Attenuation effects in spiral galaxies: Multiwavelength photometry and disk radiative transfer models

We present a quantitative investigation of the optical depth through spiral disks using BVRIJHK colors of 15 highly inclined Sab-Sc spirals and new models for radiative transfer in stellar disks, The models include exponential stellar and dust disks, exponential stellar bulge components, multiple scattering, and both homogeneous and clumpy dust distributions. Preliminary comparisons of the observed optical and near-infrared (NIR) color gradients across galaxy dust lanes with predictions from radiative transfer models with slab and spherical shapes underscore the need for these realistic exponential disk geometries. When compared with the extent of reddening predicted by the disk models, the maximum optical and NIR color excesses in galaxy dust lanes imply central 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 insensitive to the difference between clumpy and homogeneous dust distributions. Comparisons of galaxy color gradients to models with high central optical depths contradict the often-stated claim that spiral disks are opaque out to D(25). Our derived optical depths show that the predicted radial color changes caused by attenuation in face-on galaxies are much smaller than the observed color gradients in such systems, which suggests that the observed changes are the result of variations in stellar content. The face-on optical depths also imply that, when viewed edge-on, galaxy dust lanes have optical depths greater than 1.0 even in the K band and, thus, the NIR mass-to-light ratio changes across dust features.