2017ApJ...838..165V

UV absorption studies with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite have made important observations of H₂ molecular gas in Galactic interstellar translucent and diffuse clouds. Observations of the 158 µm [C II] fine-structure line with Herschel trace the same H₂ molecular gas in emission. We present [C II] observations along 27 lines of sight (LOSs) toward target stars of which 25 have FUSE H₂ UV absorption. Two stars have only HST STIS C II λ2325 absorption data. We detect [C II] 158 µm emission features in all but one target LOS. For three target LOSs that are close to the Galactic plane, | b|   1°, we also present position-velocity maps of [C II] emission observed by Herschel Heterodyne Instrument in the Far Infrared (HIFI) in on-the-fly spectral-line mapping. We use the velocity-resolved [C II] spectra observed by the HIFI instrument toward the target LOSs observed by FUSE to identify [C II] velocity components associated with the H₂ clouds. We analyze the observed velocity integrated [C II] spectral-line intensities in terms of the densities and thermal pressures in the H₂ gas using the H₂ column densities and temperatures measured by the UV absorption data. We present the H₂ gas densities and thermal pressures for 26 target LOSs and from the [C II] intensities derive a mean thermal pressure in the range of ∼6100-7700 K cm^–3 in diffuse H₂ clouds. We discuss the thermal pressures and densities toward 14 targets, comparing them to results obtained using the UV absorption data for two other tracers C i and CO. Our results demonstrate the richness of the far-IR [C II] spectral data which is a valuable complement to the UV H₂ absorption data for studying diffuse H₂ molecular clouds. While the UV absorption is restricted to the directions of the target star, far-IR [C II] line emission offers an opportunity to employ velocity-resolved spectral-line mapping capability to study in detail the clouds' spatial and velocity structures.