The Fraction of Ionizing Photons Escaping from High-Redshift Galaxies

The fraction (f_esc) of ionizing photons that escape their host galaxy and so are able to ionize hydrogen in the intergalactic medium is a critical parameter in studies of the reionization era. In this paper we constrain f_esc at high redshift by modelling the observed Lyα transmission. Using an N-body simulation of the cosmic web, we first model an ionizing background due to star-bursting galaxies. We then extract an ensemble of mock absorption spectra from the simulation and compute the mean transmission. We adjust the free parameter, f_esc in our model of the ionizing background to reproduce the observed transmission. We find that f_esc increases with the minimum galaxy mass considered to contribute to the ionizing background, and that at high redshift f_esc must be greater than 5% irrespective of galaxy mass. Based on our numerical results we calibrate a semi-analytic model which shows that the ionizing background 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 >~10⁹ M_solar, the ionizing background would be over-produced at z <~ 5. In summary, our results indicate that the Universe was reionized by low mass galaxies.