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Astron. Astrophys. 339, 19-33 (1998)

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FIR and C+ emissions of spiral galaxies disks

The example of NGC 6946

S. Sauty 1, M. Gerin 2, 1 and F. Casoli 1

1 DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire, F-75014 Paris, France; and URA 336 du CNRS
2 Radioastronomie Millimétrique, ENS, 24 Rue Lhomond, F-75231 Paris Cedex 05, France; and URA 336 du CNRS

Received 29 September 1997 / Accepted 25 May 1998


We present numerical simulations of radiative transfer in the spiral galaxy NGC 946. The interstellar medium is represented as a two phases medium, with molecular clouds and a smooth diffuse phase. The molecular gas distribution is calculated in a self-consistent way from the distribution of an ensemble of molecular clouds evolving in the gravitational potential of NGC 946. 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 (912-2000 Å), in the H[FORMULA] and C+ [FORMULA] lines as well as in the continuum at far infrared wavelengths, 60, 100 & 200 µm.

It is possible to obtain a consistent picture of this galaxy with a global star formation rate of 4 [FORMULA]yr-1 (for stars with masses in the range 2-60 [FORMULA]) occuring mostly in the spiral arms. The close spatial association of massive stars and molecular clouds has a profound impact on the transfer of UV radiation in the galactic disk and on the dust emission. The median distance travelled by UV photons is about 120 pc. However, when they have escaped 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 Å and 0.7 at 2000 Å.

For radii less than 4 kpc, the C+ 158 [FORMULA] 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 ([FORMULA] kpc). Molecular clouds and diffuse atomic gas have almost equal contributions to the total far infrared emission from 60 to 200 [FORMULA]. As a whole, 72% of the 60-200 [FORMULA] 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.

Key words: ISM: molecules – ISM: dust, extinction – galaxies: individual: NGC 6946 – galaxies: ISM

Send offprint requests to: M. Gerin

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998