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Published October 10, 2001 | Published
Journal Article Open

On the Threshold of the Reionization Epoch


Discovery of the cosmic reionization epoch would represent a significant milestone in cosmology. We present Keck spectroscopy of the quasar SDSS 1044-0125, at z = 5.73. The spectrum shows a dramatic increase in the optical depth at observed wavelengths λ ≳ 7550 Å, corresponding to z_(abs) ≳ 5.2. Only a few small, narrow transmission regions are present in the spectrum beyond that point and out to the redshifts where the quasar signal begins. We interpret this result as a signature of the trailing edge of the cosmic reionization epoch, which we estimate to occur around 〈z〉~ 6 (as indeed confirmed by subsequent observations by Becker et al.) and extending down to z ~ 5.2. This behavior is expected in the modern theoretical models of the reionization era, which predict a patchy and gradual onset of reionization. The remaining transmission windows we see may correspond to the individual reionization bubbles (Strömgren spheres) embedded in a still largely neutral intergalactic medium, intersected by the line of sight to the quasar. Future spectroscopic observations of quasars at comparable or larger redshifts will provide a more detailed insight into the structure and extent of the reionization era.

Additional Information

© 2001 The American Astronomical Society. Received 2001 July 26; accepted 2001 August 30; published 2001 September 13. Based on the observations obtained at the W. M. Keck Observatory, which is operated by the California Association for Research in Astronomy, a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. We thank the staff of the W. M. Keck Observatory for their expert assistance. The LRIS data were obtained in the course of a collaborative project with F. Harrison and P. Mao. We thank numerous colleagues whose constructive comments helped us improve the discussion presented in this Letter. S. G. D. acknowledges partial funding from the Bressler Foundation. The work of D. S. was carried out at the Jet Propulsion Laboratory, Caltech, under a contract with NASA.

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