MILLIMETRIC OBSERVATIONS OF DIFFUSE GALACTIC EMISSION - FURTHER IDEAS ON THE INTERSTELLAR DUST

In this paper we have analyzed some millimetric observations of the Galactic plane previously published by our group in addition with new unpublished 3 mm data. Both sets of data were carried out at the Italian base in Antarctica. The observed emission fluxes can be matched by either T = 7 K and a lambda(-2) emissivity or T = 15 K and a lambda(-1.1) emissivity. These results imply that either silicates or amorphous/disordered carbon grains are responsible for the detected fluxes. Laboratory measured optical constants of these kinds of materials have been used in the computations. T = 15 K, lambda(-1.1) emissivity might be produced by particles in the diffuse medium; T = 7 K characterizes grains in giant molecular clouds. The experimental data do not allow us to discriminate between these two cases. However, a search in both the Dame et al. (1987) CO sky survey and the Desert, Bazell, & Boulanger (1988) list of molecular clouds does not show any coincidence between the individual sources in these catalogs and the Galactic structures we have detected in our experiment. The negative result as well as a check on the atomic abundances which are needed to reproduce the observed fluxes tend to support the idea that our mm data are actually due to thermal emission by dust sitting in a diffuse medium. Furthermore, if we consider the model by Desert, Boulanger, & Puget (1990) for the composition of the interstellar dust, our results do not contradict that large particles (similar to 0.1 mu m) dominated by silicates with an additional dark refractory mantle account for the FIR emission, but another dust component should be introduced in order to model the emission beyond about 440 mu m.