Galaxy blending effects in deep imaging cosmic shear probes of cosmology

TL;DR

This paper investigates how galaxy blending in deep imaging surveys like Rubin Observatory affects cosmic shear measurements and cosmological parameter estimation, revealing a significant bias in the structure growth parameter.

Contribution

It provides a detailed emulation of galaxy blending effects in mock catalogues and quantifies their impact on shear correlation functions and cosmological parameters.

Findings

Approximately 12% of galaxies are unrecognized blends.

Unrecognized blending causes a ~0.025 decrease in S_8.

Bias from blending exceeds 2σ statistical errors.

Abstract

Upcoming deep imaging surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time will be confronted with challenges that come with increased depth. One of the leading systematic errors in deep surveys is the blending of objects due to higher surface density in the more crowded images; a considerable fraction of the galaxies which we hope to use for cosmology analyses will overlap each other on the observed sky. In order to investigate these challenges, we emulate blending in a mock catalogue consisting of galaxies at a depth equivalent to 1.3 yr of the full 10-yr Rubin Observatory that includes effects due to weak lensing, ground-based seeing, and the uncertainties due to extraction of catalogues from imaging data. The emulated catalogue indicates that approximately 12 per cent of the observed galaxies are ``unrecognized'' blends that contain two or more objects but are detected as one. Using the positions and shears of half a billion distant galaxies, we compute shear--shear correlation functions after selecting tomographic samples in terms of both spectroscopic and photometric redshift bins. We examine the sensitivity of the cosmological parameter estimation to unrecognized blending employing both jackknife and analytical Gaussian covariance estimators. An $\sim0.025$ decrease in the derived structure growth parameter $S_8 = \sigma_8 (\Omega_{\rm m}/0.3)^{0.5}$ is seen due to unrecognized blending in both tomographies with a slight additional bias for the photo-$z$-based tomography. This bias is greater than the $2\sigma$ statistical error in measuring $S_8$.