The Large-Scale Orientations of Disk Galaxies

TL;DR

This study uses high-resolution hydrodynamic simulations to analyze the orientations of galactic disks within cosmic filaments, revealing alignment patterns with their host halos and large-scale structures that vary with mass, redshift, and environment.

Contribution

It provides new insights into the alignment behaviors of galactic disks with their halos and the large-scale tidal field across different cosmic conditions.

Findings

Inner halo regions show strong alignment with galactic disks.

Massive disks tend to align along cosmic filaments.

Alignment with the tidal field depends on environment and redshift.

Abstract

We use a 380 h-1 pc resolution hydrodynamic AMR simulation of a cosmic filament to investigate the orientations of a sample of ~100 well-resolved galactic disks spanning two orders of magnitude in both stellar and halo mass. We find: (i) At z=0, there is an almost perfect alignment at a median angle of 18 deg, in the inner dark matter halo regions where the disks reside, between the spin vector of the gaseous and stellar galactic disks and that of their inner host haloes. The alignment between galaxy spin and spin of the entire host halo is however significantly weaker, ranging from a median of ~46 deg at z=1 to ~50 deg at z=0. (ii) The most massive galaxy disks have spins preferentially aligned so as to point along their host filaments. (iii) The spin of disks in lower-mass haloes shows, at redshifts above z~0.5 and in regions of low environmental density, a clear signature of alignment with the intermediate principal axis of the large-scale tidal field. This behavior is consistent with predictions of linear tidal torque theory. This alignment decreases with increasing environmental density, and vanishes in the highest density regions. Non-linear effects in the high density environments are plausibly responsible for establishing this density-alignment correlation. We expect that our numerical results provide important insights for both understanding intrinsic alignment in weak lensing from the astrophysical perspective and formation and evolution processes of galactic disks in a cosmological context.