IslandFAST: A Semi-numerical Tool for Simulating the Late Epoch of
  Reionization
  We present the algorithm and main results of our semi-numerical simulation,
islandFAST, which is developed from the 21cmFAST and designed for the late
stage of reionization. The islandFAST predicts the evolution and size
distribution of the large scale under-dense neutral regions (neutral islands),
and we find that the late Epoch of Reionization (EoR) proceeds very fast,
showing a characteristic scale of the neutral islands at each redshift. Using
islandFAST, we compare the impact of two types of absorption systems, i.e. the
large scale under-dense neutral islands versus small scale over-dense
absorbers, in regulating the reionization process. The neutral islands dominate
the morphology of the ionization field, while the small scale absorbers
dominate the mean free path of ionizing photons, and also delay and prolong the
reionization process. With our semi-numerical simulation, the evolution of the
ionizing background can be derived self-consistently given a model for the
small absorbers. The hydrogen ionization rate of the ionizing background is
reduced by an order of magnitude in the presence of dense absorbers.
