Monte Carlo Simulations of Dusty Spiral Galaxies: Extinction and Polarization Properties

We present Monte Carlo simulations of dusty spiral galaxies, modelled asbulge + disk systems, aimed to study their extinction and polarizationproperties. The extinction parameters (absorption and scattering) of dustgrains are calculated from Mie's theory for a full distribution of sizes andmaterials; the radiation transfer is carried on for the four Stokes parameters.Photometric and polarimetric maps of galaxies of different optical depths,inclinations and bulge-to-total ratios have been produced in the B and Ibandpasses. As expected, the effect of scattering is to reduce substantiallythe extinction for a given optical depth, in particular for what concerns theobscuration of bright bulge cores. For the same reason, scattering reduces alsothe reddening, as evaluated from B-I maps. On the other hand the bluingdirectly due to forward scattering is hardly appreciable. Radial colorgradients are often found. A comparison with ``sandwich'' models shows thatthey fail dramatically to reproduce the extinction - optical depth relation.The degree of linear polarization produced by scattering is usually of theorder of a few percent; it increases with optical depth, and with inclination(less than 80 degrees). The polarization pattern is always perpendicular to themajor axis, unless the dust distribution is drastically modified. There islittle local correlation between extinction and polarization degree and thereis a trend of increasing polarization from the B to the I band. We discussimplications and relevance of the results for studies of the structure andmorphology of spiral galaxies and of their interstellar medium.Comment: 43 pages, 13 Postscript figures, Latex AAS manuscript substyle, Ap. J. Accepte