FIR AND C- THE EXAMPLE OF NGC-6946( EMISSIONS OF SPIRAL GALAXIES DISKS )

We present numerical simulations of radiative transfer in the spiral g alaxy NGC 946. The interstellar medium is represented as a two phases medium, with molecular clouds and a smooth diffuse phase. The molecula r gas distribution is calculated in a self-consistent way from the dis tribution of an ensemble of molecular clouds evolving in the gravitati onal potential of NGC 946. We simulate star formation by creating OB a ssociations in molecular clouds. The transfer of UV radiation is calcu lated in the clumpy interstellar medium, to determine the local UV ill umination of molecular clouds. We compute the emergent intensity in th e UV continuum (912-2000 Angstrom), in the H alpha and C+ P-2(3/2) - P -2(1/2) lines as well as in the continuum at far infrared wavelengths, 60, 100 & 200 mu m. It is possible to obtain a consistent picture of this galaxy with a global star formation rate of 4 M(circle dot)yr(-1) (for stars with masses in the range 2-60 M-circle dot) occuring mostl y in the spiral arms. The close spatial association of massive stars a nd molecular clouds has a profound impact on the transfer of UV radiat ion in the galactic disk and on the dust emission. The median distance travelled by UV photons is about 120 pc. However, when they have esca ped from the vicinity of their parent OB associations, UV photons may travel quite far in the disk, up to 1 kpc. The UV opacity of the model spiral galaxy disk, observed face-on, is 0.8 at 1000 Angstrom and 0.7 at 2000 Angstrom. For radii less than 4 kpc, the C+ 158 mu m line is mostly produced in photodissociation regions at the surfaces of molecu lar clouds. The C+ emission from diffuse atomic gas accounts for about 20% of the total. It becomes significant at large distance from the n ucleus (r greater than or equal to 4 kpc). Molecular clouds and diffus e atomic gas have almost equal contributions to the total far infrared emission from 60 to 200 mu m. As a whole, 72% of the 60-200 mu m FIR emission can be attributed to dust grains heated by the UV radiation o f massive stars and 28% by the radiation field of the old stellar popu lation.