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 13 CO, C 18 O 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 $\la$1 pc in size. The on-the-fly images obtained in 13 CO and C 18 O confirmed that the cores are dense, compact molecular emitters, significantly more massive than local dark clouds (more than 1000 $M_\odot$) and lie within low activity Giant Molecular Clouds (GMC's). Ratios of the emission in the $J=(2{-}1)$ and $(1{-}0)$ transitions of 13 CO and C 18 O show a remarkable uniformity within each cloud, with a significant portion of the sample represented well by a ratio of $0.67\pm0.12$. 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 ($A_{\rm v} > 50$, Hennebelle et al. [CITE]), the molecular lines are surprisingly weak, indicating likely depletion onto cold grains.