Reionization on Large Scales I: A Parametric Model Constructed from
  Radiation-Hydrodynamic Simulations
  We present a new method for modeling inhomogeneous cosmic reionization on
large scales. Utilizing high-resolution radiation-hydrodynamic simulations with
2048^3 dark matter particles, 2048^3 gas cells, and 17 billion adaptive rays in
a L = 100 Mpc/h box, we show that the density and reionization-redshift fields
are highly correlated on large scales (>~ 1 Mpc/h). This correlation can be
statistically represented by a scale-dependent linear bias. We construct a
parametric function for the bias, which is then used to filter any large-scale
density field to derive the corresponding spatially varying
reionization-redshift field. The parametric model has three free parameters
which can be reduced to one free parameter when we fit the two bias parameters
to simulations results. We can differentiate degenerate combinations of the
bias parameters by combining results for the global ionization histories and
correlation length between ionized regions. Unlike previous semi-analytic
models, the evolution of the reionization-redshift field in our model is
directly compared cell by cell against simulations and preforms well in all
tests. Our model maps the high-resolution, intermediate-volume
radiation-hydrodynamic simulations onto lower-resolution, larger-volume N-body
simulations (>~ 2 Gpc/h) in order to make mock observations and theoretical
predictions.
