Euclid preparation XC. Calibrated intrinsic galaxy alignments in the Euclid Flagship simulation

The intrinsic alignments of galaxies are a potentially major contaminant among cosmological analyses of weak gravitational lensing. In this work, we constructed a semi-analytic model of galaxy ellipticities and alignments in the Euclid Flagship simulation to predict this contamination in Euclid's weak lensing observations. Galaxy shapes and orientations were determined by the corresponding properties of the host haloes in the underlying N-body simulation, as well as the relative positions of galaxies within their halo. Alignment strengths were moderated via stochastic misalignments, separately for central and satellite galaxies, and were dependent on the galaxy's redshift, luminosity, and rest-frame colour. The resulting model was calibrated against galaxy ellipticity statistics from the COSMOS Survey, selected alignment measurements based on Sloan Digital Sky Survey samples, and galaxy orientations extracted from the Horizon-AGN hydrodynamic simulation at redshift z = 1. The best-fit model has a total of 12 alignment parameters and generally reproduces the calibration data sets well within the 1 sigma statistical uncertainties of the observations and the Flagship simulation, with notable exceptions for the most luminous subsamples on small physical scales. The statistical power of the calibration data and the volume of the single Flagship realisation are still too small to provide informative prior ranges for intrinsic alignment amplitudes in relevant galaxy samples. Given these uncertainties, we predict that Euclid end-of-mission tomographic weak gravitational lensing two-point statistics will end up modified due to intrinsic alignments, by up to 10%.