SKA weak lensing - III. Added value of multiwavelength synergies for the mitigation of systematics

In this third paper of a series on radio weak lensing for cosmology with the Square Kilometre Array, we scrutinize synergies between cosmic shear measurements in the radio and optical/near-infrared (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, multiwavelength analysis provides a self-calibration method for experimental systematic effects, only implying < 50 per cent 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 10^-5, 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 polarization 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 2-5 times less contaminated by non-linear perturbations.