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.Comment: To be published in A&A HIFI Special Editio
