Weak Lensing Reconstruction by Counting DECaLS Galaxies

TL;DR

This paper demonstrates a novel method to reconstruct weak lensing maps using cosmic magnification in DECaLS galaxies, achieving significant detection of lensing signals through cross-correlation with shear data.

Contribution

It introduces a new approach to weak lensing reconstruction via cosmic magnification, effectively handling galaxy bias and shot noise, and validates the method with real survey data.

Findings

Detected lensing convergence-shear cross-correlation with S/N ≈ 10

Suppressed galaxy intrinsic clustering by a factor of 100

Imaging systematics correction improved detection by 30%

Abstract

Alternative to weak lensing measurements through cosmic shear, we present a weak lensing convergence $\hat{\kappa}$ map reconstructed through cosmic magnification effect in DECaLS galaxies of the DESI imaging surveys DR9. This is achieved by linearly weighing $12$ maps of galaxy number overdensity in different magnitude bins of $grz$ photometry bands. The weight is designed to eliminate the mean galaxy deterministic bias, minimize galaxy shot noise while maintaining the lensing convergence signal. We also perform corrections of imaging systematics in the galaxy number overdensity. The $\hat{\kappa}$ map has $8365$ deg$^2$ sky coverage. Given the low number density of DECaLS galaxies, the $\hat{\kappa}$ map is overwhelmed by shot noise and the map quality is difficult to evaluate using the lensing auto-correlation. Alternatively, we measure its cross-correlation with the cosmic shear catalogs of DECaLS galaxies of DESI imaging surveys DR8, which has $8365$ deg$^2$ overlap in sky coverage with the $\hat{\kappa}$ map. We detect a convergence-shear cross-correlation signal with $S/N\simeq 10$. The analysis also shows that the galaxy intrinsic clustering is suppressed by a factor $\mathcal{O}(10^2)$ and the residual galaxy clustering contamination in the $\hat{\kappa}$ map is consistent with zero. Various tests with different galaxy and shear samples, and the Akaike information criterion analysis all support the lensing detection. So is the imaging systematics corrections, which enhance the lensing signal detection by $\sim 30\%$. We discuss various issues for further improvement of the measurements.