Galactic Porosity and a Star-Formation
Threshold for the Escape of Ionising Radiation from Galaxies
C.J. Clarke & M.S. Oey
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 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
SFRcrit, at which the ISM porosity is unity. This
critical value describes the case in which the SN mechanical energy
output over a timescale te , is comparable with the
ISM "thermal" energy contained in random motions;
te is the duration of SN mechanical input per
superbubble. This condition also defines a critical gas consumption
timescale texh, which for a Salpeter IMF and
random velocities of ~10 km s-1 is roughly 1010 years.
We draw a link between porosity and the escape of ionising radiation
from galaxies, arguing that high escape fractions are expected
if SFR > SFRcrit. The Lyman Break Galaxies,
which are presumably subject to infall on a timescale <
texh , meet this criterion, as is consistent with
the significant leakage of ionising photons inferred in these systems.
We suggest the utility of this simple parameterisation of escape
fraction in terms of SFR for semi-empirical models of galaxy formation
and evolution and for modeling mechanical and chemical feedback effects.
MNRAS, 337, 1299
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