1702.03913
Galaxy-halo alignments in the Horizon-AGN cosmological hydrodynamical simulation
Chisari, et al
Intrinsic alignments of galaxies are a significant astrophysical systematic affecting cosmo constraints from WL. Obtaining numerical predictions from hydro sims of expected survey volumes is expensive, and a cheaper alternative relies on populating large DM-only sims with accurate models of alignments calibrated on smaller hydro runs. This requires connecting the shapes and orientations of galaxies to those of DM haloes and to the LSS. In this paper, characterize galaxy-halo alignments in the Horizon-AGN cosmo hydrodynamical sim. Compare the shapes and orientations of galaxies in 0<z<3 to those of their embedding DM haloes, and to the matching haloes of a twin DM only run with identical initial conditions. 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. Discuss the implications of these results for current halo models of IA and suggest several avenues for improvement.
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