Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-09-20
Astronomy and Astrophysics
Astrophysics
8 pages, 5 figures. To be published in "Extra-planar Gas", ed. R. Braun, held June 2004 in Dwingeloo. Higher resolution versio
Scientific paper
We report on observations of several optical emission lines toward a variety of newly revealed faint, large-scale H-alpha-emitting structures in the warm ionized medium (WIM) of the Galaxy. The lines include [N II] 6583, [N II] 5755, [S II] 6716, [O III] 5007, and He I 5876 and were obtained with the Wisconsin H-Alpha Mapper (WHAM) toward sightlines 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]/H-alpha and [S II]/H-alpha are generally high, while [O III]/H-alpha and He I/H-alpha 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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