Reionization on Large Scales I: A Parametric Model Constructed from Radiation-Hydrodynamic Simulations

Abstract: 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.