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 ionising 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
tau measurements, from the constraints provided by quasar absorption spectra
and from the prevalence of Ly-alpha emission in z~7-8 galaxies. We find the
redshift evolution in the emissivity dot{N}_{ion}(z) required by the
observations to be d(log Nion)/dz=-0.15(-0.11)(+0.08), largely independent of
the assumed clumping factor C_{HII} and entirely independent of the nature of
the ionising sources. The trend in dot{N}_{ion}(z) is well-matched by the
evolution of the galaxy UV-luminosity density (dlog_{10}
rho_UV/dz=-0.11+/-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 rho_UV to dot(N)_{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/AGN 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-luminosity density is
8(-4)(+15)x 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.
