A Star-formation Threshold for the Escape of Ionizing Radiation from Galaxies

We consider structuring of the interstellar medium (ISM) by supernovae (SNe) originating in spatially distributed clusters described by the observed HII region luminosity function. We derive a critical star formation rate SFRcrit at which the ISM porosity is unity; this describes the case in which the SN mechanical energy output is comparable with the ISM "thermal" energy contained in random motions. We argue that ionizing radiation has a high escape fraction for the simple parameterization SFR > SFRcrit.