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 } _ { \textrm { 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 } _ { \textrm { ion } } , allowing us to flexibly explore the entire prior volume .
We infer a declining \dot { N } _ { \textrm { 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 } _ { \textrm { ion } } evolution ( d \log _ { 10 } \dot { \mathbf { N } } _ { \textbf { ion } } / dz| _ { z = 6 \rightarrow 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 { \mathbf { N } } _ { \textbf { ion } } / dz| _ { z = 6 \rightarrow 8 } = -0.44 \pm 0.22 dex , steeper than the declining galaxy luminosity density ( if extrapolated beyond M _ { \textsc { uv } } \hbox to 0.0 pt { \lower 3.5 pt \hbox { $ \mathchar 0 \sim$ } } \raise 1. % 0 pt \hbox { $ > $ } -13 ) , and constrains the mid-point of reionization to z = 6.93 \pm 0.14 .