Radio-millimetre investigation of galactic infrared dark clouds^*

Laboratoire de radioastronomie millimétrique, URA 336 du CNRS, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France

Corresponding author: D. Teyssier, teyssier@lra.ens.fr

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, C¹⁸O 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 C¹⁸O 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 and 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 . 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. [CITE]), the molecular lines are surprisingly weak, indicating likely depletion onto cold grains.