Infrared dark clouds from the ISOGAL survey

P. Hennebelle; M. Pérault; D. Teyssier; S. Ganesh

doi:doi:10.1051/0004-6361:20000052

Free access

Issue		A&A Volume 365, Number 3, January III 2001


Page(s)		598 - 611
Section		Diffuse matter in space
DOI		http://dx.doi.org/10.1051/0004-6361:20000052

A&A 365, 598-611 (2001)
DOI: 10.1051/0004-6361:20000052

Constraints on the interstellar extinction curve

P. Hennebelle¹, M. Pérault¹, D. Teyssier¹ and S. Ganesh²

¹ Laboratoire de Radioastronomie Millimétrique, UMR 8540 du CNRS, École Normale Supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
² Physical Research Laboratory, Navarangpura, Ahmedabad 380009, India

(Received 28 July 2000 / Accepted 24 October 2000)

Abstract
The ISO galactic survey provides images of the inner disk in two broad filters (around 7 and 15 $\mu$ m) 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 $\mu$ m contrast ratio is $0.75\pm 0.15$ for the clouds located away from the Galactic Centre ( $\vert l\vert > 1 ^\circ$ ) and $1.05 \pm 0.15$ for the clouds closest to the Galactic Centre ( $\vert l\vert< 1^\circ, \vert b\vert < 0.2 ^ \circ$ ). Using a simple absorption model, we derive a 7 to 15 $\mu$ m opacity ratio equal to $0.7\pm 0.1$ for the clouds located away from the Galactic Centre and estimate the opacity, $\tau$ , of a few objects at 15 $\mu$ m 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.

Key words: ISM: clouds, dust, structure -- Galaxy: structure -- infrared: general

Offprint request: M. Pérault, perault@lra.ens.fr

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