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

We examine the constraints on the epoch of reionization (redshift z_r) set by recent WMAP-3 observations of tau_e = 0.09 +/- 0.03, the electron-scattering optical depth of the cosmic microwave background (CMB), combined with models of high-redshift galaxy and black hole formation. Standard interpretation begins with the computed optical depth, tau_e = 0.042 +/- 0.003, for a fully ionized medium out to z_r = 6.1 +/- 0.15, including ionized helium, which recombines at z ~ 3. At z > z_r, one must also consider scattering off electrons produced by from early black holes (X-ray pre-ionization) and from residual electrons left from incomplete recombination. Inaccuracies in computing the ionization history, x_e(z) add systematic sources of uncertainty in tau_e. The required scattering at z > z_r 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 Delta-tau_e < 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 (Ly-alpha) absorption in quasar spectra are consistent with an H~I reionization epoch at z_r ~ 6 providing 50% of the total tau_e at z < z_r, preceded by a partially ionized medium at z ~ 6-20.