The Luminosities of Bright H N Regions in Ngc 628 and Its O Star Formation Rate

Using narrow band CCD imaging we have identified 183 bright H n regions in the Sc galaxy NGC 628. These H n regions have luminosities which indicate that they are powered by young star clusters which contain ^ 100 ionizing stars. We attempt to reproduce the H ii region LF with and without models including stellar evolution. We find stellar evolution an important parameter in understanding the H n region LF and find that it allows star-forming galaxies to be more robust to star bursting episodes. Ranking the H n regions by luminosity and assuming that luminosity evolution is due to the temporal evolution of the ionizing stars allows us to crudely determine the slope of the IMF for massive stars by matching the data with O star model atmospheres. This procedure also allows us to determine the O star formation rate and therefore predict the Type n supernova rate and the numbers of Wolf-Rayet stars in NGC 628. Overall, the data are most consistent with an IMF slope in massive stars for NGC 628 that is very near the Salpeter value found for the solar neighborhood. Our model also indicates that ^ 1 % of the total supply of disk H I is presently participating in active star formation.