2019A&A...631L..12O

Context. [¹³CII] observations in several Galactic sources show that the fine-structure [¹²CII] emission is often optically thick (the optical depths around 1 to a few).
Aims. Our goal was to test whether this also affects the [¹²CII] emission from nearby galaxies like the Large Magellanic Cloud (LMC).
Methods. We observed three star-forming regions in the LMC with upGREAT on board SOFIA at the frequency of the [CII] line. The 4GHz bandwidth covers all three hyperfine lines of [¹³CII] simultaneously. For the analysis, we combined the [¹³CII] F=1-0 and F=1-1 hyperfine components as they do not overlap with the [¹²CII] line in velocity.
Results. Three positions in N159 and N160 show an enhancement of [¹³CII] compared to the abundance-ratio-scaled [¹²CII] profile. This is likely due to the [¹²CII] line being optically thick, supported by the fact that the [¹³CII] line profile is narrower than [¹²CII], the enhancement varies with velocity, and the peak velocity of [¹³CII] matches the [OI] 63 µm self-absorption. The [¹²CII] line profile is broader than expected from a simple optical depth broadening of the [¹³CII] line, supporting the scenario of several PDR components in one beam having varying [¹²CII] optical depths. The derived [¹²CII] optical depth at three positions (beam size of 14'', corresponding to 3.4 pc) is 1-3, which is similar to values observed in several Galactic sources shown in previous studies.If this also applies to distant galaxies, the [CII] intensity will be underestimated by a factor of approximately 2.