C${}^{+}$ detection of warm dark gas in diffuse clouds

Abstract

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 (A${}_V$ < 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${}_2$, 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.

To be published in A&A HIFI Special Edition

References in corpus (4)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• The VLA Galactic Plane Survey
• [CII] observations of H${}_2$ molecular layers in transition clouds
• A Sample of [CII] Clouds Tracing Dense Clouds in Weak FUV Fields observed by Herschel

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