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L.M. Haffner, R.J. Reynolds, S.L. Tufte (UW--Madison)
A large region of the Galaxy toward the Perseus arm (\ell = 123\arcdeg to 164\arcdeg, b = -6\arcdeg to -35\arcdeg) has been mapped with the Wisconsin H-Alpha Mapper (WHAM) in the [\ion{S}{2}] \lambda 6716 and [\ion{N}{2}] \lambda 6583 lines. By comparing this data with the maps from the WHAM H\alpha Sky Survey, we begin a global investigation of the physical properties of the Warm Ionized Medium (WIM) in the Galaxy. Several trends noticed in emission-line investigations of diffuse gas in other galaxies are confirmed in the Milky Way and extended to much fainter emission levels. In particular, we find that the [\ion{S}{2}]/H\alpha and [\ion{N}{2}]/H\alpha ratios increase as absolute H\alpha intensities decrease. Examining only the more distant Perseus arm emission, we find that the increase in these ratios is a strong function of b and thus, of height above the Galactic plane, z. We also find that the [\ion{N}{2}]/[\ion{S}{2}] ratio appears to be independent of H\alpha intensity---although a slight increase with z is also seen in this ratio. Scatter in this ratio appears to be physically significant and maps suggest regions with similar [\ion{N}{2}]/[\ion{S}{2}] ratios are spatially correlated. The large variation of [\ion{S}{2}]/H\alpha and [\ion{N}{2}]/H\alpha with nearly constant [\ion{N}{2}]/[\ion{S}{2}] may indicate variations in the local electron temperature. Such an interpretation suggests WIM temperatures that range from 6,000 K to 10,000 K, with regions having fainter H\alpha emission (and thus higher [\ion{S}{2}]/H\alpha and [\ion{N}{2}]/H\alpha) at higher temperatures. The smaller variations in [\ion{N}{2}]/[\ion{S}{2}] may reflect changes in the local ionization conditions (which could affect S++/S+) or in elemental abundance.
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The author(s) of this abstract have provided an email address for comments about the abstract: haffner@astro.wisc.edu