Euclid preparation. Impact of galaxy intrinsic alignment modelling choices on Euclid 3x2pt cosmology

TL;DR

This paper evaluates how different galaxy intrinsic alignment models impact Euclid's 3x2pt cosmological analysis, highlighting the TATT model's flexibility and suitability for unbiased results.

Contribution

It compares the performance of NLA and TATT intrinsic alignment models, including variations, for Euclid's cosmological parameter estimation.

Findings

TATT model offers the most flexible IA description.

TATT provides similar constraining power to simpler models.

Using TATT reduces bias in cosmological parameters.

Abstract

The Euclid galaxy survey will provide unprecedented constraints on cosmology, but achieving unbiased results will require an optimal characterisation and mitigation of systematic effects. Among these, the intrinsic alignments (IA) of galaxies are one of the dominant contaminants of the weak lensing (WL) and galaxy-galaxy lensing (GGL) probes. In this work, we assess IA modelling choices for Euclid DR1 3x2pt analyses by comparing the performance of the two most commonly used IA models, nonlinear alignment (NLA) and tidal alignment tidal torquing (TATT), along with several variations. Our analyses combine three perspectives: i) the constraining power on the IA and cosmological parameters for each IA model, ii) the bias that results when the IA analysis model differs from the model used to generate the synthetic data vector, and iii) the degeneracies between IA and photometric redshift (photo-z) nuisance parameters. Among the IA models analysed, the redshift-dependent TATT model (zTATT) provides the most flexible description of IA, with a similar constraining power compared to simpler IA models, making it a well-motivated choice for Euclid DR1 3x2pt analyses.