Cosmic reionization of hydrogen and helium: Contribution from both mini-quasars and stars

Abstract

Observations on the high-redshift galaxies at z < 6 imply that their ionizing emissivity is unable to fully reionize the Universe at z ~ 6. Either a high escape fraction of ionizing photons from these galaxies or a large population of faint galaxies below the detection limit is required. However, these requirements are somewhat in tension with present observations. In this work, we explored the combined contribution of mini-quasars and stars to the reionization of cosmic hydrogen and helium. Our model is roughly consistent with (1) the low escape fractions of ionizing photons from the observed galaxies, (2) the optical depth of cosmic microwave background measured by the 7-yr Wilkinson Microwave Anisotropy Probe and (3) the redshift of the end of hydrogen and helium reionization at z ≈ 6 and z ≈ 3, respectively. Neither an extremely high escape fraction nor a large population of fainter galaxies is required in this scenario. In our most optimistic model, more than ~20 per cent of the cosmic helium is reionized by z ~ 6, and the ionized fraction of cosmic helium rapidly climbs to more than 50 per cent by z ~ 5. These results may imply that better measurements of helium reionization, especially at high redshifts, could be helpful in constraining the growth of intermediate-mass black holes in the early Universe, which would shed some light on the puzzles concerning the formation of supermassive black holes.