21-cm signatures of residual HI inside cosmic HII regions during
  reionization
  We investigate the impact of sinks of ionizing radiation on the
reionization-era 21-cm signal, focusing on 1-point statistics. We consider
sinks in both the intergalactic medium and inside galaxies. At a fixed filling
factor of HII regions, sinks will have two main effects on the 21-cm
morphology: (i) as inhomogeneous absorbers of ionizing photons they result in
smaller and more widespread cosmic HII patches; and (ii) as reservoirs of
neutral gas they contribute a non-zero 21-cm signal in otherwise ionized
regions. Both effects damp the contrast between neutral and ionized patches
during reionization, making detection of the epoch of reionization with 21-cm
interferometry more challenging. Here we systematically investigate these
effects using the latest semi-numerical simulations. We find that sinks
dramatically suppress the peak in the redshift evolution of the variance,
corresponding to the midpoint of reionization. As previously predicted,
skewness changes sign at midpoint, but the fluctuations in the residual HI
suppress a late-time rise. Furthermore, large levels of residual HI
dramatically alter the evolution of the variance, skewness and power spectrum
from that seen at lower levels. In general, the evolution of the large-scale
modes provides a better, cleaner, higher signal-to-noise probe of reionization.
