Dust and molecules in the local group galaxy NGC 6822. III. The first-ranked HII region complex Hubble V

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 μm transition of C⁺, the 21-cm line of HI, the Paβ line of HII, and the continuum at 21 cm and 2.2 μm 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 (T_kin = 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 M_⊙ 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.