2020A&A...636A..16G

Context. The [CII] 158 µm far-infrared fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [CII] emission.
Aims. Our aim is to determine whether the complex profiles observed in [¹²CII] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [¹²CII] and isotopic [¹³CII] line profiles.
Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [¹³CII] emission lines, along with the [¹²CII] emission lines, with a high signal-to-noise ratio. We first derived the [¹²CII] optical depth and the [CII] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the [CII] gas: column density and excitation temperature.
Results. We find moderate to high [¹²CII] optical depths in all four sources and self-absorption of [¹²CII] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent A_v of up to 41mag. The foreground absorption requires substantial column densities of cold and dense [CII] gas, with an equivalent A_v ranging up to about 13mag.
Conclusions. The column density of the warm background material requires multiple photon-dominated region surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [CII] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins.