Starburst and cirrus models for submillimeter galaxies

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 160 μm 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~L_⊙) and cirrus (median L=10^12.4~L_⊙) components. This could arise if the galaxies have been forming stars continuously for the last 250 Myr with the star formation occurring in the last 5 Myr being shrouded by high-optical-depth molecular cloud dust, whereas the rest of the starlight is attenuated by diffuse dust or cirrus with an A_V of about 1 mag.