Quantifying Density-Ionization Correlations with the 21cm Power Spectrum

TL;DR

This paper introduces a new way to quantify the relationship between density and ionization during the Epoch of Reionization and forecasts how current and future observations can constrain this correlation through the 21cm power spectrum.

Contribution

It proposes a parametrization for density-ionization correlation and analyzes its impact on the 21cm power spectrum, enabling better interpretation of observational data.

Findings

Upper limits on the power spectrum can rule out uncorrelated reionization at 99% credibility.

HERA can place strong constraints on the sign and magnitude of the correlation.

Error bars of ~20 mK^2 at certain scales are achievable with current data.

Abstract

The Epoch of Reionization (EoR)---when neutral hydrogen in the intergalactic medium was systematically ionized---is a period in our Universe's history that is currently poorly understood. However, a key prediction of most models is a correlation between the density and ionization field during the EoR. This has consequences for the 21cm power spectrum. Here, we propose a parametrization for the density-ionization correlation and study the dependence of the 21cm power spectrum on this parameterization. We use this formalism to forecast the ability of current and future observations to constrain these correlations. We find that upper limits on the dimensionless power spectrum at redshifts $7.5 < z < 8.5$ using $k$ bins between $0.1\,\textrm{Mpc}^{-1} < k < 0.75\,\textrm{Mpc}^{-1}$ with error bars at the level of $\sim\!\!20\,\textrm{mK}^2$ about our fiducial model would rule out uncorrelated reionization at $99\%$ credibility. Beyond upper limits, we find that at its full sensitivity, the Hydrogen Epoch of Reionization Array (HERA) will be able to place strong constraints on the sign and magnitude of density-ionization correlations.