Intrinsic alignment of disks and ellipticals across hydrodynamical simulations
The correlations between the positions and shapes of galaxies, i.e. intrinsic alignments, have been measured in many observational studies and hydrodynamical simulations. The position-shape correlation measurements of disk galaxies with varying methodologies, samples and hydrodynamical simulations are inconsistent in amplitude and sign. This work compares the correlations of disk and elliptical shapes around all galaxy positions and disk shapes around the positions of ellipticals at $z=0$ and $z=1$ for two different shape definitions in TNG300, Horizon-AGN and EAGLE for multiple morphological definitions in a consistent way. All types of signals are positive and robust in TNG300 and EAGLE and positive or null in Horizon-AGN. A re-weighting of the ellipticals around all galaxies correlations in TNG300, according to the underlying stellar mass distributions of the samples, suggests that stellar mass is the driving factor determining the amplitude of the correlations. The exception to this is the negative correlation of disks around ellipticals in Horizon-AGN. This arises for reduced shapes, which down-weight the outskirts of galaxies, at $z=1$, when disks are identified via a threshold in $|v/σ|$, the rotational velocity over the velocity dispersion.