A thermal plume in NGC 2024

We present 1-arcmin resolution radio images of the Galactic H ii region NGC 2024 made at 0.33 and 10.55 GHz. These are the highest dynamic range images of the nebula to date. The peak brightness in the 0.33-GHz radio continuum image indicates that the electron temperature is 6800–7600 K. The images clarify the structure of a thermal plume that is directed north from the H ii region and opposite to a unipolar molecular outflow found in the molecular cloud near the H ii region. We report the detection of H91α recombination lines from the plume: these data confirm that the plume is blueshifted relative to the molecular cloud. Finally, we discuss the relative merits of three possible explanations for the plume: that the plume results from the blueshifted counterjet to the CO outflow; that the plume is the continuation of the champagne flow observed in the denser parts of the H ii region; and that the plume is produced by the photoevaporation of molecular clumps apparent in a 13CO map of the region.