The Evolution of Optical Depth in the Lyα Forest: Evidence Against Reionization at z~6
Abstract
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.
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.Attached Files
Published - BECapj07.pdf
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Additional details
- Eprint ID
- 13094
- Resolver ID
- CaltechAUTHORS:BECapj07
- National Science Foundation
- AST 05-06845
- National Science Foundation
- AST 06-06866
- W.M. Keck Foundation
- Created
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2009-01-17Created from EPrint's datestamp field
- Updated
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2021-11-08Created from EPrint's last_modified field