High Resolution Observations of the Near-Infrared Emission from NGC 6822 Hubble V

Details High Resolution Observations of the Near-Infrared Emission from NGC 6822 Hubble V High Resolution Observations of the Near-Infrared Emission from NGC 6822 Hubble V

2004-10-24

arxiv.org/abs/astro-ph/0410572v2

Mon.Not.Roy.Astron.Soc. 361 (2005) 1273-1280

Astronomy and Astrophysics

Astrophysics

9 pages, 3 figures, MNRAS in press, Replacement: much condensation in the length but small change in the conclusions

Scientific paper

10.1111/j.1365-2966.2005.09259.x

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 Br_gamma emission (2.1661 um), is surrounded by an outer layer traced by molecular hydrogen (H_2) emission. The measured line ratios of H_2 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 H_2 emission originates purely from a PDR. By comparing the H_2 results with a PDR model, we conclude that Hubble V includes dense (10^{4.5} cm^{-3}) and warm PDRs. In this environment, most of the H_2 molecules are excited by far-UV photons (with a field strength of 10^{2-4} times that of the average interstellar field), although collisional processes de-excite H_2 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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