Foreground-immune CMB lensing reconstruction with polarization

Noah Sailer, Simone Ferraro, Emmanuel Schaan

Submitted on 7 November 2022


Extragalactic foregrounds are known to generate significant biases in temperature-based CMB lensing reconstruction. Several techniques, which include ``source hardening'' and ``shear-only estimators'' have been proposed to mitigate contamination and have been shown to be very effective at reducing foreground-induced biases. Here we extend both techniques to polarization, which will be an essential component of CMB lensing reconstruction for future experiments, and investigate the ``large-lens'' limit analytically to gain insight on the origin and scaling of foreground biases, as well as the sensitivity to their profiles.Using simulations of polarized point sources, we estimate the expected bias to both Simons Observatory and CMB-S4 like (polarization-based) lensing reconstruction, finding that biases to the former are minuscule while those to the latter are potentially non-negligible at small scales (L10002000). In particular, we show that for a CMB-S4 like experiment, an optimal linear combination of point-source hardened estimators can reduce the (point-source induced) bias to the CMB lensing power spectrum by up to two orders of magnitude, at a 4% noise cost relative to the global minimum variance estimator.


Comment: 18 pages, 7 figures

Subject: Astrophysics - Cosmology and Nongalactic Astrophysics


\textit{Top:} The angular averaged temperature-only lensing weights ($\times\,2\pi N^\kappa_{\bm{0}}\ell$) in the large-lens limit $(L\to0)$ when hardening against a point source (blue), hardening against a Gaussian profile with a $2'$ FHWM (purple) and when choosing the weights to minimize the variance (red). Note that since the point source hardened estimator has zero response to point-sources (when $w_\ell = 1$) the area under the blue curve vanishes. \textit{Bottom:} The response to a Gaussian foreground $w_\ell$ with a full width half max $\theta_{\text{FHHM}}$, normalized to the response of the MV estimator to point sources. In both plots we take $\ell_{\text{max},T}=3500$. See section \ref{sec:noise} for details regarding the instrumental assumptions.