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

TL;DR

This study uses Herschel's [CII] line observations to identify warm dark gas in diffuse interstellar clouds, revealing that [CII] emission traces a significant component of warm H$_2$ not detectable by CO.

Contribution

First detection of warm dark gas via [CII] emission in diffuse clouds, highlighting [CII] as a key tracer for this elusive component.

Findings

[CII] emission exceeds expectations for diffuse atomic clouds.

Excess [CII] indicates presence of warm H$_2$ dark gas.

[CII] is effective for mapping warm dark gas in the Milky Way.

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.