Reionization with galaxies and active galactic nuclei
  In this work we investigate the properties of the sources that reionized the
intergalactic medium (IGM) in the high-redshift Universe. Using a
semi-analytical model aimed at reproducing galaxies and black holes in the
first 1.5 Gyr of the Universe, we revisit the relative role of star formation
and black hole accretion in producing ionizing photons that can escape into the
IGM. Both star formation and black hole accretion are regulated by supernova
feedback, resulting in black hole accretion being stunted in low-mass halos. We
explore a wide range of combinations for the escape fraction of ionizing
photons (redshift-dependent, constant and scaling with stellar mass) from both
star formation ($\langle f_{\rm esc}^{\rm sf} \rangle$) and AGN ($f_{\rm
esc}^{\rm bh}$) to find: (i) the ionizing budget is dominated by stellar
radiation from low stellar mass ($M_*<10^9 {\rm M_\odot}$ ) galaxies at $z>6$
with the AGN contribution (driven by $M_{bh}>10^6 {\rm M_\odot}$ black holes in
$M_* > 10^9 {\rm M_\odot}$ galaxies) dominating at lower redshifts; (ii) AGN
only contribute $10-25\%$ to the cumulative ionizing emissivity by $z=4$ for
the models that match the observed reionization constraints; (iii) if the
stellar mass dependence of $\langle f_{\rm esc}^{\rm sf} \rangle$ is shallower
than $f_{\rm esc}^{\rm bh}$, at $z<7$ a transition stellar mass exists above
which AGN dominate the escaping ionizing photon production rate; (iv) the
transition stellar mass decreases with decreasing redshift. While AGN dominate
the escaping emissivity above the knee of the stellar mass function at $z \sim
6.8$, they take-over at stellar masses that are a tenth of the knee mass by
$z=4$.
