Bias hardened estimators of patchy screening profiles

TL;DR

This paper reviews and develops bias-hardening techniques to mitigate lensing bias in estimators of anisotropic screening of the CMB, crucial for understanding cosmic gas distribution and galaxy formation.

Contribution

It introduces strategies to null lensing bias in stacked estimators, including the optimal approach, applicable to any large-scale structure tracer.

Findings

Lensing bias dominates the signal if unmitigated when stacking on unWISE galaxies.

Existing hardening techniques for quadratic estimators are adapted for stacked estimators.

Proposed strategies effectively null lensing contamination in CMB screening measurements.

Abstract

Detecting anisotropic screening of the cosmic microwave background (CMB) holds the promise of revealing the distribution of gas in the Universe, characterizing the complex processes of galaxy formation and feedback, and studying the epoch of reionization. Estimators for inhomogeneous screening, including some recently proposed small-scale (stacked) estimators, are quadratic or higher order in the CMB temperature or polarization fields and are therefore subject to contamination from CMB lensing. We review the origin of this lensing bias and show that, when stacking on unWISE galaxies, the expected lensing bias dominates the signal if left unmitigated. Hardening techniques that null the lensing bias have been proposed for standard quadratic estimators, whereas only approximate methods have been proposed for stacked estimators. We review these techniques and apply the former to stacked estimators, presenting several strategies (including the optimal strategy) to null lensing contamination when stacking on any large-scale structure (LSS) tracer.