# Galaxy-halo alignments in the Horizon-AGN cosmological hydrodynamical simulation

## TL;DR

This study characterizes galaxy-halo alignments in the Horizon-AGN simulation, revealing complex relationships between galaxy shapes, halo properties, and redshift, with implications for modeling intrinsic alignments in cosmology.

## Contribution

It provides a detailed analysis of galaxy-halo alignments in a hydrodynamical simulation, highlighting the limitations of predicting galaxy shapes from halo ellipticities and exploring redshift evolution.

## Key findings

- Galaxy ellipticities are not directly predictable from halo ellipticities.
- Misalignment angle depends on halo mass and galaxy type.
- Haloes are more strongly aligned among themselves than galaxies.

## Abstract

Intrinsic alignments of galaxies are a significant astrophysical systematic affecting cosmological constraints from weak gravitational lensing. Obtaining numerical predictions from hydrodynamical simulations of expected survey volumes is expensive, and a cheaper alternative relies on populating large dark matter-only simulations with accurate models of alignments calibrated on smaller hydrodynamical runs. This requires connecting the shapes and orientations of galaxies to those of dark matter haloes and to the large-scale structure. In this paper, we characterise galaxy-halo alignments in the Horizon-AGN cosmological hydrodynamical simulation. We compare the shapes and orientations of galaxies in the redshift range $0<z<3$ to those of their embedding dark matter haloes, and to the matching haloes of a twin dark-matter only run with identical initial conditions. We find that galaxy ellipticities in general cannot be predicted directly from halo ellipticities. The mean misalignment angle between the minor axis of a galaxy and its embedding halo is a function of halo mass, with residuals arising from the dependence of alignment on galaxy type, but not on environment. Haloes are much more strongly aligned among themselves than galaxies, and they decrease their alignment towards low redshift. Galaxy alignments compete with this effect, as galaxies tend to increase their alignment with haloes towards low redshift. We discuss the implications of these results for current halo models of intrinsic alignments and suggest several avenues for improvement.