The size-line width relation and the mass of molecular hydrogen

A size-line-width relation has been found in a number of previous studies of molecular clouds. Most recently (in a paper by Solomon et al.), such a relation has been used in conjunction with the virial theorem to obtain cloud masses. Here, some difficulties associated with the problem of cloud definition are considered, with particular regard to the crowded distribution of clouds and the difficulty of choosing an appropriate boundary in such circumstances. A number of tests carried out on the original data suggest that the σ_v - S relation found by Solomon et al. is not a genuine reflection of the dynamical state of Giant Molecular Clouds (GMCs). The Solomon et al. parameters, σ_v and S, are insensitive to the actual cloud properties and are unable to distinguish true clouds from the consequences of sampling any crowded region of emission down to a low threshold temperature. The overall effect of such problems is to overestimate both the masses of GMCs and the number of very large clouds. In particular, we consider that there is no evidence favoring the presence of any gravitationally bound GMCs of individual mass greater than a few times 10⁵ M_⊙.