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 QSO emissivity and star formation history for the escape fraction (f_esc) of hydrogen ionizing photons from galaxies. We estimate the f_esc that is required to reionize the Universe and to maintain the ionization state of the intergalactic medium in the post-reionization era. At z>5.5, we show that a constant f_esc of 0.14 to 0.22 is sufficient to reionize the Universe. At z<3.5, consistent with various observations, we find that f_esc can have values from 0 to 0.05. However, a steep rise in f_esc, of at least a factor of ~3, is required between z=3.5 to 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 f_esc 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 f_esc 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.