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 contributions from high-z galaxies and active galactic nuclei (AGNs) and calculate reionization and thermal histories with the model. To keep the model general and realistic, we vary the escape fraction of ionizing photons, f(esc), and the faint-end slope of the AGN luminosity function at high redshifts, alpha(hz), within constraints from the observed cosmic star formation history and observed bright-end UV luminosity functions at z <= 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 HI fractions and the optical depth for Thomson scattering from Planck, we find that alpha(hz) > -1.5 and f(esc) < 0.15 are favoured when we employ the bright-end luminosity function obtained by Giallongo et al. Our result suggests that an AGN-dominated model with an abundance of faint AGNs 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 the SED has a significant impact on the thermal history. Therefore it is expected that measurements of the thermal state of the intergalactic medium (IGM) will provide useful information on the properties of ionizing sources.