SKA Weak Lensing III: Added Value of Multi-Wavelength Synergies for the Mitigation of Systematics

TL;DR

This paper explores how combining radio and optical/near-IR weak lensing data enhances systematic error mitigation, improves cosmological parameter accuracy, and extends the redshift range for studying dark energy.

Contribution

It demonstrates the added value of multi-wavelength synergies in reducing systematics and probing higher redshifts in radio weak lensing surveys for cosmology.

Findings

Multi-wavelength analysis provides effective self-calibration for systematics.

Radio-optical cross-correlation can detect residual systematics at very low variance.

Radio surveys reach higher redshifts, reducing non-linear contamination in cosmological measurements.

Abstract

In this third paper of a series on radio weak lensing for cosmology with the Square Kilometre Array, we scrutinise synergies between cosmic shear measurements in the radio and optical/near-IR bands for mitigating systematic effects. We focus on three main classes of systematics: (i) experimental systematic errors in the observed shear; (ii) signal contamination by intrinsic alignments; and (iii) systematic effects due to an incorrect modelling of non-linear scales. First, we show that a comprehensive, multi-wavelength analysis provides a self-calibration method for experimental systematic effects, only implying <50% increment on the errors on cosmological parameters. We also illustrate how the cross-correlation between radio and optical/near-IR surveys alone is able to remove residual systematics with variance as large as 0.00001, i.e. the same order of magnitude of the cosmological signal. This also opens the possibility of using such a cross-correlation as a means to detect unknown experimental systematics. Secondly, we demonstrate that, thanks to polarisation information, radio weak lensing surveys will be able to mitigate contamination by intrinsic alignments, in a way similar but fully complementary to available self-calibration methods based on position-shear correlations. Lastly, we illustrate how radio weak lensing experiments, reaching higher redshifts than those accessible to optical surveys, will probe dark energy and the growth of cosmic structures in regimes less contaminated by non-linearities in the matter perturbations. For instance, the higher-redshift bins of radio catalogues peak at z~0.8-1, whereas their optical/near-IR counterparts are limited to z<0.5-0.7. This translates into having a cosmological signal 2 to 5 times less contaminated by non-linear perturbations.