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, ${\dot{N}}_{\text{ion}}$) 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 ${\dot{N}}_{\text{ion}}$, allowing us to flexibly explore the entire prior volume. We infer a declining ${\dot{N}}_{\text{ion}}$ 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 Ly$\alpha$ and Ly$\beta$ forest dark pixel fraction produce ${\dot{N}}_{\text{ion}}$ evolution ($d{\log }_{10}{\dot{N}}_{\text{ion}}/dz{|}_{z=6\to 8}=-0.31\pm 0.35$ dex) consistent with the declining UV luminosity density of galaxies, assuming constant ionizing photon escape fraction and efficiency. Including measurements from Ly$\alpha$ damping of galaxies and quasars produces a more rapid decline: $d{\log }_{10}{\dot{N}}_{\text{ion}}/dz{|}_{z=6\to 8}=-0.44\pm 0.22$ dex, steeper than the declining galaxy luminosity density (if extrapolated beyond $M_{\mathrm{UV}}\lesssim -13$), and constrains the mid-point of reionization to $z=6.93\pm 0.14$.

submitted to MNRAS

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