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A&A 382, 624-638 (2002)
DOI: 10.1051/0004-6361:20011646
Radio-millimetre investigation of galactic infrared dark clouds
D. Teyssier, P. Hennebelle and M. PéraultLaboratoire de radioastronomie millimétrique, URA 336 du CNRS, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
(Received 29 December 2000 / Accepted 13 November 2001 )
Abstract
We present follow-up observations of the mid-Infrared dark clouds selected from the ISOGAL inner Galaxy sample. On-the-fly
maps of
13CO, C
18O and the 1.2 mm continuum emission were conducted at the IRAM 30-m telescope, showing spectacular correlation with the mid-IR
absorption. The dark clouds are distributed as far as the prominent molecular ring at a distance of 3 to 7 kpc from the Sun.
The clouds exhibit shapes ranging from globules to thin filaments down to
1 pc in size. The on-the-fly images obtained in
13CO and C
18O confirmed that the cores are dense, compact molecular emitters, significantly more massive than local dark clouds (more
than 1000
) and lie within low activity Giant Molecular Clouds (GMC's). Ratios of the emission in the
J=(2-1) and
(1-0) transitions of
13CO and C
18O show a remarkable uniformity within each cloud, with a significant portion of the sample represented well by a ratio of
. Preliminary analysis of temperature and density measurements reveals that most of the cores have densities above
105 cm
-3 and temperatures between 8 and 25 K, these latter clouds being associated with young embedded stars. Despite the high extinction
inferred from mid-IR (
, Hennebelle et al. 2001), the molecular lines are surprisingly weak, indicating likely depletion onto cold grains.
Key words: ISM: clouds -- molecules -- structure -- radio continuum: ISM -- radio lines: ISM
Offprint request: D. Teyssier, teyssier@lra.ens.fr
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