Extended Carbon Emission in the Galaxy: Dark Gas along the G328 Sightline

We present spectral data cubes of the [CI] 809 GHz, (CO)-C-12 115 GHz, (CO)-C-13 110 GHz and HI 1.4 GHz line emission from a similar to 1 degrees region along the l = 328 degrees (G328) sightline in the Galactic Plane. The [CI] data comes from the High Elevation Antarctic Terahertz telescope at Ridge A on the summit of the Antarctic plateau, where the extremely low levels of precipitable water vapour open atmospheric windows for THz observations. The CO data comes from the Southern Galactic Plane Survey being conducted with the Mopra telescope. Emission arises principally from gas in three spiral arm crossings along the sight line. The distribution of the emission in the CO and [CI] lines is found to be similar, with the [CI] slightly more extended, and both are enveloped in extensive HI. Spectral line ratios are similar across the entire extent of the Galaxy. However, towards the edges of the molecular clouds the [CI]/(CO)-C-13 and (CO)-C-12/(CO)-C-13 line ratios rise by similar to 50%, and the [CI]/HI ratio falls by similar to 10%. We attribute this to sightlines passing predominantly through the surfaces of photodissociation regions (PDRs), where the carbon is found mainly as C or C+ rather than CO, while the gas is mostly molecular. This is the signature of dark molecular gas.