Constraining the Evolution of the Ionizing Background and the Epoch of Reionization with z ∼ 6 Quasars II: A Sample of 19 Quasars

We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate resolution spectra of a sample of nineteen quasars at 5.74 < zem < 6.42 discovered in the Sloan Digital Sky Survey. Three methods are used to trace IGM properties: (a) the evolution of the Gunn-Peterson (GP) optical depth in the Lyα, β, and γ transitions; (b) the distribution of lengths of dark absorption gaps, and (c) the size of HII regions around luminous quasars. Using this large sample, we find that the evolution of the ionization state of the IGM accelerated at z > 5.7: the GP optical depth evolution changes from τ eff GP ∼ (1 + z) 4.3 to (1 + z)∼>11, and the average length of dark gaps with τ > 3.5 increases from < 10 to > 80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of ∼> 4 in the UV background Based on observations obtained with the Sloan Digital Sky Survey; at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration, made possible by the generous financial support of the W.M. Keck Foundation, with the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution, and with the Kitt Peak National Observatory 4-meter Mayall Telescope. This paper is dedicated to the memory of John N. Bahcall. Steward Observatory, The University of Arizona, Tucson, AZ 85721; [email protected] Princeton University Observatory, Princeton, NJ 08544 Physics Department, University of California, Davis, CA 95616 IGPP/Lawrence Livermore National Laboratory, Livermore, CA 94550 Space Telescope Science Institute, Baltimore, MD 21218 Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 Apache Point Observatory, P. O. Box 59, Sunspot, NM 88349-0059 Institute for Cosmic Ray Research, University of Tokyo, Midori, Tanashi, Tokyo 188-8502, Japan