Detection of Diffuse Interstellar [O ii ] Emission from the Milky Way Using Spatial Heterodyne Spectroscopy

Using a newly developed spatial heterodyne spectrometer (SHS), we have obtained the first radial velocity resolved emission-line profiles of diffuse [O II] 3726 and 3729 ? emission lines from the warm (104 K) ionized component of our Galaxy's interstellar medium. These [O II] lines are a principal coolant for this widespread, photoionized gas and are a potential tracer of variations in the gas temperature resulting from unidentified heating processes that appear to be acting within the Galaxy's disk and halo. By spectrally isolating for the first time Galactic [O II] from atmospheric [O II] emission, we were able to detect interstellar [O II] out to 20? from the Galactic equator with intensities that range from tens of rayleighs near the Galactic plane to less than 1 rayleigh at high Galactic latitudes. The [O II] line profiles clearly show structure indicating emission along the lines of sight from both local and more distant interstellar gas. Comparisons of the [O II] intensities with the intensities of [N II] 6584 ? and H? 6563 ? observed with WHAM indicate that the observed variations in [N II]/H? and [O II]/H? in the diffuse interstellar gas are consistent with variations in temperature and confirm the value of the [O II] observations as a temperature diagnostic for the WIM.