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Thomson optical depth tau measurements from Planck provide new insights intothe reionization of the universe. In pursuit of model-independent constraintson the properties of the ionising sources, we determine the empirical evolutionof the cosmic ionizing emissivity. We use a simple two-parameter model to mapout the evolution in the emissivity at z>~6 from the new Planck optical depthtau measurements, from the constraints provided by quasar absorption spectraand from the prevalence of Ly-alpha emission in z~7-8 galaxies. We find theredshift evolution in the emissivity dot{N}_{ion}(z) required by theobservations to be d(log Nion)/dz=-0.15(-0.11)(+0.08), largely independent ofthe assumed clumping factor C_{HII} and entirely independent of the nature ofthe ionising sources. The trend in dot{N}_{ion}(z) is well-matched by theevolution of the galaxy UV-luminosity density (dlog_{10}rho_UV/dz=-0.11+/-0.04) to a magnitude limit >~-13 mag, suggesting thatgalaxies are the sources that drive the reionization of the universe. The roleof galaxies is further strengthened by the conversion from the UV luminositydensity rho_UV to dot(N)_{ion}(z) being possible for physically-plausiblevalues of the escape fraction f_{esc}, the Lyman-continuum photon productionefficiency xi_{ion}, and faint-end cut-off $M_{lim}$ to the luminosityfunction. Quasars/AGN appear to match neither the redshift evolution nornormalization of the ionizing emissivity. Based on the inferred evolution inthe ionizing emissivity, we estimate that the z~10 UV-luminosity density is8(-4)(+15)x lower than at $z~6, consistent with the observations. The presentapproach of contrasting the inferred evolution of the ionizing emissivity withthat of the galaxy UV luminosity density adds to the growing observationalevidence that faint, star-forming galaxies drive the reionization of theuniverse.

Reionization after Planck: The Derived Growth of the Cosmic Ionizing Emissivity now matches the Growth of the Galaxy UV Luminosity Density