The Optical Depth of H II Regions in the Magellanic Clouds

Details The Optical Depth of H II Regions in the Magellanic Clouds The Optical Depth of H II Regions in the Magellanic Clouds

2012-02-13

arxiv.org/abs/1202.3334v3

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

31 pages, 24 figures, 6 tables, accepted to ApJ Errors in Tables B1 and B2 object types corrected

Scientific paper

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.

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