C⁺detection of warm dark gas in diffuse clouds

Abstract

We present the first results of the <i>Herschel<i/> open time key program, Galactic Observations of Terahertz C<sup>+<sup/> (GOT C+) survey of the [CII] <sup>2<sup/>P<sub>3/2<sub/>–<sup>2<sup/>P<sub>1/2<sub/> fine-structure line at 1.9 THz (158 <i>μ<i/>m) using the HIFI instrument on <i>Herschel<i/>. 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 (<i>A<i/><sub>V<sub/> < 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 H<sub>2<sub/>, dark gas, component. This first [CII] 158 <i>μ<i/>m detection of warm dark gas demonstrates the value of this tracer for mapping this gas throughout the Milky Way and in galaxies.