The Impact of Light Polarization Effects on Weak Lensing Systematics

TL;DR

This paper investigates how light polarization effects from galaxy observations and optical system interactions can introduce systematic errors in weak lensing measurements, particularly affecting the WFIRST mission.

Contribution

The study provides the first estimates of polarization-induced systematics on PSF shapes and galaxy ellipticity measurements relevant to weak lensing surveys.

Findings

Systematic uncertainties are $8\times 10^{-5}$ and $1.1 \times 10^{-4}$ for selection bias and PSF errors.

These uncertainties are close to WFIRST's tolerance level of $4.7\times 10^{-4}$ per component.

More detailed studies or stricter polarization control are needed for WFIRST.

Abstract

A fraction of the light observed from edge-on disk galaxies is polarized due to two physical effects: selective extinction by dust grains aligned with the magnetic field, and scattering of the anisotropic starlight field. Since the reflection and transmission coefficients of the reflecting and refracting surfaces in an optical system depend on the polarization of incoming rays, this optical polarization produces both (a) a selection bias in favor of galaxies with specific orientations and (b) a polarization-dependent PSF. In this work we build toy models to obtain for the first time an estimate for the impact of polarization on PSF shapes and the impact of the selection bias due to the polarization effect on the measurement of the ellipticity used in shear measurements. In particular, we are interested in determining if this effect will be significant for WFIRST. We show that the systematic uncertainties in the ellipticity components are $8\times 10^{-5}$ and $1.1 \times 10^{-4}$ due to the selection bias and PSF errors respectively. Compared to the overall requirements on knowledge of the WFIRST PSF ellipticity ($4.7\times 10^{-4}$ per component), both of these systematic uncertainties are sufficiently close to the WFIRST tolerance level that more detailed studies of the polarization effects or more stringent requirements on polarization-sensitive instrumentation for WFIRST are required.