Extinction and Dust Geometry in M83 HII Regions: A Hubble Space
  Telescope/WFC3 Study
  We present HST/WFC3 narrow-band imaging of the starburst galaxy M83 targeting
the hydrogen recombination lines (H$\beta$, H$\alpha$ and Pa$\beta$), which we
use to investigate the dust extinction in the HII regions. We derive extinction
maps with 6 parsec spatial resolution from two combinations of hydrogen lines
(H$\alpha$/H$\beta$ and H$\alpha$/Pa$\beta$), and show that the longer
wavelengths probe larger optical depths, with $A_V$ values larger by $\gtrsim$1
mag than those derived from the shorter wavelengths. This difference leads to a
factor $\gtrsim$2 discrepancy in the extinction-corrected H$\alpha$ luminosity,
a significant effect when studying extragalactic HII regions. By comparing
these observations to a series of simple models, we conclude that a large
diversity of absorber/emitter geometric configurations can account for the
data, implying a more complex physical structure than the classical foreground
"dust screen" assumption. However, most data points are bracketed by the
foreground screen and a model where dust and emitters are uniformly mixed. When
averaged over large ($\gtrsim$100--200 pc) scales, the extinction becomes
consistent with a "dust screen", suggesting that other geometries tend to be
restricted to more local scales. Moreover, the extinction in any region can be
described by a combination of the foreground screen and the uniform mixture
model with weights of 1/3 and 2/3 in the center ($\lesssim$2 kpc),
respectively, and 2/3 and 1/3 for the rest of the disk. This simple
prescription significantly improves the accuracy of the dust extinction
corrections and can be especially useful for pixel-based analyses of galaxies
similar to M83.
