Extinction and dust/gas ratio in LMC molecular clouds

¹ Centre d'Étude Spatiale des Rayonnements, CNRS, 9 Avenue du Colonel Roche, 31028 Toulouse, France e-mail: dobashi@u-gakugei.ac.jp
² Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan
³ Centre for Supercomputing and Astrophysics, Swinburne University of Technology, Hawthorn VIC 3122, Australia
⁴ CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
⁵ IPAC/Caltech, MS 220-6, Pasadena, CA 91125, USA
⁶ Nagoya University, Dept. of Astrophysics, Chikusa-Ku, Nagoya, 464-8602, Japan

Received: 27 November 2007
Accepted: 3 March 2008

Abstract

Aims. The goal of this paper is to measure the dust content and distribution in the Large Magellanic Cloud (LMC) by comparing extinction maps produced in the near-infrared wavelengths and the spatial distribution of the neutral and molecular gas, as traced by H i and CO observations.

Methods. In order to derive an extinction map of the LMC, we have developed a new method to measure the color excess of dark clouds, using the 2MASS all-sky survey. Classical methods to measure the color excess (including the NICE method) tend to underestimate the true color excess if the clouds are significantly contaminated by unreddened foreground stars, as is the case in the LMC. We propose a new method that uses the color of the X percentile reddest stars and which is robust against such contamination. Using this method, it is possible to infer the positions of dark clouds with respect to the star distribution by comparing the observed color excess as a function of the percentile used and that predicted by a model.

Results. On the basis of the resulting extinction map, we perform a correlation analysis for a set of dark molecular clouds. Assuming similar infrared absorption properties for the dust in the neutral and molecular phases, we derive the absorption-to-column density ratio  and the CO-to-H₂ conversion factor X_CO. We show that increases from the outskirts of the LMC towards the 30 Dor star-forming region. This can be explained either by a systematic increase of the dust abundance, or by the presence of an additional gas component not traced by H i or CO, but strongly correlated to the H i distribution. If dust abundance is allowed to vary, the derived X_CO factors for the selected regions are several times lower than those derived from a virial analysis of the CO data. This could indicate that molecular clouds in the LMC are not gravitationally bound, or that they are bounded by substantial external pressure. However, the X_CO values derived from absorption can be reconciled with the virial results assuming a constant value for the dust abundance and the existence of an additional, unseen gas component. These results are in agreement with those derived for the LMC from diffuse far-infrared emission.