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Description
The forbidden fine structure transition of [CII] at 158 micron is one of the major cooling lines of the interstellar medium (ISM). We aim to understand the contribution of the ionized, atomic and molecular phases of the ISM to the [CII] emission from clouds near the dynamical center and the BCLMP302 HII region in the north of the nearby galaxy M33 at a spatial resolution of 50pc. We combine high resolution [CII] spectra taken with the Herschel/HIFI spectrometer onboard the Herschel satellite with [CII] Herschel-PACS maps and ground-based observations of CO(2-1) with IRAM 30m and HI with VLA, all at the same spatial resolution. We decomposed the [CII] spectra in terms of contribution from molecular and atomic gas detected in CO(2-1) and HI, respectively. Typically, the [CII] lines have widths intermediate between the narrower CO(2-1) and broader HI line profiles. Comparison of spectra show that the relative contribution of molecular and atomic gas traced by CO(2-1) and HI varies substantially between positions and depends mostly on the local physical conditions and geometry. In both regions a larger fraction of the molecular gas is traced by [C II] than by the canonical tracer CO. These results emphasize the need for velocity-resolved observations to discern the contribution of different components of the ISM to [C II] emission.
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