The Effect of Light Scattering by Dust in Galactic Halos on Emission Line Ratios

We present results from Monte Carlo simulations describing the radiation transfer of Hα line emission, produced both by HII regions in the disk and in the diffuse ionized gas (DIG), through the dust layer of the galaxy NGC891. This allows us to calculate the amount of light originating in the HII regions of the disk and scattered by dust at high z, and compare it with the emission produced by recombinations in the DIG. The cuts of photometric and polarimetric maps along the z-axis show that scattered light from HII regions is still 10% of that of the DIG at z ∼ 600 pc, whereas the the degree of linear polarization is small (< 1%). The importance of these results for the determination of intrinsic emission line ratios is emphasized, and the significance and possible implications of dust at high z are discussed. Subject headings: dust, extinction—ISM: abundances—galaxies: spiral—HII: regions—polarization—radiative transfer—scattering