  Previous studies have shown the existence of an excess of emission at
submillimeter (submm) and millimeter (mm) wavelengths in the spectral energy
distribution (SED) of many low-metallicity galaxies. The goal of the present
study is to model separately the emission from the star forming (SF) component
and the emission from the diffuse interstellar medium (ISM) in the nearby
spiral galaxy M33. We decomposed the observed SED of M33 into its SF and
diffuse components. Mid-infrared (MIR) and far-infrared (FIR) fluxes were
extracted from Spitzer and Herschel data. At submm and mm wavelengths, we used
ground-based observations from APEX to measure the emission from the SF
component and data from the Planck space telescope to estimate the diffuse
emission. Both components were separately fitted using radiation transfer
models based on standard dust properties and a realistic geometry. Both modeled
SEDs were combined to build the global SED of M33. In addition, the radiation
field necessary to power the dust emission in our modeling was compared with
observations from GALEX, Sloan, and Spitzer. Our modeling is able to reproduce
the observations at MIR and FIR wavelengths, but we found a strong excess of
emission at submm and mm wavelengths, where the model expectations severely
underestimate the LABOCA and Planck fluxes. We also found that the ultraviolet
(UV) radiation escaping the galaxy is 70% higher than the model predictions. We
determined a gas-to-dust mass ratio Gdust~100, significantly lower than the
value expected from the sub-solar metallicity of M33. We discussed different
hypotheses to explain the discrepancies found in our study (i.e., excess of
emission at submm and mm wavelengths, deficit of UV attenuation, and abnormally
low value of Gdust), concluding that different dust properties in M33 is the
most plausible explanation.
