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.