3.3. Correspondence with Star Formation
Modern studies that compare the spatial distribution of H I shells and star-forming regions show perhaps surprisingly ambiguous results. The galaxy that has been most actively studied in this respect recently is Holmberg II. At a distance of 3 Mpc, Ho II is not a member of the Local Group, and the conflicting results in the literature may be symptomatic of the difficulty with spatial resolution at that distance. While Rhode et al. (1999) failed to identify H I shell progenitor populations from BVR aperture photometry, Stewart et al. (2000) did find a positive spatial correlation with star-forming regions using FUV data from the Ultraviolet Imaging Telescope (UIT)and H observations. Tongue & Westpfahl (1995) also found that the SN rate implied by the radio continuum emission is consistent with the total, integrated superbubble energy in Ho II.
The Magellanic Clouds, and other nearby Local Group galaxies, are clearly superior candidates for investigating quantitative spatial correlations. Kim et al. (1999) have carried out a preliminary study that compares the H I shell properties with those of the OB associations and nebular emission. They are able to identify an evolutionary sequence such that the shells with associated H emission show higher expansion velocities than those showing only the presence of OB stars, and the latter in turn show higher v than the remainder of the shells. The H emission also shows smaller radial extent, remaining within that of the H I shells. These trends are consistent with an age sequence and feedback origin for the neutral shells. We are currently carrying out a more detailed follow-up of this study using UIT and new optical data (Oey et al. 2003, in preparation).