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S.D. Points, Y.-H. Chu, R.A. Gruendl (Univ. Illinois), R.C. Smith (CTIO)
Supergiant shells are the largest coherent interstellar structures observed in galaxies. They are thought to be the result of fast stellar winds and multiple supernova explosions from massive stars in multiple OB associations. Their physical sizes (~1000~pc) can be comparable to the thickness of gas in galactic disks. It may be possible for these structures to break out of the gaseous disk and vent hot gas into the galactic halo. High-resolution spectral observations of supergiant shells in distant galaxies (d \ge 4~Mpc), give ambiguous results as to whether they are even expanding.
The Large Magellanic Cloud (LMC) offers a unique laboratory where we can investigate the kinematic and physical structure of supergiant shells in great detail. We have observed two LMC supergiant shells, LMC 1 and LMC 4, to better understand these objects. LMC 1 has the most coherent shell structure in both ionized gas and neutral atomic gas, and LMC 4 is the largest supergiant shell in the LMC. These observations triple our sample of kinematic studies of LMC supergiant shells and provide insight into the controversy about the expansion of supergiant shells. We find no evidence for coherent expansion in either of these supergiant shells. Instead, we find localized, high-velocity features that can be associated with interior supernova remnant shocks. In distant galaxies, these velocity features would appear to be indicative of global expansion because lower spatial resolution makes their interpretation difficult.
The author(s) of this abstract have provided an email address for comments about the abstract: points@astro.uiuc.edu