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 Ly alpha 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 similar to 5.5-6 implies a sufficient number of ionizing photons to have reionized the Universe by z similar to 6. However, if the minimum mass for star-formation were greater than or similar to 10(9) M-circle dot, the ionizing background would be over-produced at z less than or similar to 5. In summary, our results indicate that the Universe was reionized by low mass galaxies.