ARE SPIRAL GALAXIES OPTICALLY THIN OR THICK

The opacity of spiral galaxies is examined by mod- elling the dust and stellar content of individual galaxies. The three dimensional model that we use assumes exponential dis- tributions for the dust and the stars in the disk, while the R 1=4 law is used to describe the bulge. In this model, both absorption and scattering by the dust are taken into account. The model is applied to five late-type spiral galaxies (NGC 4013, IC 2531, UGC 1082, NGC 5529 and NGC 5907) using their optical (and near infrared for IC 2531) surface photometry. For these galax- ies we have determined the scalelengths and scaleheights of the stars and the dust in the disk, the bulge characteristics, the in- clination angle and the face-on optical depth. Computation of the dust masses, as well as the extinction as a function of the wavelength, are also reported. Having analyzed a total of seven galaxies thus far, the five galaxies mentioned above plus UGC 2048 and NGC 891 pre- sented in (Xilouris et al. 1997, 1998), we are able to draw some general conclusions, the most significant of which are: 1) The face-on central optical depth is less than one in all optical bands, indicating that typical spiral galaxies like the ones that we have modelled would be completely transparent if they were to be seen face-on. 2) The dust scaleheight is about half that of the stars, which means that the dust is more concentrated near the plane of the disk. 3) The dust scalelength is about 1.4 times larger than that of the stars and the dust is more radially extended than the stars. 4) The dust mass is found to be about an order of a magnitude more than previously measured using the IRAS fluxes, indicat- ing the existence of a cold dust component. The gas-to-dust mass ratio calculated is close to the value derived for our Galaxy. 5) The derived extinction law matches quite well the Galactic extinction law, indicating a universal dust behaviour.