Reionization after Planck : the derived growth of the cosmic ionizing emissivity now matches the growth of the galaxy UV luminosity density

Abstract

Thomson optical depth τ measurements from Planck provide new insights into the reionization of the universe. In pursuit of model-independent constraints on the properties of the ionizing sources, we determine the empirical evolution of the cosmic ionizing emissivity. We use a simple two-parameter model to map out the evolution in the emissivity at z ≳ 6 from the new Planck optical depth τ measurements, from the constraints provided by quasar absorption spectra and from the prevalence of Lyα emission in z ∼ 7–8 galaxies. We find the redshift evolution in the emissivity ${\dot{N}}_{\mathrm{ion}}(z)$ required by the observations to be ${({\text{}}d{\mathrm{log}}_{10}\;{\dot{N}}_{\mathrm{ion}}/{dz})}_{z=8}=-{0.15}_{-0.11}^{+0.08}$ (${({\text{}}d{\mathrm{log}}_{10}\;{\dot{N}}_{\mathrm{ion}}/{dz})}_{z=8}=-{0.19}_{-0.11}^{+0.09}$ for a flat prior), largely independent of the assumed clumping factor CH ii and entirely independent of the nature of the ionizing sources. The trend in ${\dot{N}}_{\mathrm{ion}}(z)$ is well-matched by the evolution of the galaxy UV-luminosity density ($d{\mathrm{log}}_{10}{\rho }_{\mathrm{UV}}/{dz}=-0.11\pm 0.04$) to a magnitude limit ≳−13 mag, suggesting that galaxies are the sources that drive the reionization of the universe. The role of galaxies is further strengthened by the conversion from the UV luminosity density ρUV to ${\dot{N}}_{\mathrm{ion}}(z)$ being possible for physically plausible values of the escape fraction fesc, the Lyman-continuum photon production efficiency ξion, and faint-end cut-off Mlim to the luminosity function. Quasars/active galactic nuclei appear to match neither the redshift evolution nor normalization of the ionizing emissivity. Based on the inferred evolution in the ionizing emissivity, we estimate that the z ∼ 10 UV-Iuminosity density is 8−4+15× lower than at z ∼ 6, consistent with the observations. The present approach of contrasting the inferred evolution of the ionizing emissivity with that of the galaxy UV luminosity density adds to the growing observational evidence that faint, star-forming galaxies drive the reionization of the universe.