Model-independent constraints on the hydrogen-ionizing emissivity at z > 6

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, (N) over dot(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 (N) over dot(ion), allowing us to flexibly explore the entire prior volume. We infer a declining. N 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 alpha and Ly beta forest dark pixel fraction produce. (N) over dot(ion) evolution (d log(10) (N) over dot(ion)/dz vertical bar(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 Ly a damping of galaxies and quasars produces a more rapid decline: d log10 (N) over dot(ion)/dz vertical bar(z=6 -> 8) =-0.44 +/- 0.22 dex, steeper than the declining galaxy luminosity density (if extrapolated beyond M-UV greater than or similar to -13), and constrains the mid-point of reionization to z = 6.93 +/- 0.14.