Advances in Constraining Intrinsic Alignment Models with Hydrodynamic Simulations

TL;DR

This paper investigates galaxy intrinsic alignments using hydrodynamic simulations across multiple redshifts, revealing significant IA signals and their dependence on galaxy properties, and introduces a new method for constraining alignment models directly from the tidal field.

Contribution

It provides a comprehensive analysis of intrinsic alignments in simulations and proposes a novel pixel-based method for constraining alignment model parameters.

Findings

Non-zero intrinsic alignments detected at several sigma levels.

Strong redshift dependence of IA amplitude, increasing by a factor of ~2 from z=0 to z=1.

New pixel-based method for constraining the TATT model shows promise.

Abstract

We use galaxies from the IllustrisTNG, MassiveBlack-II and Illustris hydrodynamic simulations to investigate the behaviour of large scale galaxy intrinsic alignments. Our analysis spans four redshift slices over the approximate range of contemporary lensing surveys $z=0-1$. We construct comparable weighted samples from the three simulations, which we then analyse using an alignment model that includes both linear and quadratic alignment contributions. Our data vector includes galaxy-galaxy, galaxy-shape and shape-shape projected correlations, with the joint covariance matrix estimated analytically. In all of the simulations, we report non-zero IAs at the level of several $\sigma$. For a fixed lower mass threshold, we find a relatively strong redshift dependence in all three simulations, with the linear IA amplitude increasing by a factor of $\sim 2$ between redshifts $z=0$ and $z=1$. We report no significant evidence for non-zero values of the tidal torquing amplitude, $A_2$, in TNG, above statistical uncertainties, although MassiveBlack-II favours a moderately negative $A_2\sim-2$. Examining the properties of the TATT model as a function of colour, luminosity and galaxy type (satellite or central), our findings are consistent with the most recent measurements on real data. We also outline a novel method for constraining the TATT model parameters directly from the pixelised tidal field, alongside a proof of concept exercise using TNG. This technique is shown to be promising, although the comparison with previous results obtained via other methods is non-trivial.