Zusammenfassung
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.
Nutzer