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The discovery of HI holes and shells in our Galaxy and in nearby spiral galaxies has stimulated interest in the relation between these neutral atomic structures and star formation sites such as HII regions and OB associations. Results have already been published for the nearby spirals M31, M33, and M101. However, these studies were typically based on comparison of HI holes with existing catalogs of star forming regions -- not on direct comparison of H$\alpha$ and HI images. We have started a program to compare high resolution HI maps with continuum-subtracted H$\alpha$ images for 11 spiral galaxies of various star formation character, Hubble type (Sab - Sd), and inclination angle.
Specific questions we wish to address are: Does shell size vary predictably with galactocentric radius? Can a relation between local conditions in the gaseous atomic disk and shell morphology be deduced? More fundamentally, do the Hubble type and star formation rate of the parent galaxy exhibit any notable influence on the observational character of HI shells?
The interferometric HI data for this study were obtained using several antenna configurations of the VLA. Our synthesized beam measures 10"~x~10" -- substantially improved with respect to previous HI surveys. Furthermore, all of the galaxies in our sample lie within 5 Mpc, allowing the detection of 100 pc structures in each case. Deep narrow-band H$\alpha$ images have been obtained using the Burrell Schmidt and 36" telescopes at Kitt Peak. We present initial results for a few representative systems.
