The Evolution of Optical Depth in the Lyα Forest: Evidence Against Reionization atz∼6

We examine the evolution of the IGM Ly-alpha optical depth distribution usingthe transmitted flux probability distribution function (PDF) in a sample of 63QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to twotheoretical optical depth distributions: a model distribution based on thedensity distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormaldistribution. We assume a uniform UV background and an isothermal IGM for theMHR00 model, as has been done in previous works. Under these assumptions, theMHR00 model produces poor fits to the observed flux PDFs at redshifts where theoptical depth distribution is well sampled, unless large continuum correctionsare applied. However, the lognormal optical depth distribution fits the data atall redshifts with only minor continuum adjustments. We use a simpleparametrization for the evolution of the lognormal parameters to calculate theexpected mean transmitted flux at z > 5.4. The lognormal optical depthdistribution predicts the observed Ly-alpha and Ly-beta effective opticaldepths at z > 5.7 while simultaneously fitting the mean transmitted flux downto z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects aslowly-evolving density field, temperature, and UV background, then no suddenchange in the IGM at z ~ 6 due to late reionization appears necessary. We haveused the lognormal optical depth distribution without any assumption about theunderlying density field. If the MHR00 density distribution is correct, then anon-uniform UV background and/or IGM temperature may be required to produce thecorrect flux PDF. We find that an inverse temperature-density relation greatlyimproves the PDF fits, but with a large scatter in the equation of state index.[Abridged]Comment: 45 pages, 16 figures, submitted to Ap