Papers › Model-independent constraints on the hydrogen-ionizing emissivity at z>6
Model-independent constraints on the hydrogen-ionizing emissivity at z>6
Charlotte A. Mason, Rohan P. Naidu, Sandro Tacchella, Joel Leja
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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, Ṅᵢₒₙ) 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 Ṅᵢₒₙ, allowing us to flexibly explore the entire prior volume. We infer a declining Ṅᵢₒₙ with redshift at z>6, which can be used as a benchmark for reionization models. Model-independent reionization constraints from the CMB optical depth and Lyα and Lyβ forest dark pixel fraction produce Ṅᵢₒₙ evolution (dlog₁₀Ṅᵢₒₙ/dz|_(z=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 Lyα damping of galaxies and quasars produces a more rapid decline: dlog₁₀Ṅᵢₒₙ/dz|_(z=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.
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