Origin and z-distribution of Galactic diffuse [C II] emission

Context. The [CII] emission is an important probe of star formation in the Galaxy and in external galaxies. The GOT C+ survey and its follow up observations of spectrally resolved 1.9 THz [C II] emission using Herschel HIFI provides the data needed to quantify the Galactic interstellar [CII] gas components as tracers of star formation. Aims. We determine the source of the diffuse [CII] emission by studying its spatial (radial and vertical) distributions by separating and evaluating the fractions of [C II] and CO emissions in the Galactic ISM gas components. Methods. We used the HIFI [C II] Galactic survey (GOT C+), along with ancillary HI, (CO)-C-12, (CO)-C-13, and (CO)-O-18 data toward 354 lines of sight, and several HIFI [C II] and [C I] position-velocity maps. We quantified the emission in each spectral line profile by evaluating the intensities in 3 kms(-1) wide velocity bins, spaxels. Using the detection of [CII] with CO or [C I], we separated the dense and diffuse gas components. We derived 2D Galactic disk maps using the spaxel velocities for kinematic distances. We separated the warm and cold H-2 gases by comparing CO emissions with and without associated [CII]. Results. We find evidence of widespread diffuse [C II] emission with a z-scale distribution larger than that for the total [C II] or CO. The diffuse [C II] emission consists of (i) diffuse molecular (CO-faint) H-2 clouds and (ii) diffuse H I clouds and/or WIM. In the inner Galaxy we find a lack of [C II] detections in a majority (similar to 62%) of Hi spaxels and show that the diffuse component primarily comes from the WIM (similar to 21%) and that the Hi gas is not a major contributor to the diffuse component (similar to 6%). The warm-H-2 radial profile shows an excess in the range 4 to 7 kpc, consistent with enhanced star formation there. Conclusions. We derive, for the first time, the 2D [C II] spatial distribution in the plane and the z-distributions of the individual [C II] gas component. From the GOT C+ detections we estimate the fractional [C II] emission tracing (i) H-2 gas in dense and diffuse molecular clouds as similar to 48% and similar to 14%, respectively, (ii) in the Hi gas similar to 18%, and (iii) in the WIM similar to 21%. Including non-detections from Hi increases the [C II] in Hi to similar to 27%. The z-scale distributions FWHM from smallest to largest are [C II] sources with CO, similar to 130 pc, (CO-faint) diffuse H-2 gas, similar to 200 pc, and the diffuse Hi and WIM, similar to 330 pc. When combined with [C II], CO observations probe the warm-H-2 gas, tracing star formation.