The Mass Spectra of Giant Molecular Clouds in the Local Group

Abstract

We reanalyze the catalogs of molecular clouds in the Local Group to determine the parameters of their mass distributions in a uniform manner. The analysis uses the error-in-variables method of parameter estimation which accounts not only for the variance of the sample when drawn from a parent distribution but also for errors in the mass measurements. Testing the method shows that it recovers the underlying properties of cumulative mass distribution without bias while accurately reflecting uncertainties in the parameters. Clouds in the inner disk of the Milky Way follow a truncated power-law distribution with index γ=-1.5 +/- 0.1 and maximum mass of 10^6.5 M_sun. The distributions of cloud mass for the outer Milky Way and M33 show significantly steeper indices (γ_OMW= -2.1 +/- 0.2 and γ_M33= -2.9 +/- 0.4) with no evidence of a cutoff. The mass distribution of clouds in the Large Magellanic Cloud has a marginally steeper distribution than the inner disk of the Milky Way (γ=-1.7 +/- 0.2) and also shows evidence of a truncation with a maximum mass of 10^6.5 M_sun. The mass distributions of molecular clouds vary dramatically across the Local Group, even after accounting for the systematic errors that arise in comparing heterogeneous data and catalogs. These differences should be accounted for in studies that aim to reproduce the molecular cloud mass distributions or in studies that use the mass spectrum as a parameter in a model.