Monte Carlo simulations of dusty spiral galaxies: Extinction and polarization properties

We present Monte Carlo simulations of dusty spiral galaxies, modeled as bulge-plus-disk systems, aimed at studying their extinction and polarization properties. The extinction parameters (absorption and scattering) of dust grains are calculated from Mie's theory for a full distribution of sizes and materials; the radiative transfer is carried out for the four Stokes parameters. Photometric and polarimetric maps of galaxies of different optical depths, inclinations, and bulge-to-total luminosity ratios have been produced in the B and I bands. As expected, the effect of scattering is to reduce substantially the extinction for a given optical depth, in particular concerning the obscuration of bright bulge cores. For the same reason, scattering also reduces the reddening, as evaluated from B-l maps. On the other hand, the blueing directly due to forward scattering is hardly appreciable. Radial color gradients are often found. A comparison with ''sandwich'' models shows that they dramatically fail to reproduce the extinction-optical depth relation. The degree of linear polarization produced by scattering is usually of the order of a few percent; it increases with optical depth and with inclination (i less than or equal to 80 degrees). The polarization pattern is always perpendicular to the major axis unless the dust distribution is drastically modified. There is little local correlation between extinction and polarization degree, and there is a trend of increasing polarization from the B to the I band. We discuss the implications and relevance of the results for studies of the structure and morphology of spiral galaxies and of their interstellar mediums.