H [CSC]i[/CSC] Shells in the Large Magellanic Cloud

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Abstract
A recent high-resolution H I survey of the Large Magellanic Cloud (LMC) shows that the structure of the neutral atomic interstellar gas is dominated by numerous shells and holes, as well as complex filamentary and spiral-type structure. We present an up-to-date catalog of candidate H I supergiant and giant shells in the LMC. The candidates are visually selected from the H I data cube using selection and classification criteria that are described. Twenty-three supergiant shells, defined as those regions whose extent is much larger than the H I scale height, are cataloged; 103 giant shells (radii less than the scale height of the H I gas) are cataloged. We further classify the H I shells into five different types, based on the comparison of the H I with their associated Hα emission. For this purpose, we obtained new wide-field Hα images of the LMC with a CCD camera mounted on a 16 inch (0.41 m) telescope at Siding Spring Observatory. The pixel size of 20'' and the field of view of 12° are well matched to the H I survey. The size distribution of H I shells follows a crude power law, N(log R) ∝ R-1.5. For constant energy input to the H I shells and a constant shell creation rate, a shell luminosity spectrum of the form (L) ∝ L-β, where β = 1.75 ± 0.2, is obtained. This agrees well with the observed H II region luminosity spectrum for the LMC of Kennicut, Edgar, & Hodge, which has β = 1.75 ± 0.15. H I shells containing H II regions and OB associations seem to expand more rapidly than those without, providing direct evidence for substantial input of mechanical energy from regions of star formation.

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