Emulating redshift mixing due to blending in weak gravitational lensing

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians Universität München Scheinerstr. 1 81679 München, Germany
² Excellence Cluster ORIGINS Boltzmannstr. 2 85748 Garching, Germany
³ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing 44780 Bochum, Germany
⁴ Leiden Observatory, Leiden University Einsteinweg 55 2333 CC Leiden, The Netherlands
⁵ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 24 July 2025
Accepted: 27 December 2025

Abstract

Context. Galaxies whose images overlap in the focal plane of a telescope, commonly referred to as blends, are often located at different redshifts. Blending introduces a challenge to weak-lensing cosmology probes since such blends are subject to shear signals from multiple redshifts.

Aims. This effect can be described by joining shear bias and redshift characterisation in the effective redshift distribution, n_γ(z), which includes the response of apparent shapes of detected objects to shear of galaxies at redshift, z. In this work, we propose a novel method to correct n_γ(z) for redshift-mixed blending by emulating the shear response to neighbouring galaxies.

Methods. We designed a ‘half-sky-shearing’ simulation with Subaru Hyper Suprime Cam (HSC) wide-like specifications, which allowed us to extract the response of a detected object’s measured ellipticity to the shearing of neighbouring galaxies among numerous galaxy pairs.

Results. We demonstrate the feasibility of accurately emulating these pairwise responses and validate the robustness of our approach under varying observing conditions and galaxy population uncertainties. We find that the effective redshift of sources at the high-redshift tail of the distribution is about 0.05 lower than expected when the effect is not modelled.

Conclusions. Given adequately processed image simulations, our correction method can be readily incorporated into future cosmological analyses to mitigate this source of systematic error.