Runaway stars and the escape of ionizing radiation from high-redshift galaxies

Approximately 30% of all massive stars in the Galaxy are runaways with velocities exceeding 30 km s^-1. Their high speeds allow them to travel 0.1-1 kpc away from their birthplace before they explode at the end of their several Myr lifetimes. At high redshift, when galaxies were much smaller than in the local universe, runaways could venture far from the dense inner regions of their host galaxies. From these large radii, and therefore low column densities, much of their ionizing radiation is able to escape into the intergalactic medium. Runaways may therefore significantly enhance the overall escape fraction of ionizing radiation, f _esc, from small galaxies at high redshift. We present simple models of the high-redshift runaway population and its impact on f _esc as a function of halo mass, size, and redshift. We find that the inclusion of runaways enhances f _esc by factors of ≈1.1-8, depending on halo mass, galaxy geometry, and the mechanism of runaway production, implying that runaways may contribute 50%-90% of the total ionizing radiation escaping from high-redshift galaxies. Runaways may therefore play an important role in reionizing the universe.