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 \lesssim 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 \pm 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 .