Position-dependent power spectra of the 21-cm signal from the epoch of
  reionization
  The 21-cm signal from the epoch of reionization is non-Gaussian. Current
radio telescopes are focused on detecting the 21-cm power spectrum, but in the
future the Square Kilometre Array is anticipated to provide a first measurement
of the bispectrum. Previous studies have shown that the position-dependent
power spectrum is a simple and efficient way to probe the squeezed-limit
bispectrum. In this approach, the survey is divided into subvolumes and the
correlation between the local power spectrum and the corresponding mean density
of the subvolume is computed. This correlation is equivalent to an integral of
the bispectrum in the squeezed limit, but is much simpler to implement than the
usual bispectrum estimators. It also has a clear physical interpretation: it
describes how the small-scale power spectrum of tracers such as galaxies and
the 21-cm signal respond to a large-scale environment. Reionization naturally
couples large and small scales as ionizing radiation produced by galactic
sources can travel up to tens of Megaparsecs through the intergalactic medium
during this process. Here we apply the position-dependent power spectrum
approach to fluctuations in the 21-cm background from reionization. We show
that this statistic has a distinctive evolution in time that can be understood
with a simple analytic model. We also show that the statistic can easily
distinguish between simple "inside-out" and "outside-in" models of
reionization. The position-dependent power spectrum is thus a promising method
to validate the reionization signal and to extract higher-order information on
this process.
