Effects of overlapping sources on cosmic shear estimation: Statistical sensitivity and pixel-noise bias
Javier Sánchez, Ismael Mendoza, D. Kirkby, P. R. Burchat
Abstract
In Stage-IV imaging surveys, a significant amount of the cosmologically\nuseful information is due to sources whose images overlap with those of other\nsources on the sky. The cosmic shear signal is primarily encoded in the\nestimated shapes of observed galaxies and thus directly impacted by overlaps.\nWe introduce a framework based on the Fisher formalism to analyze effects of\noverlapping sources (blending) on the estimation of cosmic shear. For the Rubin\nObservatory Legacy Survey of Space and Time (LSST), we present the expected\nloss in statistical sensitivity for the ten-year survey due to blending. We\nfind that for approximately 62% of galaxies that are likely to be detected in\nfull-depth LSST images, at least 1% of the flux in their pixels is from\noverlapping sources. We also find that the statistical correlations between\nmeasures of overlapping galaxies and, to a much lesser extent the higher shot\nnoise level due to their presence, decrease the effective number density of\ngalaxies, $N_{eff}$, by $\\sim$18%. We calculate an upper limit on $N_{eff}$ of\n39.4 galaxies per arcmin$^2$ in $r$ band. We study the impact of varying\nstellar density on $N_{eff}$ and illustrate the diminishing returns of\nextending the survey into lower Galactic latitudes. We extend the Fisher\nformalism to predict the increase in pixel-noise bias due to blending for\nmaximum-likelihood (ML) shape estimators. We find that noise bias is sensitive\nto the particular shape estimator and measure of ensemble-average shape that is\nused, and properties of the galaxy that include redshift-dependent quantities\nsuch as size and luminosity.