The Optical Depth of [CII]: the Implications for Galaxies both Near and Far

Average spectra of the [CII] emission for M17 SW superposed on a composite image with Herschel (70 µm, red), Spitzer (3.6 µm, white), and SOFIA (37 µm, green) data. Compare the [ ¹² CII] spectrum (blue) with the [ ¹³ CII] spectrum (red), which has been scaled up (by the ¹² C/ ¹³ C abundance ratio of 40 and the relative satellite weight of 1/0.25). The intensity difference indicates that [ ¹² CII] is optically thick, with absorption dips mimicking velocity components.

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