Quantifying the impact of detection bias from blended galaxies on cosmic shear surveys

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing, 44780 Bochum, Germany
² Argelander-Institut f. Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ University of Innsbruck, Institute for Astro- and Particle Physics, Technikerstraße 25, 6020 Innsbruck, Austria

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

Received: 26 June 2025
Accepted: 20 October 2025

Abstract

Increasingly large areas in cosmic shear surveys lead to a reduction in statistical errors, necessitating increasingly accurate control of systematic errors. Previous studies have investigated one of these systematic effects: image overlap with bright foreground galaxies may cause some distant source galaxies to remain undetected. Since this overlap is more likely to occur in regions of high foreground density – which tend to be the regions in which the shear is largest – this detection bias would cause an underestimation of the shear correlation function. This detection bias adds to the possible systematic of image blending, in which nearby pairs or multiplets of images render shear estimates more uncertain, and thus may cause a reduction in their statistical weight. Based on simulations with data from the Kilo-Degree Survey, we investigated the conditions under which images are not detected. We find an approximate analytic expression for the detection probability in terms of the separation and brightness ratio relative to the neighbouring galaxies. Applying this fitting formula to weak-lensing ray tracing through the galaxy distribution in the Millennium Simulation, we estimate that the detection bias alone causes an underestimation of S₈ = σ₈√(Ω_m/0.3) by almost 2%, and therefore cannot be neglected in current and forthcoming cosmic shear surveys.