The volume densities of giant molecular clouds in M 83 ^*,**

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: heiner@astro.rug.nl
² Space Telescope Science Institute, Baltimore, MD 21218, USA
³ Astronomy Department, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA

Received: 19 June 2008
Accepted: 24 July 2008

Abstract

Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm HI column densities, along with estimates of the local dust abundances, we measure the volume densities of a sample of actively star-forming giant molecular clouds (GMCs) in the nearby spiral galaxy M 83 on a typical resolution scale of 170 pc. Our approach is based on an equilibrium model for the cycle of molecular hydrogen formation on dust grains and photodissociation under the influence of the FUV radiation on the cloud surfaces of GMCs. We find a range of total volume densities on the surface of GMCs in M 83, namely 0.1–400 cm^-3 inside R₂₅, 0.5–50 cm^-3 outside R₂₅. Our data include a number of GMCs in the HI ring surrounding this galaxy. Finally, we discuss the effects of observational selection, which may bias our results.