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 H alpha-emitting regions in the Galaxy. The lines include [N II] lambda 6583, [N II] lambda 5755, [S II] lambda 6716, [O III] lambda 5007, and He I lambda 5876 obtained with theWisconsin H alpha Mapper (WHAM) toward sight lines 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 60 degrees x 20 degrees bipolar superbubble centered in Perseus, plus several classical H II 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 [N II]/H alpha and [S II]/H alpha are generally high, while [O III]/H alpha and HeI/H alpha are generally low compared to the bright classical H II regions. This suggests that the gas producing this faint widespread emission is warmer, in a lower ionization state, and ionized by a softer spectrum than gas in classical H II 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 [N II]/H alpha and [S II]/H alpha.