05-0.15 mag dispersion from low to high B/T. We measured a median 0.3 mag bulge-disc colour difference in rest-frame M-g - M-i that originates from the disc-dominated galaxies, whereas bulge-dominated galaxies have median colours similar to those of their components. Remarkably, we also measured redder inside disc colour gradients based on 5 to 10% systematic variations of disc effective radii between the optical and near-infrared bands. This analysis demonstrates the usefulness and limitations of single-Sersic profile modelling and the power of bulge-disc decomposition for characterising the morphology of lenticulars and spirals in Euclid images. We make available the catalogues of best-fit parameters for the morphological and SED fits.