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We present detailed modeling of the spectral energy distribution (SED) of thespiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared and submmwavelengths. The model takes into account the emission produced by the diffusedust and the star forming HII complexes. The dust mass is constrained byradiative transfer simulations in the optical (Xilouris et al. 1999). We findthat the submm emission predicted by our model cannot account for the observedfluxes. Two scenarios may account for the "missing" submm flux. In the firstscenario (Popescu et al. 2000), additional dust (to that derived from theoptical, and associated with young stars) is embedded in the galaxy in the formof a thin disk and gives rise to additional submm emission. The other scenarioinvestigates whether the average submm emissivity of the dust grains is higherthan the values widely used in Galactic environments. In this case, the dustmass is equal to that derived from the optical observations, and the submmemissivity is treated as a free parameter calculated by fitting our model tothe observed SED. We find the submm emissivity value to be ~3 times that oftenused for our Galaxy. While both scenarios reproduce the observed 850 micronsurface brightness, the extra embedded dust model is not supported by the nearinfrared observations. We, thus, find that the enhanced dust submm emissivityscenario is the most plausible. [abridged]Comment: 12 pages, 10 figures, accepted for publication in Astronomy and  Astrophysic

Is the Galactic submillimeter dust emissivity underestimated?