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

Becker, George D. and Rauch, Michael and Sargent, Wallace L. W. (2007) The Evolution of Optical Depth in the Lyα Forest: Evidence Against Reionization at z~6. Astrophysical Journal, 662 (1). pp. 72-93. ISSN 0004-637X.

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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.

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Additional Information:© 2007. The American Astronomical Society. Received 2006 July 4; accepted 2007 February 9. Print publication: Issue 1 (2007 June 10). The authors would like to thank Martin Haehnelt for stimulating conversations, as well as Tom Barlow and Rob Simcoe for reducing much of the data. We especially thank the Hawaiian people for the opportunity to observe from Mauna Kea. Without their hospitality, this work would not have been possible. G.D.B. and M.R. have been supported by the NSF under grant AST 05-06845. W.L.W.S. has been supported by the NSF under grant AST 06-06866. 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.
Funding AgencyGrant Number
National Science FoundationAST 05-06845
National Science FoundationAST 06-06866
W.M. Keck FoundationUNSPECIFIED
Subject Keywords:cosmology: observations; early universe; intergalactic medium; quasars: absorption lines
Issue or Number:1
Record Number:CaltechAUTHORS:BECapj07
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13094
Deposited By: Archive Administrator
Deposited On:17 Jan 2009 07:01
Last Modified:03 Oct 2019 00:34

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