How to Fix a Trillion-Dollar Space Telescope's Biggest Blind Spot
A new study identifies which mathematical models will let the Euclid space observatory achieve its mission: measuring the universe's expansion without being fooled by galaxies' mysterious alignments. Getting this right determines whether a €1.4 billion investment yields trustworthy answers about dark energy or introduces systematic errors that skew results for decades.
Originaltitel: Euclid preparation. Impact of galaxy intrinsic alignment modelling choices on Euclid 3x2pt cosmology
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.