Estimating the weak-lensing rotation signal in radio cosmic shear surveys

TL;DR

This paper introduces a novel estimator for the weak-lensing rotation mode in radio surveys using polarization data, enabling new tests of cosmological models and systematics control.

Contribution

The paper develops the first estimator for the rotation mode in weak lensing, utilizing radio polarization measurements to detect physics beyond $\Lambda$CDM.

Findings

Estimator can detect rotation signals consistent with Planck constraints

Simulation results demonstrate potential for systematics control in radio surveys

Rotation mode offers a new probe for physics beyond standard cosmology

Abstract

Weak lensing has become an increasingly important tool in cosmology and the use of galaxy shapes to measure cosmic shear has become routine. The weak-lensing distortion tensor contains two other effects in addition to the two components of shear: the convergence and rotation. The rotation mode is not measurable using the standard cosmic shear estimators based on galaxy shapes, as there is no information on the original shapes of the images before they were lensed. Due to this, no estimator has been proposed for the rotation mode in cosmological weak-lensing surveys, and the rotation mode has never been constrained. Here, we derive an estimator for this quantity, which is based on the use of radio polarisation measurements of the intrinsic position angles of galaxies. The rotation mode can be sourced by physics beyond $\Lambda$CDM, and also offers the chance to perform consistency checks of $\Lambda$CDM and of weak-lensing surveys themselves. We present simulations of this estimator and show that, for the pedagogical example of cosmic string spectra, this estimator could detect a signal that is consistent with the constraints from Planck. We examine the connection between the rotation mode and the shear $B$-modes and thus how this estimator could help control systematics in future radio weak-lensing surveys.