Thermal Pressure in Diffuse H2 Gas Measured by Herschel [C II] Emission
  and FUSE UV H2 Absorption
  UV absorption studies with FUSE have observed H2 molecular gas in translucent
and diffuse clouds. Observations of the 158 micron [C II] fine structure line
with Herschel also trace the same H2 molecular gas in emission. We present [C
II] observations along 27 lines of sight (LOSs) towards target stars of which
25 have FUSE H2 UV absorption. We detect [C II] emission features in all but
one target LOS. For three Target LOSs, which are close to the Galactic plane,
we also present position-velocity maps of [C II] emission observed by HIFI in
on-the-fly spectral line mapping. We use the velocity resolved [C II] spectra
towards 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 6100 to 7700 K cm^-3 in diffuse H2 clouds. We discuss the thermal
pressures and densities towards 14 targets, comparing them to results obtained
using the UV absorption data for two other tracers CI and CO.
