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Paper: Multi-wavelength WHAM Observations of Extra-planar Warm Ionized Gas in the Galaxy
Volume: 331, Extra-Planar Gas
Page: 33
Authors: Madsen, G.J.
Abstract: We report on observations of several optical emission lines toward a variety of newly revealed faint, large-scale Hα-emitting structures in the warm ionized medium (WIM) of the Galaxy. The lines include [N II] λ6583, [N II] λ5755, [S II] λ6716, [O III] λ5007, and He I λ5876 and were obtained with the Wisconsin H-Alpha Mapper (WHAM) toward sight-lines that probe both large filamentary features and the more diffuse WIM, from the outer Perseus spiral arm to the inner Galaxy. These emission lines allow us to explore the temperature and ionization conditions within the emitting gas and their variations between the different emission regions. We compare the relative intensities of the emission lines toward these faint H II structures to brighter, classical O star H II regions. We find that the line ratios of [N II]/Hα and [S II]/Hα are generally high, while [O III]/Hα and He I/Hα are generally low in the diffuse WIM and in the faint filamentary structures. This suggests that the gas is warmer, in a lower ionization state, and ionized by a softer spectrum than the gas in classical H II regions surrounding O stars, the presumed ionization source for the WIM. In addition, we find differences in the physical conditions between the large filamentary structures and the more diffuse WIM, suggesting that the filaments are regions of higher density, not geometrical projections of folds in large sheet-like or shell-like structures.
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