Intrinsic Alignments in IllustrisTNG and their implications for weak lensing: Tidal shearing and tidal torquing mechanisms put to the test

TL;DR

This study tests the tidal shearing and tidal torquing models of intrinsic galaxy alignments using IllustrisTNG simulations, revealing significant alignments for ellipticals and massive spirals, and assessing their impact on weak lensing measurements.

Contribution

It provides the first detailed test of linear and quadratic intrinsic alignment models with simulation data across galaxy types and redshifts, including environmental dependencies.

Findings

Elliptical galaxies show significant linear alignment signals that increase with mass and redshift.

Spiral galaxies exhibit quadratic alignment signals only at high mass and redshift, with the model breaking down at its assumptions.

The study estimates intrinsic alignment contamination in Euclid's weak lensing measurements.

Abstract

Accurate measurements of the cosmic shear signal require a separation of the true weak gravitational lensing signal from intrinsic shape correlations of galaxies. These `intrinsic alignments' of galaxies originate from galaxy formation processes and are expected to be correlated with the gravitational field through tidal processes affecting the galaxies, such as tidal shearing for elliptical galaxies and tidal torquing for spiral galaxies. In this study, we use morphologically selected samples of elliptical and spiral galaxies from the IllustrisTNG simulation at z=0 and z=1 to test the commonly employed linear (tidal shearing) and quadratic (tidal torquing) models for intrinsic alignments. We obtain local measurements of the linear and quadratic alignment parameters, including corrections for large-scale anisotropies of the cosmologically small simulation volume, and study their dependence on galaxy and environmental properties. We find a significant alignment signal for elliptical galaxies (linear model), that increases with mass and redshift. Spiral galaxies (quadratic model) on the other hand exhibit a significant signal only for the most massive objects at z=1. We show the quadratic model for spiral galaxies to break down at its fundamental assumptions, and simultaneously obtain a significant signal of spiral galaxies to align according to the linear model. We use the derived alignment parameters to compute intrinsic alignment spectra and estimate the expected contamination in the weak lensing signal obtained by Euclid.