2002A&A...382..624T

We present follow-up observations of the mid-Infrared dark clouds selected from the ISOGAL inner Galaxy sample. On-the-fly maps of ¹³CO, C¹⁸O and the 1.2mm 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 7kpc from the Sun. The clouds exhibit shapes ranging from globules to thin filaments down to ≲1pc in size. The on-the-fly images obtained in ¹³CO and C¹⁸O confirmed that the cores are dense, compact molecular emitters, significantly more massive than local dark clouds (more than 1000M_☉) 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 C¹⁸O 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 10⁵cm^–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., 2001A&A...365..598H), the molecular lines are surprisingly weak, indicating likely depletion onto cold grains.