Effects of Small-Scale Absorption Systems on Neutral Islands during the
  Late Epoch of Reionization
  The reionization process is expected to be prolonged by the small-scale
absorbers (SSAs) of ionizing photons, which have been seen as Lyman-limit
systems in quasar absorption line observations. We use a set of semi-numerical
simulations to investigate the effects of absorption systems on the
reionization process, especially their impacts on the neutral islands during
the late Epoch of Reionization (EoR). Three models are studied, i.e. the
extreme case of no-SSA model with a high level of ionizing background, the
moderate-SSA model with a relatively high level of ionizing background, and the
dense-SSA model with a low level of ionizing background. We find that while the
characteristic scale of neutral regions decreases during the early and middle
stages of reionization, it stays nearly unchanged at about 10 comoving Mpc
during the late stage for the no-SSA and moderate-SSA models. However, in the
case of weak ionizing background in the dense-SSA model, the characteristic
island scale shows obvious evolution, as large islands break into many small
ones that are slowly ionized. The evolutionary behavior of neutral islands
during the late EoR thus provides a novel way to constrain the abundance of
SSAs. We discuss the 21-cm observation with the upcoming Square Kilometre
Array. The different models can be distinguished by either the 21-cm imaging or
the 21-cm power spectrum measurements.
