Contribution of galaxies to the background hydrogen-ionizing flux

Abstract

We estimate the evolution of the contribution of galaxies to the cosmic background flux at 912 Å by means of a semi-analytic model of galaxy formation and evolution. Such modelling has been quite successful in reproducing the optical properties of galaxies. We assume that high-redshift damped Lyman α systems are the progenitors of present-day galaxies, and we design a series of models that are consistent with the evolution of cosmic comoving emissivities in the available near-infrared, optical, ultraviolet and far-infrared bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman α systems. We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for 0~=z<=4. We take into account the absorption of Lyman-limit photons by HI and dust in the interstellar medium of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by HI in the interstellar medium is neglected. Such HI absorption would result in a severe diminishing of this flux - by almost three orders of magnitude at high redshifts and by one to two orders at z~=0. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at z~=0.