Are spiral galaxies optically thin or thick?

Computer Science

Scientific paper

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Galaxies, Dust, Extinction

Scientific paper

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 disk we use exponential distributions in both directions, radially and perpendicular to the plane of the disk, while the de Vaucouleurs (R1/4) profile is used for the description of the bulge. From this analysis we were able to determine all the parameters of the axisymmetric 3D distributions of stars and dust that best fit the surface photometry of the galaxies and thus obtain 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 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 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 quite well with the Galactic extinction law, indicating a universal dust behaviour.

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