Structure of Supergiant Shells in the Large Magellanic Cloud

TL;DR

This study investigates the physical structure and kinematics of supergiant shells in the Large Magellanic Cloud, revealing that optical H-alpha observations alone are insufficient for unambiguous identification of these shells.

Contribution

The paper provides a detailed analysis combining H-alpha and HI data to clarify the true nature of supergiant shells and highlights the limitations of optical observations for their identification.

Findings

Seven out of nine optically identified SGSs are confirmed as true shells.

H-alpha SGSs are often ionized inner walls of larger HI structures.

Presence of OB associations correlates with shell visibility in H-alpha.

Abstract

Nine supergiant shells (SGSs) have been identified in the Large Magellanic Cloud (LMC) based on H-alpha images, and twenty-three SGSs have been reported based on HI 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 HI 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 HI SGSs, while three are an ionized portion of a larger and more complex HI 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 Myrs, 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 HI and those also seen in H-alpha. Based on our analysis, H-alpha observations alone cannot unambiguously identify SGSs, especially in distant galaxies.