The redshift evolution of escape fraction of hydrogen ionizing photons from galaxies

Abstract

Using our cosmological radiative transfer code, we study the implications of the updated quasi-stellar object (QSO) emissivity and star formation history for the escape fraction (fesc) of hydrogen ionizing photons from galaxies. We estimate the fesc that is required to reionize the Universe and to maintain the ionization state of the intergalactic medium in the postreionization era. At z > 5.5, we show that a constant fesc of 0.14-0.22 is sufficient to reionize the Universe. At z < 3.5, consistent with various observations, we find that fesc can have values from 0 to 0.05. However, a steep rise in fesc, of at least a factor of ~3, is required between z = 3.5 and 5.5. It results from a rapidly decreasing QSO emissivity at z > 3 together with a nearly constant measured H I photoionization rates at 3 < z < 5. We show that this requirement of a steep rise in fesc over a very short time can be relaxed if we consider the contribution from a recently found large number density of faint QSOs at z ≥ 4. In addition, a simple extrapolation of the contribution of such QSOs to high-z suggests that QSOs alone can reionize the Universe. This implies, at z > 3.5, that either the properties of galaxies should evolve rapidly to increase the fesc or most of the low-mass galaxies should host massive black holes and sustain accretion over a prolonged period. These results motivate a careful investigation of theoretical predictions of these alternate scenarios that can be distinguished using future observations. Moreover, it is also very important to revisit the measurements of H I photoionization rates that are crucial to the analysis presented here.