We present Monte Carlo simulations of dusty spiral galaxies, modelled as
bulge + disk systems, aimed to study 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 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 I
bandpasses. As expected, the effect of scattering is to reduce substantially
the extinction for a given optical depth, in particular for what concerns the
obscuration of bright bulge cores. For the same reason, scattering reduces also
the reddening, as evaluated from B-I maps. On the other hand the bluing
directly due to forward scattering is hardly appreciable. Radial color
gradients are often found. A comparison with ``sandwich'' models shows that
they fail dramatically 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
(less than 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
implications and relevance of the results for studies of the structure and
morphology of spiral galaxies and of their interstellar medium.Comment: 43 pages, 13 Postscript figures, Latex AAS manuscript substyle, Ap.
J. Accepte