The Influence of Supershells and Galactic Outflows on the Escape of Ionizing Radiation from Dwarf Starburst Galaxies

We study the escape of Lyman continuum radiation from the disks of dwarf starburst galaxies, by solving the radiation transfer problem of stellar radiation through them. We model disks with Md=10^{8-10}Msun, with exponential surface density profiles as a function of redshift, consistent with the predictions of standard hierarchical cosmogonies, and model the effects of repeated supernova explosions in the disks, using the hydrodynamic simulation code, ZEUS-3D. The amount of star formation is proportional to critical mass above a density threshold. We vary the threshold to explore the range of star formation efficiencies, f*. We show that a large fraction of ionizing photons escape from disks without bubbles at low redshift, but the bubbles sweep up the interstellar gas into dense shells which can effectively trap the ionizing photons before the bubbles blow out of the disks. On the other hand, only a small fraction of ionizing photons escape from the undisturbed disks at high redshift, but the bubbles quickly blow out of the highly stratified disks in the dense, high-redshift universe, and form chimneys for the radiation to escape directly to the IGM. We suggest that our results are consistent with escape fractions of less than 10% observed in local dwarf starburst galaxies, as long as the time observed is before the bubbles break out of the galactic disks. We also suggest that dwarf starburst galaxies may play an important role in reionizing the universe with total escape fractions >=20% at redshift above 5