The impact of extragalactic foregrounds on internal delensing of CMB B-mode polarization

TL;DR

This paper investigates how extragalactic foregrounds contaminate matter maps used for CMB B-mode delensing, causing biases in primordial gravitational wave constraints, and proposes mitigation strategies to improve measurement accuracy.

Contribution

It demonstrates the impact of extragalactic foregrounds on delensing bias and introduces effective methods to model and reduce these biases in CMB B-mode analysis.

Findings

Foregrounds induce non-Gaussian biases in delensed B-modes.

Point-source-hardened and shear-only estimators reduce biases below acceptable levels.

Temperature-based reconstructions remain effective for measuring the tensor-to-scalar ratio.

Abstract

The search for primordial $B$-mode polarization of the CMB is limited by the sample variance of $B$-modes produced at later times by gravitational lensing. Constraints can be improved by `delensing': using some proxy of the matter distribution to partially remove the lensing-induced $B$-modes. Current and soon-upcoming experiments will infer a matter map -- at least in part -- from the temperature anisotropies of the CMB. These reconstructions are contaminated by extragalactic foregrounds: radio-emitting galaxies, the cosmic infrared background, or the Sunyaev--Zel'dovich effects. Using the Websky simulations, we show that the foregrounds add spurious power to the angular auto-spectrum of delensed $B$-modes via non-Gaussian higher-point functions, biasing constraints on the tensor-to-scalar ratio, $r$. We consider an idealized experiment similar to the Simons Observatory, with no Galactic or atmospheric foregrounds. After removing point sources detectable at 143 GHz and reconstructing lensing from CMB temperature modes $l<3500$ using a Hu-Okamoto quadratic estimator (QE), we infer a value of $r$ that is $1.5\,\sigma$ higher than the true $r=0$. Reconstructing instead from a minimum-variance ILC map only exacerbates the problem, bringing the bias above $3\,\sigma$. When the $TT$ estimator is co-added with other QEs or with external matter tracers, new couplings ensue which partially cancel the diluted bias from $TT$. We provide a simple and effective prescription to model these effects. In addition, we demonstrate that the point-source-hardened or shear-only QEs can not only mitigate the biases to acceptable levels, but also lead to lower power than the Hu-Okamoto QE after delensing. Thus, temperature-based reconstructions remain powerful tools in the quest to measure $r$.