Paβ, Hα, and Attenuation in NGC 5194 and NGC 6946

Abstract

We combine Hubble Space Telescope Paschen β (Pa β ) imaging with ground-based, previously published H α maps to estimate the attenuation affecting H α , A (H α ), across the nearby, face-on galaxies NGC 5194 and NGC 6946. We estimate A (H α ) in ∼2000 independent 2″ ∼ 75 pc diameter apertures in each galaxy, spanning out to a galactocentric radius of almost 10 kpc. In both galaxies, A (H α ) drops with radius, with a bright, high-attenuation inner region, though in detail the profiles differ between the two galaxies. Regions with the highest attenuation-corrected H α luminosity show the highest attenuation, but the observed H α luminosity of a region is not a good predictor of attenuation in our data. Consistent with much previous work, the IR-to-H α color does a good job of predicting A (H α ). We calculate the best-fit empirical coefficients for use combining H α with 8, 12, 24, 70, or 100 μ m to correct for attenuation. These agree well with previous work, but we also measure significant scatter around each of these linear relations. The local atomic plus molecular gas column density, N (H), also predicts A (H α ) well. We show that a screen with magnitude ∼0.2 times that expected for a Milky Way gas-to-dust value does a reasonable job of explaining A (H α ) as a function of N (H). This could be expected if only ∼40% of gas and dust directly overlap regions of H α emission.