Structure of supergiant shells in the Large Magellanic Cloud

Nine supergiant shells (SGSs) have been identified in the Large Magellanic Cloud (LMC) based on H alpha images, and 23 SGSs have been reported based on H I 21 cm line observations, but these sets do not always identify the same structures. We have examined the physical structure of the optically identified SGSs using H I channel maps and P-V diagrams to analyze the gas kinematics. There is good evidence for seven of the nine optically identified SGSs to be true shells. Of these seven H alpha SGSs, four are the ionized inner walls of H I SGSs, while three are an ionized portion of a larger and more complex H I structure. All of the H alpha SGSs are identified as such because they have OB associations along the periphery or in the center, with younger OB associations more often found along the periphery. After roughly 12 Myr, if no new OB associations have been formed, a SGS will cease to be identifiable at visible wavelengths. Thus, the presence and location of ionizing sources is the main distinction between shells seen only in H I and those also seen in H alpha. Based on our analysis, H alpha observations alone cannot unambiguously identify SGSs, especially in distant galaxies.