Thermal Pressure in Diffuse H 2 Gas Measured by Herschel Emission and FUSE UV H 2 Absorption

Abstract

UV absorption studies with the Far Ultraviolet Spectroscopic Explorer ( FUSE ) satellite have made important observations of H 2 molecular gas in Galactic interstellar translucent and diffuse clouds. Observations of the 158 μ m [C ii ] fine-structure line with Herschel trace the same H 2 molecular gas in emission. We present [C ii ] observations along 27 lines of sight (LOSs) toward target stars of which 25 have FUSE H 2 UV absorption. Two stars have only HST STIS C ii   λ 2325 absorption data. We detect [C ii ] 158 μ m emission features in all but one target LOS. For three target LOSs that are close to the Galactic plane, 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 H 2 clouds. We analyze the observed velocity integrated [C ii ] spectral-line intensities in terms of the densities and thermal pressures in the H 2 gas using the H 2 column densities and temperatures measured by the UV absorption data. We present the H 2 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 H 2 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 H 2 absorption data for studying diffuse H 2 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.