IMAGING AND SPECTROSCOPY OF IONIZED SHELLS AND SUPERSHELLS IN THE LARGE MAGELLANIC CLOUD

Deep H alpha images of the Large Magellanic Cloud (LMC) have revealed the presence of numerous supergiant ( > 300 pc radius) and giant shells of ionized gas. These structures are generally believed to be the result of the action of encircled massive stars on the surrounding interstellar medium. This paper examines the spectral and kinematic signature of this interaction through low and high dispersion spectra obtained for three supergiant and three giant shells in the LMC. One of the giant shells is an x-ray bubble embedded in the 30 Doradus nebula. The emission line ratios, including the lines [O II]lambda 3727, [O III]lambda 5007, [N II]lambda 6584, [S II]lambda 6717,31, in all but the embedded x-ray bubble, are found to be unusual compared to typical H II regions and supernova remnants in the same galaxy. However, the emission lines and surface brightnesses of these structures are generally consistent with models of photoionized gas having a very low ionization parameter due to the large distance between the encircled stars and the gas. Thus, emission from both the supergiant and giant shell structures appears to be dominated by photoionization processes. High dispersion spectra reveal that the profiles of the ionized gas at the edges of supershells are narrow and contain a single velocity component; spectra of the giant shells reveal broad profiles with multiple velocity components.