Dust and molecules in the Local Group galaxy NGC 6822

Abstract

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 12CO, the 158 mu m transition of C+, the 21-cm line of HI, the Pabeta 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 H2 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 (Tkin = 150 K) and have high molecular gas densities (n( H2) ~ 104 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 106 Msun 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- H2 conversion factor X found earlier, characteristic of starforming low-metallicity regions.