FIR AND C+ EMISSIONS OF SPIRAL GALAXIES DISKS : THE EXAMPLE OF NGC 6946

We present numerical simulations of radiative transfer in the spiral galaxy NGC 6946. The interstellar medium is represented as a two phases medium, with molecular clouds and a smooth diffuse phase. The molecular gas distribution is calculated from the distribution of an ensemble of molecular clouds evolving in the gravitational potential of NGC 6946. We simulate star formation by creating OB associations in molecular clouds. The transfer of UV radiation is calculated in the clumpy interstellar medium, to determine the local UV illumination of molecular clouds. We compute the emergent intensity in the UV continuum, in the Halpha and C+ lines as well as in the continuum at far infrared wavelengths, 60, 100 & 200 microns. It is possible to obtain a consistent picture of this galaxy with a global star formation rate of 4 Mo/yr occuring mostly in the spiral arms. The UV opacity of the model spiral galaxy disk, observed face-on, is 0.8 at 100 nm. For radii less than 4 kpc, the C+ line is mostly produced in photodissociation regions at the surfaces of molecular clouds. The C+ emission from diffuse atomic gas accounts for about 20% of the total. It becomes significant at large distance from the nucleus (r > 4 kpc). Molecular clouds and diffuse atomic gas have almost equal contributions to the total far infrared emission from 60 to 200 microns. As a whole, 72% of the 60 - 200 microns FIR emission can be attributed to dust grains heated by the UV radiation of massive stars and 28% by the radiation field of the old stellar population.