Origin and z-distribution of Galactic diffuse [CII] emission

Abstract: We determine the source of the diffuse [CII] emission by studying its spatial (radial and vertical) distributions. We used the HIFI [CII] Galactic survey (GOT C+), along with HI, 12CO, and 13CO data toward 354 lines of sight, and several HIFI [CII] and [CI] position-velocity maps. We quantified the emission in each spectral line profile by evaluating the intensities in 3 km/s wide velocity bins, 'spaxels'. Using the detection of [CII] with CO or [CI], we separated the dense and diffuse gas components. We derived 2-D Galactic disk maps using the spaxel velocities for kinematic distances. We separated the warm and cold H2 gases by comparing CO emissions with and without associated [CII]. We find evidence of widespread diffuse [CII] emission with a z-scale distribution larger than that for the total [CII] or CO. and it consists of (i) diffuse molecular (CO-faint) H2 clouds and (ii) diffuse HI clouds and/or WIM. In the inner Galaxy we find a lack of [CII] detections in a majority (~62%) of HI spaxels and show that the diffuse component primarily comes from the WIM (~21%) and that the HI gas is not a major contributor to the diffuse component (~6%). The warm-H2 radial profile shows an excess in the range 4 to 7 kpc, consistent with enhanced star formation there. We derive, for the first time, the 2-D [CII] spatial distribution in the plane and the z-distributions of the individual [CII] gas component. We estimate the fractional [CII] emission tracing (i) H2 gas in dense and diffuse molecular clouds as ~48% and ~ 14%, respectively, (ii) in the HI gas ~18%, and (iii) in the WIM ~21%. Including non-detections from HI increases the [CII] in HI to ~ 27%. The z-scale distributions FWHM are [CII] sources with CO, ~130 pc, (CO-faint) diffuse H2 gas, ~200 pc, and the diffuse HI and WIM, ~330 pc. CO observations, when combined with [CII], probe the warm-H2 gas, tracing star formation.