A Comparison between Paα and Hα Emission: The Relation between Mean H II Region Reddening, Local Gas Density, and Metallicity^*

We measure reddenings to H II regions in NGC 1512, 2903, 4449, and 6946, and M51 from Hubble Space Telescope (HST) Paα and Hα images. Extinctions range from A_V ∼ 5 to 0 depending upon the galaxy. For the galaxies with HST images in both lines, NGC 1512, NGC 2903, and M51, the Paα and Hα emission are almost identical in morphology, which implies that little emission from bright H II regions is hidden from view by regions of comparatively high extinction. The scatter in the measured extinctions in each galaxy is only ±0.5 mag. We compare the reddenings we measure in five galaxies using the Paα-to-Hα ratios to those measured previously from the Balmer decrement in the Large Megallanic Cloud and as a function of radius in M101 and M51. We find that luminosity-weighted mean extinctions of these ensembles of H II regions are correlated with gas surface density and metallicity. The correlation is consistent with the mean extinction depending on dust density, where the dust-to-gas mass ratio scales with the metallicity. This trend is expected if H II regions tend to be located near the midplane of a gas disk and emerge from their parent molecular clouds soon after birth. In environments with gas densities below a few hundred solar masses per square parsec, star formation rates estimated from integrated line fluxes and mean extinctions are likely to be fairly accurate.