Weak Lensing with Sizes, Magnitudes and Shapes

TL;DR

This paper develops a Bayesian method to infer cosmic convergence from galaxy size, magnitude, and redshift data, demonstrating its potential to enhance dark energy studies by combining magnification with shear measurements.

Contribution

It introduces a novel Bayesian approach for convergence inference using size and magnitude data, including a model for their joint distribution and an analysis of systematic effects.

Findings

Estimated convergence dispersion ~0.8, larger than shear (~0.38).

Magnification can significantly improve dark energy constraints when combined with shear.

Different systematics for shear and magnification make their combination advantageous.

Abstract

Weak lensing can be observed through a number of effects on the images of distant galaxies; their shapes are sheared, their sizes and fluxes (magnitudes) are magnified and their positions on the sky are modified by the lensing field. Galaxy shapes probe the shear field whilst size, magnitude and number density probe the convergence field. Both contain cosmological information. In this paper we are concerned with the magnification of the size and magnitude of individual galaxies as a probe of cosmic convergence. We develop a Bayesian approach for inferring the convergence field from a measured size, magnitude and redshift and demonstrate that the inference on convergence requires detailed knowledge of the joint distribution of intrinsic sizes and magnitudes. We build a simple parameterised model for the size-magnitude distribution and estimate this distribution for CFHTLenS galaxies. In light of the measured distribution, we show that the typical dispersion on convergence estimation is ~0.8, compared to ~0.38 for shear. We discuss the possibility of physical systematics for magnification (similar to intrinsic alignments for shear) and compute the expected gains in the Dark Energy Figure-of-Merit (FoM) from combining magnification with shear for different scenarios regarding systematics: when accounting for intrinsic alignments but no systematics on the magnification signal, including magnification could improve the FoM by upto a factor of ~2.5, whilst when accounting for physical systematics in both shear and magnification we anticipate a gain between ~25% and ~65%. In addition to the statistical gains, the fact that cosmic shear and magnification are subject to different systematics makes magnification an attractive complement to any cosmic shear analysis.