Radio-millimetre investigation of galactic infrared dark clouds

We present follow-up observations of the mid-Infrared dark clouds selected from the ISOGAL inner Galaxy sample. On-the-fly maps of (CO)-C-13, (CO)-O-18 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 less than or similar to1 pc in size. The on-the-fly images obtained in (CO)-C-13 and (CO)-O-18 confirmed that the cores are dense, compact molecular emitters, significantly more massive than local dark clouds (more than 1000 M.) and lie within low activity Giant Molecular Clouds (GMC's). Ratios of the emission in the J = (2-1) and (1-0) transitions of (CO)-C-13 and (CO)-O-18 show a remarkable uniformity within each cloud, with a significant portion of the sample represented well by a ratio of 0.67 +/- 0.12. Preliminary analysis of temperature and density measurements reveals that most of the cores have densities above 10(5) 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 (A(v) > 50, Hennebelle et al. 2001), the molecular lines are surprisingly weak, indicating likely depletion onto cold grains.