The wedge bias in reionization 21-cm power spectrum measurements

TL;DR

This paper quantifies the wedge bias in 21-cm power spectrum measurements during reionization, revealing significant overestimation at high redshifts and smaller biases later, impacting interpretation of observational data.

Contribution

It introduces the first calculation of the wedge bias in 21-cm power spectrum measurements, highlighting its redshift-dependent magnitude and implications for reionization studies.

Findings

Bias is up to 100% at high redshifts.

Bias becomes negative and smaller (~20%) later in reionization.

Bias shows weak dependence on scale and topology.

Abstract

A proposed method for dealing with foreground emission in upcoming 21-cm observations from the epoch of reionization is to limit observations to an uncontaminated window in Fourier space. Foreground emission can be avoided in this way, since it is limited to a wedge-shaped region in $k_{\parallel}, k_{\perp}$ space. However, the power spectrum is anisotropic owing to redshift-space distortions from peculiar velocities. Consequently, the 21-cm power spectrum measured in the foreground avoidance window---which samples only a limited range of angles close to the line-of-sight direction---differs from the full spherically-averaged power spectrum which requires an average over \emph{all} angles. In this paper, we calculate the magnitude of this "wedge bias" for the first time. We find that the bias is strongest at high redshifts, where measurements using foreground avoidance will over-estimate the power spectrum by around 100 per cent, possibly obscuring the distinctive rise and fall signature that is anticipated for the spherically-averaged 21-cm power spectrum. In the later stages of reionization, the bias becomes negative, and smaller in magnitude ($\lesssim 20$ per cent). The effect shows only a weak dependence on spatial scale and reionization topology.