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(HI) drops from similar to 0.8 at z = 10.6 to similar to 10(-4) at z = 5.8 and reionization is completed around 5.8 less than or similar to z less than or similar to 8.5 (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(esc) of ionizing photons by incorporating the high-redshift galaxy luminosity function data into our analysis. We find a non-evolving f(esc) of similar to 10 per cent in the redshift range z = 6-9.