An inventory of supershells in nearby galaxies: first results from THINGS

The HI Nearby Galaxy Survey (THINGS), is a 21-cm HI line survey of a sample of 34 nearby (3-10 Mpc) galaxies. The observations were carried out with the VLA and have a velocity resolution of less than 5 km s^-1 and an angular resolution of 7" which at this distance range corresponds to a linear resolution of 100-300pc. One of the primary goals of THINGS is to look at the fine scale structure of the Interstellar Medium (ISM) and examine how it varies as a function of Hubble type, star formation rate, galaxy mass, metallicity, etc. Previous studies have shown that the morphology of the neutral ISM is greatly affected by massive stars through the combined effects of stellar winds and supernova explosions. Because massive stars tend to form in associations they will end their lives within the same, relatively short time span and within a small volume which leads to the formation of expanding bubbles of coronal gas in the ISM. These structures compress the neutral gas and can trigger secondary or induced star formation on their rims where presumably molecular clouds are formed. We present a few early science highlights from the project, an inventory of holes and shells in NGC 6946, NGC 3184, NGC 2403, and M 74 among other galaxies, and give some preliminary results from a comparison between these galaxies as far as the properties of their HI holes are concerned. These properties include the size of the holes, their expansion velocities, energy requirements and kinematical ages