Feedback from Galaxy Formation: Escaping Ionizing Radiation from Galaxies at High Redshift

Abstract

Several observational and theoretical arguments suggest that starburst galaxies may rival quasars as sources of metagalactic ionizing radiation at redshifts z > 3. Reionization of the intergalactic medium (IGM) at z > 5 may arise, in part, from the first luminous massive stars. To be an important source of radiative feedback from star formation, a substantial fraction (\sim 1-10%) of the ionizing photons must escape the gas layers of the galaxies. Using models of smoothly distributed gas confined by dark-matter (DM) potentials, we estimate the fraction, f_esc, of Lyc flux that escapes the halos of spherical galaxies as a function of their mass and virialization redshift. The gas density profile is found by solving the equation of hydrostatic equilibrium for the baryonic matter in the potential well of a DM halo with the density profile found by Navarro et. al. (1996). We then perform a parametric study to understand the dependence of fesc on redshift, mass, star-formation efficiency (SFE), stellar density distribution, and OB association luminosity function. We give useful analytical formulas for f_esc(z,M_{DM}). Using the Press-Schechter formalism, we find that stellar reionization at z \sim 7 is probably dominated by small galactic sub-units, with M_{DM} \simlt 10^7 M_\odot and SFE \sim 10 times that in the Milky Way. This may affect the distribution of heavy elements throughout the intergalactic medium.