The escape fraction of ionizing photons from high redshift galaxies

Abstract

The fraction of ionizing photons which escape their host galaxy and so are able to ionize hydrogen in the inter-galactic medium (IGM) is a critical parameter in studies of the reionization era and early galaxy formation. In this paper we combine observations of Lyman-alpha absorption towards high redshift quasars with the measured UV luminosity function of high redshift galaxies to constrain the escape fraction (f_esc) of ionizing photons from galaxies at z ~ 5.5-6. The observed Lyman-alpha transmission constrains the escape fraction to lie in the range f_esc ~ 10-25 % (at z ~ 5.5-6). Excluding halos with M< 10^10 M_sun (as might be expected if galaxy formation is suppressed due to the reionization of the IGM) implies a larger escape fraction of f_esc ~ 20-45 %. Using the numerical results to calibrate an analytic relation between the escape fraction and minimum galaxy halo mass we also extrapolate our results to a mass (M~10^8 M_sun) corresponding to the hydrogen cooling threshold. In this case we find f_esc ~ 5-10 %, consistent with observed estimates at lower redshift. We find that the escape fraction of high redshift galaxies must be greater than 5 % irrespepctive of galaxy mass. Based on these results we use a semi-analytic description to model the reionization history of the IGM, assuming ionizing sources with escape fractions suggested by our numerical simulations. We find that the IBG observed at z ~ 5.5-6 implies a sufficient number of ionizing photons to have reionized the Universe by z ~ 6. However, if the minimum mass for star-formation were greater than 10^9 M_sun, the IBG would be over-produced at redshifts less than z ~ 5. In summary, our results support a scenario in which the IGM was reionized by low mass galaxies.Comment: 14 pages, 9 figure