A comparison between Pa alpha and H alpha emission: The relation between mean HII region reddening, local gas density, and metallicity

We measure reddenings to H II regions in NGC 1512, 2903, 4449, and 6946, an d M51 from Hubble Space Telescope (HST) Pa alpha and H alpha images. Extinc tions range from A(v) similar to 5 to 0 depending upon the galaxy. For the galaxies with HST images in both lines, NGC 1512, NGC 2903, and M51, the Pa a and Ha emission are almost identical in morphology, which implies that li ttle emission from bright H II regions is hidden from view by regions of co mparatively high extinction. The scatter in the measured extinctions in eac h galaxy is only +/-0.5 mag. We compare the reddenings we measure in five g alaxies using the Paa-to-Ha ratios to those measured previously from the Ba lmer 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 e nsembles of H II regions are correlated with gas surface density and metall icity. 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 midplan e of a gas disk and emerge from their parent molecular clouds soon after bi rth. In environments with gas densities below a few hundred solar masses pe r square parsec, star formation rates estimated from integrated line fluxes and mean extinctions are likely to be fairly accurate.