The Evolution of Optical Depth in the Lyα Forest: Evidence Against Reionization at z~6^*

We examine the evolution of the IGM Lyα optical depth distribution using the transmitted flux probability distribution function (PDF) in a high-resolution sample of 55 QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two theoretical τ distributions: a model distribution based on the density distribution of Miralda-Escudé et al. (2000; MHR00) and a lognormal distribution. Assuming a spatially uniform UV background and an isothermal IGM, as was done in previous works where transmitted flux statistics have been used to infer an end to cosmic reionization at z ~ 6, the MHR00 model fails to reproduce the observed flux PDFs at redshifts where the optical depth distribution is well sampled unless large continuum corrections are applied. A lognormal τ distribution, in contrast, fits the data well at all redshifts with only minor continuum adjustments. Extrapolating the evolution of the lognormal distribution at z < 5.4 predicts the observed upturn in the Lyα and Lyβ effective optical depths at z > 5.7, while simultaneously reproducing the mean transmitted flux down to z = 1.6. In this empirical sense, the evolution of the Lyα forest at z ~ 6 is consistent with observed trends at lower redshift. If the evolution of the forest at z ≲ 5 reflects a slowly evolving density field, temperature, and UV background, then no sudden change in the IGM, such as one due to late reionization, appears necessary to explain the disappearance of transmitted flux at z ~ 6. If the MHR00 density distribution is correct, then a nonuniform UV background and/or IGM temperature may be required to produce the correct distribution of optical depths. We find that an inverse temperature-density relation for the MHR00 model significantly improves the PDF fits, but with a large scatter in the equation-of-state index.