THE EFFECT OF LIGHT-SCATTERING BY DUST IN GALACTIC HALOS ON EMISSION-LINE RATIOS

We present results from Monte Carlo simulations describing the radiati ve transfer of H alpha line emission, produced both by H II regions in the disk and in the diffuse ionized gas (DIG), through the dust layer of the galaxy NGC 891. This allows us to calculate the amount of ligh t originating in the H II 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-ax is show that scattered light from H II regions is still 10% of that of the DIG at z similar to 600 pc whereas the degree of linear polarizat ion is small (<1%). The importance of these results for the determinat ion of intrinsic emission-line ratios is emphasized, and the significa nce and possible implications of dust at high z are discussed.