The Galactic distribution of large HI shells

We report the discovery of 19 new H I shells in the Southern Galactic Plane Survey (SGPS). These shells, which range in radius from 40 pc to 1 kpc, were found in the low-resolution Parkes portion of the SGPS data set covering Galactic longitudes l = 253degrees to l = 358degrees. Here we give the properties of individual shells, including positions, physical dimensions, energetics, masses, and possible associations. We also examine the distribution of these shells in the Milky Way and find that several of the shells are located between the spiral arms of the Galaxy. We offer possible explanations for this effect, in particular, that the density gradient away from spiral arms, combined with the many generations of sequential star formation required to create large shells, could lead to a preferential placement of shells on the trailing edges of spiral arms. Spiral density wave theory is used in order to derive the magnitude of the density gradient behind spiral arms. We find that the density gradient away from spiral arms is comparable to that out of the Galactic plane and therefore suggest that this may lead to exaggerated shell expansion away from spiral arms and into interarm regions.