Euclid and KiDS-1000: Quantifying the impact of source-lens clustering on cosmic shear analyses

TL;DR

This paper evaluates how source-lens clustering affects cosmic shear measurements in upcoming large surveys, finding that it can bias parameter estimates if not properly modeled, especially for fixed nuisance parameters.

Contribution

It quantifies the impact of source-lens clustering on weak lensing analyses for Stage III and IV surveys using simulated data, highlighting the importance of including SLC in models.

Findings

SLC has minor impact in Stage III surveys when nuisance parameters are flexible.

Including SLC shifts key cosmological parameters if nuisance parameters are fixed.

SLC can bias parameter inference if not modeled, especially with fixed nuisance parameters.

Abstract

The transition from current Stage-III surveys such as the Kilo-Degree Survey (KiDS) to the increased area and redshift range of Stage IV surveys such as Euclid will significantly increase the precision of weak lensing analyses. However, with increasing precision, the accuracy of model assumptions needs to be evaluated. In this study, we quantify the impact of the correlated clustering of weak lensing source galaxies with the surrounding large-scale structure, known as source-lens clustering (SLC), which is commonly neglected. For this, we use simulated cosmological datasets with realistically distributed galaxies and measure shear correlation functions for both clustered and uniformly distributed source galaxies. Cosmological analyses are performed for both scenarios to quantify the impact of SLC on parameter inference for a KiDS-like and a Euclid-like setting. We find for Stage III surveys, SLC has a minor impact when accounting for nuisance parameters for intrinsic alignments and shifts of tomographic bins, as these nuisance parameters absorb the effect of SLC, thus changing their original meaning. For KiDS (Euclid), the inferred intrinsic alignment amplitude $A_{IA}$ changes from $0.11_{-0.46}^{+0.44}$ ($-0.009_{-0.080}^{+0.079}$) for data without SLC to $0.28_{-0.44}^{+0.42}$ ($0.022_{-0.082}^{+0.081}$) with SLC. However, fixed nuisance parameters lead to shifts in $S_8$ and $\Omega_{m}$, emphasizing the need for including SLC in the modelling. For Euclid, we find that $\sigma_8$, $\Omega_m$, and $w_0$ are shifted by $0.19$, $0.12$, and $0.12\, \sigma$, respectively, when including free nuisance parameters, and by $0.20$, $0.16$, and $0.32\,\sigma$ when fixing the nuisance parameters. Consequently, SLC on its own has only a small impact on the inferred parameter inference when using uninformative priors for nuisance parameters.