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We present spectral line images of [CI] 809 GHz, CO J=1-0 110 & 115 GHz and HI 1.4 GHz line emission along a ~1ТА region in the l=328ТА direction of the Galactic plane. The [CI] data comes from the High Elevation Antarctic Terahertz (HEAT) telescope, a new facility on the summit of the Antarctic plateau where the precipitable water vapour falls to the lowest values found on the surface of the Earth. Some terahertz windows are opened through the atmosphere from this site almost continuously during the winter. The CO and HI datasets come from the Mopra and Parkes/ATCA Southern Galactic Plane Surveys, respectively. Together, these data cubes provide wide-field (1ТА scale) panoramic imaging at good spatial and spectral resolution (2’ and 1 km/s) of the atomic and molecular gas of the interstellar medium. We are able to calculate and compare the column densities in the carbon, carbon-monoxide and hydrogen emitting regions. Carbon is found in neutral or singly-ionized form within the first optical depth from the surfaces of molecular cloud, so that the [CI] emission provides a direct probe of the amount of dark (i.e. CO-free) molecular gas that is present. We also identify a filamentary molecular cloud, ~75 x 5pc long with mass ~4 x 10E4 MSun and a narrow velocity emission range of just 4 km/s. The morphology and kinematics of this filament are similar in CO, [CI] and HI, though in the latter appears as self-absorption. We calculate line fluxes and column densities for the three emitting species, which are broadly consistent with a PDR model for a GMC exposed to the average interstellar radiation field. The [C/CO] abundance ratio averaged through the filament is found to be approximately unity. The G328 filament is constrained to be cold (TDust < 20K) by the lack of far-IR emission, to show no clear signs of star formation, and to only be mildly turbulent from the narrow line width. We suggest that it may represent a GMC shortly after formation, or perhaps still be in the process of formation. |
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