  We present radiative transfer models for submillimeter galaxies with
spectroscopic redshifts and mid-infrared spectroscopy from Spitzer/IRS and
analyze available Spitzer/MIPS 24, 70 and 160mu data. We use two types of
starburst models, a cirrus model and a model for the emission of an AGN torus
in order to investigate the nature of these objects. We find that for three of
the objects (25%) cirrus emission alone can account for the mid-infrared
spectrum and the MIPS and submillimeter data. For the remaining objects we find
that we need a combination of starburst and cirrus in order to fit
simultaneously the multi--wavelength data.
  We find that the typical submillimeter galaxy has comparable luminosity in
the starburst (median L=10^12.5 Lo) and cirrus (median L=10^12.4 Lo)
components. This could arise if the galaxies have been forming stars
continuously for the last 250Myr with the star formation occurring in the last
5Myr being shrouded by high-optical-depth molecular cloud dust, whereas the
rest of the starlight is attenuated by diffuse dust or cirrus with an Av of
about 1mag.
