Optical Depth of the Cosmic Microwave Background and Reionization of the Intergalactic Medium

We examine the constraints on the epoch of reionization (redshift zr) set by recent WMAP-3 observations of τe = 0.09 ± 0.03, the electron-scattering optical depth of the cosmic microwave background (CMB), combined with models of highredshift galaxy and black hole formation. Standard interpretation begins with the computed optical depth, τe = 0.042± 0.003, for a fully ionized medium out to zr = 6.1±0.15, including ionized helium, which recombines at z ≈ 3. At z > zr, one must also consider scattering off electrons produced by from early black holes (Xray pre-ionization) and from residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, xe(z) add systematic sources of uncertainty in τe. The required scattering at z > zr can be used to constrain the ionizing contributions of “first light” sources. In high-z galaxies, the star-formation efficiency, the rate of ionizing photon production, and the photon escape fraction are limited to producing no more than ∆τe ≤ 0.03±0.03. The contribution of minihalo star formation and black-hole X-ray preionization at z = 10–20 are suppressed by factors of 5–10 compared to recent models. Both the CMB optical depth and H I (Lyα) absorption in quasar spectra are consistent with an H I reionization epoch at zr ≈ 6 providing ∼ 50% of the total τe at z ≤ zr, preceded by a partially ionized medium at z ≈ 6–20. Subject headings: cosmology: theory — cosmic microwave background — intergalactic medium Now at Department of Physics, 2130 Fulton St., University of San Francisco, San Francisco, CA 94117