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A Multiwavelength Optical Emission Line Survey of Warm Ionized Gas in the Galaxy

We report on observations of several optical emission lines toward a variety of newly revealed faint, large-scale Halpha-emitting regions in the Galaxy. The lines include [NII] 6583, [NII] 5755, [SII] 6716, [OIII] 5007, and HeI 5876 obtained with the Wisconsin H-Alpha Mapper (WHAM) toward sightlines that probe superbubbles, high latitude filamentary features, and the more diffuse warm ionized medium (WIM). Our observations include maps covering thousands of square degrees toward the well-known Orion-Eridanus bubble, a recently discovered 60deg x 20deg bipolar superbubble centered in Perseus, plus several classical HII regions surrounding OB stars and hot evolved stellar cores. We use the emission line data to explore the temperature and ionization conditions within the emitting gas and their variations between the different emission regions. We find that in the diffuse WIM and in the faint high latitude filamentary structures the line ratios of [NII]/Ha and [SII]/Ha are generally high, while [OIII]/Ha and HeI/Ha are generally low compared to the bright classical HII regions. This suggests that the gas producing this faint wide-spread emission is warmer, in a lower ionization state, and ionized by a softer spectrum than gas in classical HII regions surrounding O stars, the presumed ionization source for the WIM. In addition, we find differences in physical conditions between the large bubble structures and the more diffuse WIM, suggesting that the ionization of superbubble walls by radiation from interior O associations does not account entirely for the range of conditions found within the WIM, particularly the highest values of [NII]/Ha and [SII]/Ha.