RADIATIVE-TRANSFER MODELS OF DUSTY GALAXIAN DISCS

We present a set of face-on models of the radiative transfer of stella r light in exponential discs, taking into account absorption and multi ple scattering (diffusion) by dust. Attention has been directed toward s the effects on the observable photometric properties of different di stributions of stars and dust perpendicular to the galactic plane, A r ealistic model for the arm and interarm regions in the Galaxy, with wa velength-dependent star-to-dust scaleheight ratios, is presented. Face -on luminosity profiles, dust-induced colour excess profiles, and obse rvable 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 becaus e 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, an d becomes larger than 1 mag only for tau(v)(0) > 30. For colour excess es, differential extinction produces artificial radial colour gradient s 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 re ddening on the geometrical distributions of stars and dust, no standar d extinction law can be expected for external galaxies, even if the ph ysical properties of the dust are invariant. Differential extinction c auses the observed scalelength to vary with passband. The advantages a nd limitations of the so-called 'scalelength test' as a diagnostic of the total dust content of spiral galaxies are discussed.