Using data from ALMA and near-infrared (NIR) integral field
spectrographs including both SINFONI and KMOS on the VLT, we investigate
the two-dimensional distributions of H$\alpha$ and rest-frame
far-infrared (FIR) continuum in six submillimeter galaxies at $z\sim2$.
At a similar spatial resolution ($\sim$0.5" FWHM; $\sim$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 $2.0\pm0.4$ larger. Having
explored various ways to correct for the attenuation, we find that the
attenuation-corrected H$\alpha$-based SFRs are systematically lower than
the IR-based SFRs by at least a median factor of $3\pm1$, 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 that
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, considering all our findings along with the studies
in the literature, we postulate that the dust distributions in SMGs, and
possibly also in less IR luminous $z\sim2$ massive star-forming
galaxies, can be decomposed into three main components; the diffuse dust
heated by older stellar populations, the more obscured and extended
young star-forming HII 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.