Emulating redshift-mixing due to blending in weak gravitational lensing

Abstract: 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, as such blends are subject to shear signals from multiple redshifts. This effect can be described by joining shear bias and redshift characterisation in the effective redshift distribution, $n_{\gamma}(z)$, 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 $n_{\gamma}(z)$ for redshift-mixed blending by emulating the shear response to neighbouring galaxies. Specifically, we design a ``half-sky-shearing'' simulation with HSC-Wide-like specifications, in which we extract the response of a detected object's measured ellipticity to shear of neighbouring galaxies among numerous galaxy pairs. 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 not modelling the effect. Given appropriately processed image simulations, our correction method can be readily incorporated into future cosmological analyses to mitigate this source of systematic error.