Cold dust and stellar emissions in dust-rich galaxies observed with
  ALMA: a challenge for SED-fitting techniques
  Over the past few years ALMA has detected dust-rich galaxies whose cold dust
emission is spatially disconnected from the UV rest-frame emission. This
represents a challenge for modeling their spectral energy distributions with
codes based on an energy budget between the stellar and dust components. We
want to verify the validity of energy balance modeling on a sample of galaxies
observed from the UV to the sub-millimeter rest frame with ALMA and decipher
what information can be reliably retrieved from the analysis of the full SED
and from subsets of wavelengths. We select 17 sources at z~2 in the Hubble
Ultra-Deep Field and in the GOODS- South field detected with ALMA and Herschel
and for which UV to NIR. rest-frame ancillary data are available. We fit the
data with CIGALE exploring different configurations for dust attenuation and
star formation histories, considering either the full dataset or one that is
reduced to the stellar and dust emission. We compare estimates of the dust
luminosities, star formation rates, and stellar masses. The fit of the stellar
continuum alone with the starburst attenuation law can only reproduce up to 50%
of the total dust luminosity observed by Herschel and ALMA. This deficit is
found to be consistent with similar quantities estimated in the COSMOS field
and is found to increase with the specific star formation rate. The combined
stellar and dust SEDs are well fitted when different attenuation laws are
introduced. Shallow attenuation curves are needed for the galaxies whose cold
dust distribution is very compact compared to starlight. The stellar mass
estimates are affected by the choice of the attenuation law. The star formation
rates are robustly estimated as long as dust luminosities are available. The
large majority of the galaxies are above the average main sequence of star
forming galaxies and one source is a strong starburst.
