Cosmic reionization after Planck II: contribution from quasars

Sourav Mitra, T. Roy Choudhury, Andrea Ferrara

Published 2016-06-08Version 1

In the light of the recent Planck downward revision of the electron scattering optical depth, and of the discovery of a faint QSO population at $z > 4$, we reassess the actual contribution of quasar to cosmic reionization. To this aim, we extend the data-constrained semi-analytic reionization model, based on a rigorous MCMC analysis developed in our previous works, and study the implications of such high-$z$ QSOs on the reionization history. We find that, the quasars can alone reionize the Universe only for models with very high AGN emissivities at high redshift. However, these models predict a too rapid evolution of Lyman limit systems with respect to the observed one. Models with relatively lower emissivities at high-$z$, but consistent with the present datasets, still require a non-zero escape fraction of $\sim12\%$ from early-epoch galaxies. For such models, reionization happens over a shorter period of time; mean neutral hydrogen fraction becomes $\sim10^{-4}$ at $z=5.8$ from $\sim0.8$ at $z=9.0$. Future observations of faint quasars in the early universe will be necessary to put tighter constraints on the quasar-dominated reionization scenario.