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 (<it>f</it><inf>esc</inf>) of hydrogen ionizing photons from galaxies. We estimate the <it>f</it><inf>esc</inf> that is required to reionize the Universe and to maintain the ionization state of the intergalactic medium in the post-reionization era. At <it>z</it> > 5.5, we show that a constant <it>f</it><inf>esc</inf> of 0.14–0.22 is sufficient to reionize the Universe. At <it>z</it> < 3.5, consistent with various observations, we find that <it>f</it><inf>esc</inf> can have values from 0 to 0.05. However, a steep rise in <it>f</it><inf>esc</inf>, of at least a factor of ∼3, is required between <it>z</it> = 3.5 and 5.5. It results from a rapidly decreasing QSO emissivity at <it>z</it> > 3 together with a nearly constant measured H <scp>i</scp> photoionization rates at 3 < <it>z</it> < 5. We show that this requirement of a steep rise in <it>f</it><inf>esc</inf> over a very short time can be relaxed if we consider the contribution from a recently found large number density of faint QSOs at <it>z</it> ≥ 4. In addition, a simple extrapolation of the contribution of such QSOs to high-<it>z</it> suggests that QSOs alone can reionize the Universe. This implies, at <it>z</it> > 3.5, that either the properties of galaxies should evolve rapidly to increase the <it>f</it><inf>esc</inf> 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 <scp>i</scp> photoionization rates that are crucial to the analysis presented here.