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Session 48 - Spiral & Field Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
As part of a large study to map the distribution of star formation across galactic disks, we have obtained deep H\alpha images of the nearby Sculptor Group spirals NGC 247 and NGC 7793. These images are of sufficiently high quality that they allow identification and analysis of diffuse H\alpha emission at surface brightness levels ranging from those of extremely low density HII regions to those of the local Galactic disk diffuse emission. We present a study of the large scale distribution and global energetics of diffuse ionized gas (DIG) in these galaxies and investigate the association between DIG and discrete HII regions. Our results support the hypothesis that the DIG is photoionized by Lyc photons which leak out of traditional HII regions, and suggest that the local HI column density plays a role in regulating the amount of leakage which can occur. This interpretation has profound implications for the derivation of star-formation rates based on H\alpha emission--line fluxes since HII region counts alone will lead to significant underestimates of the true rate. The constancy of the diffuse fraction is rather unexpected and implies that the overall fraction of photons which can leak out of HII regions and ionize the ISM over large scales is relatively invariant from one galaxy to another.
