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Accurate measurements of the cosmic shear signal require a separation of thetrue weak gravitational lensing signal from intrinsic shape correlations ofgalaxies. These `intrinsic alignments' of galaxies originate from galaxyformation processes and are expected to be correlated with the gravitationalfield through tidal processes affecting the galaxies, such as tidal shearingfor elliptical galaxies and tidal torquing for spiral galaxies. In this study,we use morphologically selected samples of elliptical and spiral galaxies fromthe IllustrisTNG simulation at z=0 and z=1 to test the commonly employed linear(tidal shearing) and quadratic (tidal torquing) models for intrinsicalignments. We obtain local measurements of the linear and quadratic alignmentparameters, including corrections for large-scale anisotropies of thecosmologically small simulation volume, and study their dependence on galaxyand environmental properties. We find a significant alignment signal forelliptical galaxies (linear model), that increases with mass and redshift.Spiral galaxies (quadratic model) on the other hand exhibit a significantsignal only for the most massive objects at z=1. We show the quadratic modelfor spiral galaxies to break down at its fundamental assumptions, andsimultaneously obtain a significant signal of spiral galaxies to alignaccording to the linear model. We use the derived alignment parameters tocompute intrinsic alignment spectra and estimate the expected contamination inthe weak lensing signal obtained by Euclid.

Intrinsic Alignments in IllustrisTNG and their implications for weak lensing: Tidal shearing and tidal torquing mechanisms put to the test