Monte Carlo predictions of far-infrared emission from spiral galaxies

We present simulations of Far Infrared (FIR) emission by dust in spiral gal axies, based on the Monte Carlo radiative transfer code of Bianchi, Ferrara & Giovanardi (1996). The radiative transfer is carried out at several wave length in the Ultraviolet (UV), optical and Near Infrared (NIR), to cover t he range of the stellar Spectral Energy Distribution (SED). Together with t he images of the galactic model, a map of the energy absorbed by dust is pr oduced. Using Galactic dust properties, the spatial distribution of dust te mperature is derived under the assumption of thermal equilibrium. A correct ion is applied for non-equilibrium emission in the Mid Infrared (MIR). Imag es of dust emission can then be produced at any wavelength in the FIR. We show the application of the model to the spiral galaxy NGC 6946. The obs erved stellar SED is used as input and models are produced for different st ar-dust geometries. It is found that only optically thick dust disks can re produce the observed amount of FIR radiation. However, it is not possible t o reproduce the large FIR scalelength suggested by recent observation of sp irals at 200 mu m, even when the scalelength of the dust disk is larger tha n that for stars. Optically thin models have ratios of optical/FIR scalelen gths closer to the 200 mu m observations, but with smaller absolute scalele ngths than optically thick cases. The modelled temperature distributions ar e compatible with observations of the Galaxy and other spirals. We finally discuss the approximations of the model and the impact of a clumpy stellar and dust structure on the FIR simulations.