Bias to CMB lensing from lensed foregrounds

TL;DR

This paper investigates the bias in CMB lensing measurements caused by lensed extragalactic foregrounds, quantifies its impact for upcoming surveys, and explores mitigation strategies to reduce this bias.

Contribution

It provides the first quantification of the lensed foreground bias for standard CMB lensing estimators in the context of the Simons Observatory, without multi-frequency separation.

Findings

Lensed foreground bias can reach percent-level in CMB lensing measurements.

The bias significantly affects cross-correlation with LSST galaxies.

Bias-hardening methods can mitigate the bias but reduce signal-to-noise.

Abstract

Extragalactic foregrounds are known to constitute a limiting systematic in temperature-based CMB lensing with AdvACT, SPT-3G, Simons Observatory and CMB S4. Furthermore, since these foregrounds are emitted at cosmological distances, they are also themselves lensed. The correlation between this foreground lensing and CMB lensing causes an additional bias in CMB lensing estimators. In this paper, we quantify for the first time this "lensed foreground bias" for the standard CMB lensing quadratic estimator, the CMB shear and the CMB magnification estimators, in the case of Simons Observatory and in the absence of multi-frequency component separation. This percent-level bias is highly significant in cross-correlation of CMB lensing with LSST galaxies, and comparable to the statistical uncertainty in CMB lensing auto-spectrum. We discuss various mitigation strategies, and show that "lensed foreground bias-hardening" methods can reduce this bias at some cost in signal-to-noise. The code used to generate our theory curves is publicly available at https://github.com/EmmanuelSchaan/LensedForegroundBias .