X iv : a st ro - p h / 06 07 63 3 v 1 2 8 Ju l 2 00 6 The Evolution of Optical Depth in the Ly α Forest : Evidence Against Reionization

We examine the evolution of the IGM Lyα optical depth distribution using the transmitted flux probability distribution function (PDF) in a sample of 63 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. We assume a uniform UV background and an isothermal IGM for the MHR00 model, as has been done in previous works where transmitted flux statistics have been used to infer an abrupt change in the IGM at z ∼ 6. Under these assumptions, the MHR00 model produces poor fits to the observed flux PDFs at redshifts where the optical depth distribution is well sampled, unless large continuum corrections are applied. However, the lognormal τ distribution fits the data at all redshifts with only minor continuum adjustments. We use a simple parametrization for the evolution of the lognormal parameters to calculate the expected mean transmitted flux at z > 5.4. The lognormal τ distribution predicts the observed Lyα and Lyβ effective optical depths at z > 5.7 while simultaneously fitting the mean transmitted flux down to z = 1.6. In contrast, the best-fitting power-law under-predicts the amount of absorption both at z > 5.7 and at z < 2.5. If the evolution of the lognormal distribution at z < 5 reflects a slowly-evolving density field, temperature, and UV background, then no sudden change in the IGM at z ∼ 6 due to late reionization appears necessary. We have used the lognormal optical depth distribution without any assumption about the underlying density field. If the MHR00 density distribution is correct, then a non-uniform UV background and/or IGM temperature may be required to produce the correct flux The observations were made at the W.M. Keck Observatory which is operated as a scientific partnership between the California Institute of Technology and the University of California; it was made possible by the generous support of the W.M. Keck Foundation. Palomar Observatory, California Institute of Technology, Pasadena, CA 91125, USA; [email protected], [email protected] Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA; [email protected]