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R.A.M. Walterbos (NMSU)
The mechanical energy input from massive stars, in the form of stellar winds and supernova explosions, molds the diffuse interstellar medium in disk galaxies in a cauldron of expanding shells on a variety of spatial scales. On the small scales, we find individual windblown bubbles such as those surrounding Wolf-Rayet stars. Evidence for large-scale shells created by the combined effects of OB stars is most obvious in high resolution optical emission-line images of nearby spiral and irregular galaxies, which often show surprisingly large complete ionized rings especially in their outer parts where the disk gas scale heights are largest, and shells are less likely to run into each other. Neutral atomic hydrogen surveys have also provided direct evidence for expanding shell structures, although the lower resolution and sensitivity of the 21-cm data does not make it easy to characterize the properties of these structures unambiguously. In this review I will focus on the properties of shell structures in disk galaxies with a range in Hubble types, and widely different star formation rates per unit disk area. The discussion will include new results from an automated HI shell detection method, which combines theoretical predictions of supershell evolution with observations to produce more robust estimates of the physical parameters of the various shells.
This work is supported by NSF grant AST 96-17014.
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