2005ApJ...631...59P

We use a ΛCDM hydrodynamic cosmological simulation in a 9.6 comoving Mpc box to examine the evolution of the intergalactic medium transmissivity from z=2.5 up to the epoch of reionization (z≳6.5). Ionization is due to a uniform ultraviolet background from an evolving mix of stellar and QSO populations. We synthesize noiseless H I Lyα absorption spectra by casting lines of sight through our continuously evolving box and analyze the statistical properties of the transmitted flux. We find a smooth evolution of the effective optical depth of the form τ_eff∝1+z/6^4.16±0.02 up to reionization. During reionization, the mean transmission and variance sharply deviate from an extrapolated smooth evolution. This introduces large margins of error that would require an unobtainable number of lines of sight to minimize. Subsampling in ``high'' and ``low'' transmission lines of sight results in either inferring the continuation of a smooth profile or underestimating the global transmissivity during reionization, respectively. Nonetheless, the end of an opacity phase transition of the intergalactic medium correlates well with the redshift when both the mean and variance sharply deviate from a smooth profile. The lower bound estimates of the flux variance in this work also suggest that it is a more sensitive tool than the mean transmission in mapping reionization. In addition to optical transmission, we derive the ``dark gap'' length distribution, which is in agreement with observations. We also find a positive correlation between the mean gap optical depth and the gap length.