REIONIZATION AFTER PLANCK: THE DERIVED GROWTH OF THE COSMIC IONIZING EMISSIVITY NOW MATCHES THE GROWTH OF THE GALAXY UV LUMINOSITY DENSITY

Thomson optical depth tau 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 greater than or similar to 6 from the new Planck optical depth tau measurements, from the constraints provided by quasar absorption spectra and from the prevalence of Ly alpha emission in z similar to 7-8 galaxies. We find the redshift evolution in the emissivity (N) over dot(ion)( z) required by the observations to be (d log(10) (N) over dot(ionion)/dz)(z=8) = -0.15(-0.11)(+0.08) ((d log(10) (N) over dot(ion)/dz)(z=8) = -0.19(-0.11)(+0.09) for a flat prior), largely independent of the assumed clumping factor C-H II and entirely independent of the nature of the ionizing sources. The trend in (N) over dot(ion)(z) is well-matched by the evolution of the galaxy UV-luminosity density (d log(10)rho(UV/)dz = -0.11 +/- 0.04) to a magnitude limit greater than or similar to -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 rho(UV) to (N) over dot(ion)(z) being possible for physically plausible values of the escape fraction f(esc), the Lyman-continuum photon production efficiency xi(ion), and faint-end cut-off M-lim 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 similar to 10 UV-Iuminosity density is 8(-4)(+15)x lower than at z similar to 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.