Cosmic reionization after Planck II: contribution from quasars

Abstract: In the light of the recent Planck downward revision of the electron scattering optical depth, and of the discovery of a faint AGN population at $z > 4$, we reassess the actual contribution of quasars to cosmic reionization. To this aim, we extend our previous MCMC-based data-constrained semi-analytic reionization model and study the role of quasars on global reionization history. We find that, the quasars can alone reionize the Universe only for models with very high AGN emissivities at high redshift. These models are still allowed by the recent CMB data and most of the observations related to HI reionization. However, they predict an extended and early HeII reionization ending at $z\gtrsim4$ and a much slower evolution in the mean HeII Ly-$\alpha$ forest opacity than what the actual observation suggests. Thus when we further constrain our model against the HeII Ly-$\alpha$ forest data, this AGN-dominated scenario is found to be clearly ruled out at 2-$\sigma$ limits. The data seems to favour a standard two-component picture where quasar contributions become negligible at $z\gtrsim6$ and a non-zero escape fraction of $\sim10\%$ is needed from early-epoch galaxies. For such models, mean neutral hydrogen fraction decreases to $\sim10^{-4}$ at $z=6.2$ from $\sim0.8$ at $z=10.0$ and helium becomes doubly ionized at much later time, $z\sim3$. We find that, these models are as well in good agreement with the observed thermal evolution of IGM as opposed to models with very high AGN emissivities.