EDP Sciences
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Volume 397, Number 1, January I 2003
Page(s) 87 - 97
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20021464

A&A 397, 87-97 (2003)
DOI: 10.1051/0004-6361:20021464

Dust and molecules in the Local Group galaxy NGC 6822

III. The first-ranked HII region complex Hubble V
F. P. Israel1, F. Baas1, 2, R. J. Rudy3, E. D. Skillman4 and C. E. Woodward4

1  Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
2  Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, Hawaii, 96720, USA
3  Space Science Applications Laboratory, M2-266, The Aerospace Corporation, PO Box 92957, Los Angeles, California 90009-2957, USA
4  Department of Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA

(Received 12 April 2002 / Accepted 12 September 2002 )

We present maps of the first-ranked HII region complex Hubble V in the metal-poor Local Group dwarf galaxy NGC 6822 in the first four transitions of $\,{\rm ^{12}CO}$, the 158  $\mu$m transition of C +, the 21-cm line of HI, the Pa $\beta$ line of HII, and the continuum at 21 cm and 2.2 $\mu$m wavelengths. We have also determined various integrated intensities, notably of HCO + and near-IR $\,{\rm H_{2}}$ emission. Although the second-ranked HII region Hubble X is located in a region of relatively strong HI emission, our mapping failed to reveal any significant CO emission from it. The relatively small CO cloud complex associated with Hubble V is comparable in size to the ionized HII region. The CO clouds are hot ( $T_{\rm kin}
= 150$  K) and have high molecular gas densities ( $n(\,{\rm H_{2}}) \approx 10^{4}
\,{\rm cm^{-3}}$ ). Molecular hydrogen probably extends well beyond the CO boundaries. C + column densities are more than an order of magnitude higher than those of CO. The total mass of the complex is about 10 $^{6}~M_{\odot}$ and molecular gas accounts for more than half of this. The complex is excited by luminous stars reddened or obscured at visual, but apparent at near-infrared wavelengths. The total embedded stellar mass may account for about 10% of the total mass, and the mass of ionized gas for half of that. Hubble V illustrates that modest star formation efficiencies may be associated with high CO destruction efficiencies in low-metallicity objects. The analysis of the Hubble V photon-dominated region (PDR) confirms in an independent manner the high value of the CO-to- $\,{\rm H_{2}}$ conversion factor X found earlier, characteristic of starforming low-metallicity regions.

Key words: galaxies: individual: NGC 6822 -- galaxies: ISM -- galaxies: irregular -- galaxies: Local Group -- radio lines: ISM -- ISM: molecules

Offprint request: F. P. Israel, israel@strw.leidenuniv.nl

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