Extended H_alpha over compact far-infrared continuum in dusty submillimeter galaxies. Insights into dust distributions and star-formation rates at z~2

By using data from the Atacama Large Millimeter/submillimeter Array and near-infrared (NIR) integral field spectrographs, including both Spectrograph for INtegral Field Observations in the Near Infrared and K-band Multi Object Spectrograph on the Very Large Telescope, we investigate the two-dimensional distributions of H alpha and rest-frame far-infrared (FIR) continuum in six submillimeter galaxies (SMGs) at z similar to 2. At a similar spatial resolution (similar to 0 " 5 FWHM; similar to 4.5 kpc at z=2), we find that the half-light radius of H alpha is significantly larger than that of the FIR continuum in half of the sample, and on average H alpha is a median factor of 2.0 +/- 0.4 larger. Having explored various ways to correct for the attenuation, we find that the attenuation-corrected H alpha-based star-formation rates (SFRs) are systematically lower than the infrared (IR)-based SFRs by at least a median factor of 3 +/- 1, which cannot be explained by the difference in half-light radius alone. In addition, we find that in 40% of cases the total V-band attenuation (A(V)) derived from energy balance modeling of the full ultraviolet (UV)-to-FIR spectral energy distributions (SEDs) is significantly higher than what is derived from SED modeling using only the UV-to-NIR part of the SEDs, and the discrepancy appears to increase with increasing total infrared luminosity. Finally, in considering all of our findings along with the studies in the literature, we postulate that the dust distributions in SMGs, and possibly also in less IR luminous z similar to 2 massive star-forming galaxies, can be decomposed into the following three main components: the diffuse dust heated by older stellar populations, the more obscured and extended young star-forming H II regions, and the heavily obscured central regions that have a low filling factor but dominate the infrared luminosity in which the majority of attenuation cannot be probed via UV-to-NIR emissions.