E. W. Pellegrini, M. S. Oey, P. F. Winkler, S. D. Points, R. C. Smith
We exploit ionization-parameter mapping as a powerful tool to measure the
optical depth of star-forming H II regions. Our simulations based on the C
LOUDY photoionization code and our new, SURFBRIGHT surface brightness simulator
demonstrate that this technique can directly diagnose most density-bounded,
optically thin nebulae with spatially resolved emission line data. We apply
this method to the Large and Small Magellanic Clouds, using the data from the
Magellanic Clouds Emission Line Survey. We generate new H II region catalogs
based on photoionization criteria set by the observed ionization structure in
the [SII]/[OIII] ratio and H{\alpha} surface brightness. The luminosity
functions from these catalogs generally agree with those from H{\alpha}-only
surveys. We then use ionization-parameter mapping to crudely classify all the
nebulae into optically thick vs optically thin categories, yielding fundamental
new insights into the Lyman continuum radiation transfer. We find that in both
galaxies, the frequency of optically thin objects correlates with H{\alpha}
luminosity, and that the numbers of these objects dominate above L {\geq}
1037.0 . Similarly, the frequency of optically thick regions correlates with H
I column density, with optically thin objects dominating at the lowest N (HI).
The integrated escape luminosity of ionizing radiation is dominated by the
largest regions, and corresponds to luminosity-weighted, ionizing escape
fractions from the H II region population of {\geq} 0.42 and {\geq} 0.44 in the
LMC and SMC, respectively. This is sufficient to power the ionization rate of
the observed diffuse ionized gas in both galaxies. Since our optical depth
estimates tend to be underestimates, and also omit the contribution from field
stars without nebulae, our results suggest the possibility of significant
galactic escape fractions of Lyman continuum radiation.
View original:
http://arxiv.org/abs/1202.3334
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