Detection of [N II] λ5755 Emission from Low-Density Ionized Interstellar Gas

The extremely faint, temperature-sensitive "auroral" emission line [N II] λ5755 has been detected from the low-density ionized gas along the sight line toward l = 1300, b = -75 using the Wisconsin Hα Mapper. The intensity of this emission line, relative to the red nebular line [N II] λ6584, is found to be twice that observed in bright, classical H II regions surrounding O stars. This implies that the electron temperature of the ionized gas along this sight line is about 2000 K higher than the H II regions and that the enhanced [N II] λ6584/Hα and [S II] λ6716/Hα intensity ratios in this low-density gas are due at least in part to an elevated temperature.

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