The spectral signature of dust scattering and polarization in the near-infrared to far-ultraviolet. I. Optical depth and geometry effects

Spectropolarimetry from the near-IR to the far-UV of light scattered by dust provides a valuable diagnostic of the dust composition, grain size distribution and spatial distribution. To facilitate the use of this diagnostic, we present detailed calculations of the intensity and polarization spectral signature of light scattered by optically thin and optically thick dust in various geometries. The polarized light radiative transfer calculations are carried out using the adding-doubling method for a plane-parallel slab and are extended to an optically thick sphere by integrating over its surface. The calculations are for the Mathis, Rumpl, & Nordsieck Galactic dust model, and cover the range from 1 mu m to 500 Angstrom. We find that the wavelength dependence of the scattered light intensity provides a sensitive probe of the optical depth of the scattering medium, while the polarization wavelength dependence provides a probe of the grain scattering properties, which is practically independent of optical depth. We provide a detailed set of predictions, including polarization maps, which can be used to probe the properties of dust through imaging spectropolarimetry in the near-IR to far-UV of various Galactic and extragalactic objects. In a following paper we use the codes developed here to provide predictions for the dependence of the intensity and polarization on grain size distribution and composition.