High Resolution Observations of the Near-Infrared Emission from NGC 6822 Hubble V
Abstract
We observed Hubble V, the brightest HII region complex in the dwarf irregular galaxy NGC 6822, at near-infrared (1.8--2.4 um) wavelengths using the Cooled Grating Spectrometer 4 (CGS4) at the United Kingdom Infra-Red Telescope (UKIRT). The line emission maps of Hubble V show the typical structure of a photo-dissociation region (PDR) where an ionized core, traced by compact He I emission (2.0587 um) and Brgamma emission (2.1661 um), is surrounded by an outer layer traced by molecular hydrogen (H2) emission. The measured line ratios of H2 2-1 S(1) (2.2477 um) / 1-0 S(1) (2.1218 um) from 0.2 to 0.6 and the un-shifted and un-resolved line profiles suggest that the H2 emission originates purely from a PDR. By comparing the H2 results with a PDR model, we conclude that Hubble V includes dense (104.5 cm-3) and warm PDRs. In this environment, most of the H2 molecules are excited by far-UV photons (with a field strength of 102-4 times that of the average interstellar field), although collisional processes de-excite H2 and contribute significantly to the excitation of the first vibrational level. We expect that Hubble V is in the early stage of molecular cloud dissolution.
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