# Thermal Pressure in Diffuse H2 Gas Measured by Herschel [C II] Emission and FUSE UV H2 Absorption

## TL;DR

This study combines Herschel [C II] emission and FUSE UV H2 absorption data to measure thermal pressure and density in diffuse molecular clouds, providing new insights into the physical conditions of interstellar H2 gas.

## Contribution

It presents the first combined analysis of [C II] emission and UV absorption to determine physical conditions in diffuse H2 clouds, revealing mean thermal pressures.

## Key findings

- Mean thermal pressure range 6100-7700 K cm^-3 in diffuse H2 clouds
- Detection of [C II] emission in nearly all observed lines of sight
- Comparison of thermal pressures with other tracers like CI and CO

## Abstract

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.