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

TL;DR

This study maps the [O I] and [C II] emission lines in NGC 2023 to analyze the nebula's structure, revealing significant atomic oxygen absorption and gas dynamics, which impact star formation tracers and column density estimates.

Contribution

It provides the first velocity-resolved [O I] observations of NGC 2023, demonstrating widespread self-absorption and its implications for interpreting atomic oxygen in PDRs.

Findings

Detected significant [O I] absorption indicating foreground low-excitation gas.

Observed gas expansion into dense molecular cloud via spectral line wings.

Widespread [O I] 63 micron self-absorption affects atomic oxygen column density estimates.

Abstract

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.