2005MNRAS.361.1273L

We have observed Hubble V, the brightest Hii region complex in the dwarf irregular galaxy NGC 6822, at near-infrared (near-IR; 1.8-2.4 µm) wavelengths. The line emission maps of Hubble V show the typical structure of a photodissociation region (PDR) where an ionized core, traced by compact Hei emission (2.0587 µm) and Brγ emission (2.1661 µm), is surrounded by an outer layer traced by molecular hydrogen (H₂) emission. The measured line ratios of H₂2-1S(1) (2.2477 µm)/1-0S(1) (2.1218 µm) from 0.2 to 0.6 and the unshifted and unresolved line profiles suggest that the H₂ emission originates purely from a PDR. We find no evidence for shock activity. By comparing the H₂ results with a PDR model, we conclude that Hubble V includes dense (10^4.5/cm3) and warm PDRs. In this environment, most of the H₂ molecules are excited by far-ultraviolet photons (with a field strength of 10^2–4 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.