Modelling the dust content of spiral galaxies - Towards an understanding of the dust opacity problem

We compare optical and near-infrared surface photometry of seven edge-on spiral galaxies with corresponding surface photometry calculated from a realistic model of spiral galaxies which takes into account both absorption and scattering by the interstellar dust. For the stars and the dust in the disc we use exponential distributions in both directions, radially and perpendicular to the plane of the disc, while the de Vaucouleurs (R-1/4) profile is used for the description of the bulge. The effect of the spiral structure in the galactic discs is also examined and it is found that the simple exponential disc model is able to describe quite accurately the real galaxy. From this analysis we were able to obtain some general conclusions, the most significant of which are: 1) The face-on central optical depth is less than unity in all optical bands, indicating that typical spiral galaxies like those we have modelled would be completely transparent if they were to be seen face on. 2) The dust scale height is about half that of the stars, which means that dust is more concentrated near the plane of the disc. 3) The dust scale length is about 1.4 times larger than that of the stars and dust is more radially extended than the stars. 4) The gas-to-dust mass ratio calculated, with the dust mass derived from the model, is close to the value derived for our Galaxy. 5) The derived extinction law matches the Galactic extinction law quite well, indicating a universal dust behaviour.