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 $\theta_{\rm{2D}}$ can be well described by a truncated shifted exponential (TSE) distribution with only {\textit{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 $\theta_{\rm{2D}}$ distribution, with only a small bias ($<3^{\circ}$) possibly arising from unaccounted couplings between $\theta_{\rm{2D}}$ and other factors. We find that both the 2D and 3D misalignment angles $\theta_{\rm{2D}}$ and $\theta_{\rm{3D}}$ decrease with ex situ stellar mass fraction $F_{\rm{acc}}$, halo mass $M_{\rm{vir}}$ and stellar mass $M_{*}$, while increasing with disk-to-total stellar mass fraction $F_{\rm{disk}}$ and redshift. These dependences are in good agreement with our recent observational study based on the BOSS galaxy samples. Our results suggest that $F_{\rm{acc}}$ is a key factor in determining the galaxy-halo alignment. Grouping galaxies by $F_{\rm{acc}}$ nearly eliminates the dependence of $\theta_{\rm{3D}}$ on $M_{\rm{vir}}$ for all three principle axes, and also reduces the redshift dependence. For $\theta_{\rm{2D}}$, we find a more significant redshift dependence than for $\theta_{\rm{3D}}$ even after controlling $F_{\rm{acc}}$, 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.