Infrared dark clouds from the ISOGAL survey - Constraints on the interstellar extinction curve

The ISO galactic survey provides images of the inner disk in two broad filters (around 7 and 15 mum) over some 15 square degrees, away from the brightest star forming regions. A multiresolution analysis of the images leads to a catalogue of infrared dark clouds, most of which are condensed cores of large molecular clouds, several kpc away from the Sun, seen in absorption in front of the diffuse galactic emission. The longitude distributions of the background emission and of the dark clouds correlate with known tracers of young population components. We analyse the morphology of the dark clouds and the intensity fluctuations within the cloud boundaries at the two wavelengths. The 7 to 15 mum contrast ratio is 0.75 +/- 0.15 for the clouds located away from the Galactic Centre (\l\ > 1 degrees) and 1.05 +/- 0.15 for the clouds closest to the Galactic Centre (\l\ < 1, \b\ < 0.2). Using a simple absorption model, we derive a 7 to 15 mum opacity ratio equal to 0.7 +/- 0.1 for the clouds located away from the Galactic Centre and estimate the opacity, 7, of a few objects at 15 mum in the range 1 to 4. Several explanations for the variation of the contrast ratio, including absorption along the line of sight and local variations of the extinction curve are discussed.