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.

Final version accepted for pubblication on ApJ v543n1p Nov 1, 2000 issue. One error fixed, new figures and a new parameter (collapsed barion fraction) added to the calculation. 27 pages, 11 figs and 1 table