Toward a Physical Understanding of Galaxy–Halo Alignment

Abstract

We investigate the alignment of galaxy and halo orientations using the TNG300-1 hydrodynamical simulation. Our analysis reveals that the distribution of the 2D misalignment angle θ _2D can be well described by a truncated shifted exponential distribution with only one free parameter across different redshifts and galaxy/halo properties. We demonstrate that the galaxy–ellipticity (GI) correlations of galaxies can be reproduced by perturbing halo orientations with the obtained θ _2D distribution, with only a small bias (<3°) possibly arising from unaccounted for couplings between θ _2D and other factors. We find that both the 2D and 3D misalignment angles θ _2D and θ _3D decrease with ex situ stellar mass fraction F _acc , halo mass M _vir , and stellar mass M _* , while increasing with the disk-to-total stellar mass fraction F _disk and redshift. These dependences are in good agreement with our recent observational study based on BOSS galaxy samples. Our results suggest that F _acc is a key factor in determining galaxy–halo alignment. Grouping galaxies by F _acc nearly eliminates the dependence of θ _3D on M _vir for all three principle axes, and also reduces the redshift dependence. For θ _2D , we find a more significant redshift dependence than for θ _3D even after controlling F _acc , which may be attributed to the evolution of galaxy and halo shapes. Our findings present a valuable model for observational studies and enhance our understanding of galaxy–halo alignment.

Keywords

• Galaxy properties
• Hydrodynamical simulations
• Weak gravitational lensing
• Galaxy dark matter halos