We have mapped the NGC 2023 reflection nebula in the 63 and 145 micron
transitions of [O I] and the 158 micron [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 micron spectra, amounting to about 20-60% of the oxygen column
seen in emission in the [O I] 145 micron 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 micron 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 micron 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 micron as a tracer of massive star formation and secondly the
widespread self-absorption in [O I] 63 micron leads to underestimate of the
column density of atomic oxygen derived from this tracer and necessitates the
use of alternative indirect methods.Comment: Accepted for publication in MNRA