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ABSTRACT Modelling reionization often requires significant assumptions about the properties of ionizing sources. Here, we infer the total output of hydrogen-ionizing photons (the ionizing emissivity, Ṅ_ ion) at z = 4–14 from current reionization constraints, being maximally agnostic to the properties of ionizing sources. We use a Bayesian analysis to fit for a non-parametric form of Ṅ_ ion, allowing us to flexibly explore the entire prior volume. We infer a declining Ṅ_ ion with redshift at z 6, which can be used as a benchmark for reionization models. Model-independent reionization constraints from the cosmic microwave background (CMB) optical depth and Ly α and Ly β forest dark pixel fraction produce Ṅ_ ion evolution (d ₁₀ Ṅ ₈₎₍/ dz|ₙ=₆ ₈ = -0. 31 0. 35 dex) consistent with the declining UV luminosity density of galaxies, assuming constant ionizing photon escape fraction and efficiency. Including measurements from Ly α damping of galaxies and quasars produces a more rapid decline: d ₁₀ Ṅ ₈₎₍/ dz|ₙ=₆ ₈ =-0. 44 0. 22 dex, steeper than the declining galaxy luminosity density (if extrapolated beyond M_ UV -13), and constrains the mid-point of reionization to z = 6. 93 ± 0. 14.
Mason et al. (Wed,) studied this question.