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Astron. Astrophys. 317, 65-72 (1997)

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Dust and molecules in the Local Group galaxy NGC 6822

II. CO and molecular hydrogen

F.P. Israel

Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands

Received 6 September 1995 / Accepted 24 February 1996


Emission from the J=1-0 12 CO transition was detected unambiguously at 4 out of 17 observed positions in the Magellanic irregular galaxy NGC 6822; two more positions yielded marginal 3 [FORMULA] detections. These positions include the HII region complexes Hubble V and Hubble I/III, and two IRAS far-infrared emission peaks not associated with bright HII regions. The bright HII region Hubble X was not detected. The CO emission detected is rather weak, typically 0.4 - 0.8 K km s -1 in a 105 pc beam. It is well-correlated with infrared brightness, but not with that of HI emission. The sum of all observed spectra yields an integrated CO signal I [FORMULA] = 6.4 [FORMULA] 1.2 K km s -1, 70 [FORMULA] greater than the sum of the individual detected components. This implies the presence of more widespread weak CO emission not detected in individual pointings. The total CO content of NGC 6822 is argued to be 1.2 (+1.2, -0.6) [FORMULA] 10 5 K km s -1 pc 2, nominally twice the amount detected. The available HI, infrared and CO data have been used to estimate the magnitude of the empirical conversion factor X = N(H [FORMULA] )/I [FORMULA] in NGC 6822, resulting in X [FORMULA] = 8 [FORMULA] 3 [FORMULA] 10 21 cm -2 (K km s -1 ) -1, or 40 [FORMULA] 15 times higher than the Galactic conversion factor. Even with this factor, the total H [FORMULA] mass of NGC 6822 is estimated at only M(H [FORMULA] ) = 1.5 (+3, -1) [FORMULA] 10 7 M [FORMULA], roughly 15 per cent of the HI mass. In the Galactic foreground, CO emission from a diffuse cloud was detected with a narrow linewidth of about 1 km s -1 at V [FORMULA] = +5 km s -1.

Key words: galaxies: individual: NGC 6822 — galaxies: ISM; irregular; Local Group — radio lines: galaxies

Send offprint requests to: F.P. Israel

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