2002MNRAS.337.1299C

The spatial distribution of star formation within galaxies strongly affects the resulting feedback processes. Previous work has considered the case of a single, concentrated nuclear starburst, and also that of distributed single supernovae (SNe). Here, we consider interstellar medium (ISM) structuring by SNe originating in spatially distributed clusters having a cluster membership spectrum given by the observed HII region luminosity function. We show that, in this case, the volume of HI cleared per SN is considerably greater than in either of the two cases considered hitherto.

We derive a simple relationship between the `porosity' of the ISM and the star formation rate (SFR), and deduce a critical SFR<sub>crit</sub>, at which the ISM porosity is unity. This critical value describes the case in which the SN mechanical energy output over a time-scale (t<sub>e</sub>) is comparable with the ISM `thermal' energy contained in random motions; t_eis the duration of SN mechanical input per superbubble. This condition also defines a critical gas consumption time-scale t_exh, which for a Salpeter initial mass function and random velocities of ≃10 km.s^–1 is roughly 10¹⁰ yr.

We draw a link between porosity and the escape of ionizing radiation from galaxies, arguing that high escape fractions are expected if SFR ≳ SFR_crit. The Lyman break galaxies, which are presumably subject to infall on a time-scale <t_exh, meet this criterion, as is consistent with the significant leakage of ionizing photons inferred in these systems. We suggest the utility of this simple parametrization of the escape fraction in terms of the SFR for semi-empirical models of galaxy formation and evolution and for modelling mechanical and chemical feedback effects.