The Spectral Signature of Dust Scattering and Polarization in the Near IR to Far UV. I. Optical Depth and Geometry Effects

摘要

Spectropolarimetry from the near IR to the far UV of light scattered by dustprovides a valuable diagnostic of the dust composition, grain size distributionand spatial distribution. To facilitate the use of this diagnostic, we presentdetailed calculations of the intensity and polarization spectral signature oflight scattered by optically thin and optically thick dust in variousgeometries. The polarized light radiative transfer calculations are carried outusing the adding-doubling method for a plane-parallel slab, and are extended toan optically thick sphere by integrating over its surface. The calculations arefor the Mathis, Rumple & Nordsieck Galactic dust model, and cover the rangefrom 1 $\mu m$ to 500 \AA. We find that the wavelength dependence of thescattered light intensity provides a sensitive probe of the optical depth ofthe scattering medium, while the polarization wavelength dependence provides aprobe of the grain scattering properties, which is practically independent ofoptical depth. We provide a detailed set of predictions, including polarizationmaps, which can be used to probe the properties of dust through imagingspectropolarimetry in the near IR to far UV of various Galactic andextragalactic objects. In a following paper we use the codes developed here toprovide predictions for the dependence of the intensity and polarization ongrain size distribution and composition.