Constraining the geometry of the reflection nebula NGC 2023 with [O i]: Emission & Absorption

Abstract

We have mapped the NGC 2023 reflection nebula in the 63 and 145 μm transitions of [O i] and the 158 μm [C ii] spectral lines using the heterodyne receiver upGREAT on SOFIA. The observations were used to identify the diffuse and dense components of the 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 μm spectra, amounting to about 20–60% of the oxygen column seen in emission in the [O i] 145 μm 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 μm spectra show strong red- and blue-shifted wings due to photo evaporation flows especially in the southeastern 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 μm is readily explained as originating in foreground low-excitation gas associated with the source. Similar columns have also been observed recently in other Galactic photon-dominated-regions (PDRs). These results have two implications: for the velocity-unresolved extra-galactic observations this could impact the use of [O i] 63 μm as a tracer of massive star formation and secondly the widespread self-absorption in [O i] 63 μm leads to underestimate of the column density of atomic oxygen derived from this tracer and necessitates the use of alternative indirect methods.