We derive and discuss the strong dependence on metallicity of the CO to H2 conversion factor X = N(H2)/Ico = 12.2 – 2.5log[O]/[H] appropriate to extragalactic objects, as well as the weaker dependence found for such objects from interferometer measurements.

Introduction

The difficulty of directly observing molecular hydrogen (H2), the major constituent of the interstellar medium in galaxies, and ways of doing so indirectly are reviewed elsewhere in this volume (Combes 2000). Usually, H2 cloud properties are derived by extrapolation from more easily conducted CO observations. For instance, observed CO cloud sizes and velocity widths yield total molecular gas masses under the assumption of virial equilibrium. However, in extragalactic systems especially, this method is beset by pitfalls (see Israel, 1997, hereafter Is97) and requires high linear resolutions (i.e. use of interferometer arrays). More seriously, the fundamental assumption of virialization appears to be false. As individual components (‘clumps’) have velocities of only a few km s−1 and CO complex sizes are 50–100 pc, crossing times are comparable to CO complex lifetimes of only a few times 107 years or less (Leisawitz et al. 1989; Fukui et al. 2000; see also Elmegreen 2000). As equilibrium cannot be reached in a single crossing time or less, the virial theorem is not applicable to such complexes. Indeed, the elongated and interconnected filamentary appearance of many large CO cloud complexes do not suggest virialized systems (see also Maloney 1990).