Constraining the Contribution of Galaxies and Active Galactic Nuclei to
  Cosmic Reionization
  Understanding the detailed process of cosmic reionization is one of the
remaining problems in astrophysics and cosmology. Here we construct a model of
cosmic reionization that includes contribution from high-$z$ galaxies and
active galactic nuclei (AGNs), and calculate the reionization and thermal
histories with the model. To keep the model to be general and realistic, we
vary the escape fraction of ionizing photons, $f_{\rm esc}$, and the faint-end
slope of the AGN luminosity function at high redshifts, $\alpha_{\rm hz}$,
within the constraints from the observed cosmic star formation history and the
observed bright-end UV luminosity functions at $z \leq 6$. Additionally, we
model the spectral energy distribution (SED) of AGNs which depends on the
Eddington ratio and the black-hole mass. By comparing the computed reionization
histories with the observed \ion{H}{i} fractions and the optical depth for
Thomson scattering from Planck, we find that $\alpha_{\rm hz} > -1.5$ and
$f_{\rm esc} < 0.15$ are favored when we employ the bright-end luminosity
function obtained by Giallongo et al. Our result suggests that an AGN-dominated
model with the abundance of faint-AGNs being as large as the estimate by
Giallongo et al. is allowed only if the contribution from high-z galaxies is
almost negligible, while a galaxy-dominant model is also allowed. We also find
that the shape of SED has a significant impact on the thermal history.
Therefore it is expected that measurements of the thermal state of the IGM
provide useful information on properties of ionizing sources.
