Internal Alignments of Red Versus Blue Discs in Dark Matter Halos

TL;DR

This paper investigates the stability and orientation of galactic discs within dark matter halos, explaining the observed alignment differences between red and blue galaxies and their impact on cosmic shear measurements.

Contribution

It introduces a model explaining why red galaxies align with halos while blue galaxies do not, based on disc stability and gas cooling effects.

Findings

Red galaxies align with halos due to stable disc orientations along the minor axis.

Blue galaxies have arbitrary orientations due to gas cooling onto discs in various directions.

Substructure influences alignment timescales but less so when gas cooling occurs.

Abstract

Large surveys have shown that red galaxies are preferentially aligned with their halos while blue galaxies have a more isotropic distribution. Since halos generally align with their filaments this introduces a bias in the measurement of the cosmic shear from weak lensing. It is therefore vitally important to understand why this difference arises. We explore the stability of different disc orientations within triaxial halos. We show that, in the absence of gas, the disc orientation is most stable when its spin is along the minor axis of the halo. Instead when gas cools onto a disc it is able to form in almost arbitrary orientation, including off the main planes of the halo (but avoiding an orientation perpendicular to the halo's intermediate axis). Substructure helps gasless galaxies reach alignment with the halo faster, but have less effect on galaxies when gas is cooling onto the disc. Our results provide a novel and natural interpretation for why red, gas poor galaxies are preferentially aligned with their halo, while blue, star-forming, galaxies have nearly random orientations, without requiring a connection between galaxies' current star formation rate and their merger history.