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Session 45 - Interstellar Medium I.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
Previous studies of expanding HI supershells in nearby spiral galaxies have been limited by the difficulty of visually identifying such structures in moderate-resolution spectral line datacubes. We have developed a completely automated method for locating expanding supershells in HI datacubes of spiral galaxies. Our method relies on the cross-correlation of HI data with a series of 3-dimensional filter kernels designed to match the predicted appearance of supershells having a range of in-disk radii and expansion velocities. Filter kernels are computed using an analytic expression for the radial evolution of a supershell in order to first derive the approximate 3-dimensional space-velocity structure of an expanding HI shell within a plane-stratified gaseous disk. Knowledge of the instrumental beam and a galaxy's orientation then allows us to use this spatial model for the computation of a (velocity-projected) channel map representation. This step generates one model datacube for each combination of shell parameters. Model cubes are finally cross-correlated with the data using 3-D FFT routines from NRAO's Software Development Environment (SDE). Correlated data often contain peaks of high statistical significance. These peaks correspond to regions in which the local HI structure is well-matched with one of our model supershells. It is important to note that our method not only locates the HI shells, but also returns the physical properties of each detection: the mean ambient density as well as the shell age, mass, kinetic energy and wind luminosity. One of our goals is to compare the properties of the actual stellar population seen inside an HI shell with the age and wind luminosity inferred from constrained shell parameters. Tests of this nature will help to refine theoretical models of supershell expansion and of the time-dependent mechanical luminosity associated with groups of high mass stars. Initial results for M101 are presented. We also include a discussion of how this method will be applied to the study of expanding shells in a sample of different spiral galaxies, looking for systematic changes in global properties of the shell population.
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