The Initial Mass Function of GMCs

The properties of GMCs in several Local Group galaxies are quantified and compared. It is found that the mass spectrum of GMCs varies from galaxy to galaxy. The variations are significant and do not appear to be the result of systematic uncertainties. Nevertheless, it appears that all of the GMCs follow the same size-linewidth and mass-linewidth relations with little scatter. The power law indices of these relations imply that the GMCs are self-gravitating, and that the mean surface density of Local Group GMCs is approximately constant. This, in turn, implies that the mean internal pressure of GMCs is also constant. If the IMF of stars is determined by a Jeans instability, this constant internal pressure suggests that the distribution of stellar masses does not vary significantly in galactic disks when averaged over suitably large areas. Thus, although the distribution of GMC masses produced by various Local Group galaxies is quite variable, the large-scale properties of the GMCs is not.