Pa$\beta$, H$\alpha$, and Attenuation in NGC 5194 and NGC 6946
  We combine Hubble Space Telescope (HST) Paschen $\beta$ (Pa$\beta$) imaging
with ground-based, previously published H$\alpha$ maps to estimate the
attenuation affecting H$\alpha$, A(H$\alpha$), across the nearby, face-on
galaxies NGC 5194 and NGC 6946. We estimate A(H$\alpha$) in ~ 2,000 independent
2" ~75 pc diameter apertures in each galaxy, spanning out to a galactocentric
radius of almost 10 kpc. In both galaxies, A(H$\alpha$) 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$\alpha$ luminosity show the highest attenuation, but the observed H$\alpha$
luminosity of a region is not a good predictor of attenuation in our data.
Consistent with much previous work, the IR-to-H$\alpha$ color does a good job
of predicting A(H$\alpha$). We calculate the best-fit empirical coefficients
for use combining H$\alpha$ with 8, 12, 24, 70, or 100 $\mu$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$\alpha$) well. We
show that a screen with magnitude ~ 0.2 times the expected for a Milky Way
gas-to-dust value does a reasonable job of explaining A(H$\alpha$) as a
function of N(H). This could be expected if only ~ 40% of gas and dust directly
overlap regions of H$\alpha$ emission.
