2023MNRAS.525.5468M

We have mapped the NGC 2023 reflection nebula in the 63 and 145 $\mu$m transitions of [O I] and the 158 $\mu$m [C II] spectral lines using the heterodyne receiver upGREAT on SOFIA. The observations were used to identify the diffuse and dense components of the photon-dominated region (PDR) traced by the [C II] and [O I] emission, respectively. The velocity-resolved observations reveal the presence of a significant column of low-excitation atomic oxygen, seen in absorption in the [O I] 63 $\mu$m spectra, amounting to about 20-60 per cent of the oxygen column seen in emission in the [O I] 145 $\mu$m spectra. Some self-absorption is also seen in [C II], but for the most part it is hardly noticeable. The [C II] and [O I] 63 $\mu$m spectra show strong red- and blue-shifted wings due to photoevaporation flows especially in the south-eastern and southern part of the reflection nebula, where comparison with the mid- and high-J CO emission indicates that the C⁺ region is expanding into a dense molecular cloud. Using a two-slab toy model the large-scale self-absorption seen in [O I] 63 $\mu$m is readily explained as originating in foreground low-excitation gas associated with the source. Similar columns have also been observed recently in other Galactic PDRs. These results have two implications: for the velocity-unresolved extragalactic observations this could impact the use of [O I] 63 $\mu$m as a tracer of massive star formation and secondly, the widespread self-absorption in [O I] 63 $\mu$m leads to underestimate of the column density of atomic oxygen derived from this tracer and necessitates the use of alternative indirect methods.