Radiative transfer models of dusty galaxian discs

We present a set of face-on models of the radiative transfer of stellar light in exponential discs, taking into account absorption and multiple scattering (diffusion) by dust. Attention has been directed towards the effects on the observable photometric properties of different distributions of stars and dust perpendicular to the galactic plane, A realistic model for the arm and interarm regions in the Galaxy, with wavelength-dependent star-to-dust scaleheight ratios, is presented. Face-on luminosity profiles, dust-induced colour excess profiles, and observable scalelengths are computed for various photometric bands (UBVRIK) and for a wide range of on-axis optical thicknesses tau(v)(0) (from 1 to 50). For luminosity profiles, we carefully discuss the effects of neglecting scattering on the predicted surface brightness. The total flux (integrated out to three disc scalelengths), which is lost because of dust extinction, is much reduced if scattering is considered: the total extinction is in fact as small as 0.1 mag for tau(v)(0) = 4, and becomes larger than 1 mag only for tau(v)(0) > 30. For colour excesses, differential extinction produces artificial radial colour gradients in the discs. Their predicted amplitude is strongly dependent on the adopted dust parameters and the star-to-dust scaleheight ratios. Both effects can produce uncertainties in the predicted colour variations of several tenths of a magnitude. Owing to the strong dependence of reddening on the geometrical distributions of stars and dust, no standard extinction law can be expected for external galaxies, even if the physical properties of the dust are invariant. Differential extinction causes the observed scalelength to vary with passband. The advantages and limitations of the so-called 'scalelength test' as a diagnostic of the total dust content of spiral galaxies are discussed.