ATTENUATION EFFECTS IN SPIRAL GALAXIES - MULTIWAVELENGTH PHOTOMETRY AND DISK RADIATIVE-TRANSFER MODELS

We present a quantitative investigation of the optical depth through s piral disks using BVRIJHK colors of 15 highly inclined Sab-Sc spirals and new models for radiative transfer in stellar disks, The models inc lude exponential stellar and dust disks, exponential stellar bulge com ponents, multiple scattering, and both homogeneous and clumpy dust dis tributions. Preliminary comparisons of the observed optical and near-i nfrared (NIR) color gradients across galaxy dust lanes with prediction s from radiative transfer models with slab and spherical shapes unders core the need for these realistic exponential disk geometries. When co mpared with the extent of reddening predicted by the disk models, the maximum optical and NIR color excesses in galaxy dust lanes imply cent ral face-on optical depths of 0.5-2.0 in the V band. For these highly inclined systems, we find this inferred optical depth to be largely in sensitive to the difference between clumpy and homogeneous dust distri butions. Comparisons of galaxy color gradients to models with high cen tral optical depths contradict the often-stated claim that spiral disk s are opaque out to D-25. Our derived optical depths show that the pre dicted radial color changes caused by attenuation in face-on galaxies are much smaller than the observed color gradients in such systems, wh ich suggests that the observed changes are the result of variations in stellar content. The face-on optical depths also imply that, when vie wed edge-on, galaxy dust lanes have optical depths greater than 1.0 ev en in the K band and, thus, the NIR mass-to-light ratio changes across dust features.