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Impact of extragalactic foregrounds on internal delensing of the CMB B-mode polarization


The search for primordial B-mode polarization of the cosmic microwave background (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 autospectrum 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 1.5σ higher than the true r=0. Reconstructing instead from a minimum-variance internal linear combination map only exacerbates the problem, bringing the bias above 3σ. 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.

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