We present the first results of the Herschel open time key program, Galactic Observations of Terahertz C+ (GOT C+) survey of the [CII] fine-structure line at 1.9 THz (158 microns) using the HIFI instrument on Herschel. We detected 146 interstellar clouds along sixteen lines-of-sight towards the inner Galaxy. We also acquired HI and CO isotopologue data along each line-of-sight for analysis of the physical conditions in these clouds. Here we analyze 29 diffuse clouds (AV < 1.3 mag.) in this sample characterized by having [CII] and HI emission, but no detectable CO. We find that [CII] emission is generally stronger than expected for diffuse atomic clouds, and in a number of sources is much stronger than anticipated based on their HI column density. We show that excess [CII] emission in these clouds is best explained by the presence of a significant diffuse warm H2, dark gas, component. This first [CII] 158 micron detection of warm dark gas demonstrates the value of this tracer for mapping this gas throughout the Milky Way and in galaxies.
@article{arxiv.1007.3048,
title = {C$^+$ detection of warm dark gas in diffuse clouds},
author = {W. D. Langer and T. Velusamy and J. L. Pineda and P. F. Goldsmith and D. Li and H. W. Yorke},
journal= {arXiv preprint arXiv:1007.3048},
year = {2015}
}