Cosmic reionization after Planck
  Cosmic reionization holds the key to understand structure formation in the
Universe, and can inform us about the properties of the first sources, as their
star formation efficiency and escape fraction of ionizing photons. By combining
the recent release of Planck electron scattering optical depth data with
observations of high-redshift quasar absorption spectra, we obtain strong
constraints on viable reionization histories. We show that inclusion of Planck
data favors a reionization scenario with a single stellar population. The mean
$x_{\rm HI}$ drops from $\sim0.8$ at $z=10.6$ to $\sim10^{-4}$ at $z=5.8$ and
reionization is completed around $5.8\lesssim z\lesssim8.5$ (2-$\sigma$), thus
indicating a significant reduction in contributions to reionization from high
redshift sources. We can put independent constraints on the escape fraction
$f_{\rm esc}$ of ionizing photons by incorporating the high-redshift galaxy
luminosity function data into our analysis. We find a non-evolving $f_{\rm
esc}$ of $\sim10\%$ in the redshift range $z=6-9$.
