Quantifying Density-Ionization Correlations with the 21cm Power Spectrum
  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.
