The supergiant shell LMC 2. I. The kinematics and physical structure

LMC 2 has the brightest, most coherent filamentary structure of all known s upergiant shells in the Large Magellanic Cloud. The optical emission-line i mages show active star formation regions along the western edge and long fi laments to the east. ROSAT PSPC and HRI images show bright X-ray emission f rom within the shell boundary, indicating the presence of hot gas. Counteri ntuitively, neither high-resolution echelle spectra in the Hot line nor ape rture synthesis H I 21 cm emission-line observations show LMC 2 to have the kinematics expected of an expanding shell. Rather, LMC 2 appears to consis t of hot gas confined between H I sheets. The interior surfaces of these sh eets are ionized by the UV flux of massive stars in the star formation regi ons along the periphery of LMC 2, while the heating is provided by outflows of hot gas from the star formation regions and by SNRs interior to LMC 2. We have compared LMC 2 to other supergiant shells in the LMC and in more di stant galaxies. When the spatial resolution of our data are degraded, we fi nd that LMC 2 resembles supergiant shells observed at a distance of 4 Mpc t hat have previously been interpreted as expanding shells. Therefore, great caution should be exercised in the analysis and interpretation of the kinem atics of distant supergiant shells to prevent overestimates of their veloci ties and total kinetic energies.