The molecular interstellar medium of the Local Group dwarf NGC 6822

The molecular ISM of NGC 6822

¹ Laboratoire d'Astrophysique de Bordeaux, Université de Bordeaux, OASU, CNRS/INSU, 33271 Floirac, France e-mail: gratier@obs.u-bordeaux1.fr
² IRAM, 300 Rue de la piscine, 38406 St Martin d'Hères, France
³ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 26 January 2009
Accepted: 13 January 2010

Abstract

Do molecular clouds collapse to form stars at the same rate in all environments? In large spiral galaxies, the rate of transformation of H₂ into stars varies little. However, the SFE in distant objects (z ~ 1) is much higher than in the large spiral disks that dominate the local universe. Some small Local Group galaxies share at least some of the characteristics of intermediate-redshift objects, such as size or color. Recent work has suggested that the star formation efficiency (SFE, defined as the star formation rate per unit H₂) in local Dwarf galaxies may be as high as in the distant objects. A fundamental difficulty in these studies is the independent measure of the H₂ mass in metal-deficient environments. At 490 kpc, NGC 6822 is an excellent choice for this study; it has been mapped in the CO(2–1) line using the multibeam receiver HERA on the 30 m IRAM telescope, yielding the largest sample of giant molecular clouds (GMCs) in this galaxy. Despite the much lower metallicity, we find no clear difference in the properties of the GMCs in NGC 6822 and those in the Milky Way except lower CO luminosities for a given mass. Several independent methods indicate that the total H₂ mass in NGC 6822 is about 5 × 10⁶ in the area we mapped and less than 10⁷ in the whole galaxy. This corresponds to a N(H₂)/I_CO ≈ 4 × 10²¹ cm^-2/(K km s^-1) over large scales, such as would be observed in distant objects, and half that in individual GMCs. No evidence was found for H₂ without CO emission. Our simulations of the radiative transfer in clouds are entirely compatible with these N(H₂)/I_CO values. The SFE implied is a factor 5–10 higher than what is observed in large local universe spirals. The CO observations presented here also provide a high-resolution datacube (1500 a.u. for the assumed 100 pc distance, 0.41 km s^-1 velocity resolution) of a local molecular cloud along the line of sight.