Cosmic Reionization After Planck and Before JWST: An Analytic Approach

Abstract: The reionization of cosmic hydrogen marks a critical juncture in the history of structure formation in the universe. Here we present a new formulation of the standard reionization equation for the evolution of the volume-averaged HII fraction that is more consistent with the accepted conceptual model of inhomogeneous intergalactic absorption. The revised equation retains the basic terminology and simplicity of the classic calculation but explicitly accounts for the presence of the optically thick "Lyman-limit systems" that are known to determine the mean free path of ionizing radiation after overlap. Integration of this equation provides a better characterization of the timing of reionization by smoothly linking the pre-overlap with the post-overlap phases of such process. We confirm the validity of the quasi-instantaneous approximation as predictor of reionization completion/maintenance, and discuss new insights on the sources of cosmic reionization using the improved formalism. A constant emission rate into the intergalactic medium (IGM) of 3 Lyman continuum (LyC) photons per atom per Gyr leads to a reionization history that is consistent with a number of observational constraints on the ionization state of the z=5-9 universe and with the reduced Thomson scattering optical depth recently reported by the Planck Collaboration. While star-forming galaxies can dominate the reionization process if the luminosity-weighted fraction of LyC photons that escape into the IGM, f_esc, exceeds 15% (for a faint magnitude cut-off of the galaxy UV luminosity function of M_lim=-13 and a LyC photon yield per unit 1500 AA luminosity of xi_ion=10^{25.3} Hz/erg, simple models where the product of the two unknowns f_esc xi_ion is not evolving with redshift fail to reproduce the changing neutrality of the IGM observed at these epochs.