Impact of line of sight structure on weak lensing observables of galaxy clusters

¹ Laboratoire d’Astrophysique, EPFL, Observatoire de Sauverny, 1290 Versoix, Switzerland
² School of Mathematics, Statistics and Physics, Newcastle University, Herschel Building, Newcastle-upon-Tyne NE1 7RU, UK
³ Lorentz Institute for Theoretical Physics, Leiden University, PO Box 9506 NL-2300 RA, Leiden, The Netherlands
⁴ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA, Leiden, The Netherlands

^⋆ Corresponding author: felix.vecchi@epfl.ch

Received: 15 April 2025
Accepted: 26 August 2025

Abstract

Weak gravitational lensing observations of galaxy clusters are sensitive to all the mass that is present along the line of sight (LoS). Thus, the systematic and additional statistical uncertainties due to intervening structures must be taken into consideration. In this work, we quantify the impact of these structures on the recovery of mass density profile parameters using 967 clusters from the highest-resolution FLAMINGO simulation. We constructed mock weak-lensing maps, which included both single source plane mocks at redshifts up to z_s ≤ 3, along with Euclid-like mocks with a realistic source redshift distribution. Applying Bayesian inference with Nautilus, we fit spherical and elliptical Navarro-Frenk-White (NFW) models to recover the cluster mass, concentration, axis ratio, and centre. We used these parameters to measure the brightest cluster galaxy (BCG) offset from the potential centre (or BCG wobble). We find that the spherical model fits clusters along under-dense sight lines better than those along over-dense ones. This introduces a positive skew in the relative error distributions for mass and concentration, which increases with source redshift. In Euclid-like mocks, this results in a mean mass bias of +5.3 ± 1.4% (significant at 3.5σ) when assuming a spherical NFW model. We also detected a mean axis ratio bias of −2.0 ± 0.7% (2.9σ), with no significant bias in concentration. We measured a BCG wobble of ∼14 kpc in our Euclid-like mocks, with a negligible contribution from LoS structure. Furthermore, we predicted the scatter in mass estimates from future weak lensing surveys with mean source redshifts of z_s ≳ 1.2 (e.g. Nancy Grace Roman Space Telescope) would end up dominated by LoS structure. Hence, assuming a diagonal covariance matrix will lead to an overestimation in terms of precision. We conclude that cluster weak-lensing pipelines should be calibrated on simulations with light cone data to properly account for the significant impact of LoS structure.