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

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 $r$.

23 pages plus appendix and bibliography. 17 figures. Very minor changes to match version published in PRD

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• CMB-S4: Iterative Internal Delensing and $r$ Constraints
• Foreground-immune CMB lensing reconstruction with polarization
• LiteBIRD Science Goals and Forecasts: Improving Sensitivity to Inflationary Gravitational Waves with Multitracer Delensing
• CMB lensing with shear-only reconstruction on the full sky
• Subtracting the kinetic Sunyaev-Zeldovich effect from the cosmic microwave background with surveys of large-scale structure
• New probe of inflationary gravitational waves: cross-correlations of lensed primary CMB B-modes with large-scale structure
• A halo model of extragalactic contamination to CMB lensing, delensing, and cross-correlations
• Stage IV CMB forecasts for warm inflation
• Delensing for Precision Cosmology: Optimizing Future CMB B-mode Surveys to Constrain r
• LiteBIRD science goals and forecasts: improved full-sky reconstruction of the gravitational lensing potential through the combination of Planck and LiteBIRD data
• Delensing CMB B-modes using galaxy surveys: the effect of galaxy bias and matter clustering non-linearities