Mass Functions of Giant Molecular Clouds and Young Star Clusters in Six Nearby Galaxies

We compare the mass functions of young star clusters (ages $leq 10$ Myr) and giant molecular clouds (GMCs) in six galaxies that cover a large range in mass, metallicity, and star formation rate (LMC, M83, M51, NGC 3627, the Antennae, and NGC 3256). We perform maximum-likelihood fits of the Schechter function, $psi(M) = dN/dM propto M^{beta} exp(-M/M_*)$, to both populations. We find that most of the GMC and cluster mass functions in our sample are consistent with a pure power-law distribution ($M_* rightarrow infty$). M51 is the only galaxy that shows some evidence for an upper cutoff ($M_*$) in both populations. Therefore, physical upper mass cutoffs in populations of both GMCs and clusters may be the exception rather than the rule. When we perform power-law fits, we find a range of indices $beta_{rm PL}=-2.3pm0.3$ for our GMC sample and $beta_{rm PL}=-2.0pm0.3$ for the cluster sample. This result, that $beta_{rm Clusters} approx beta_{rm GMC} approx -2$, is consistent with theoretical predictions for cluster formation and suggests that the star-formation efficiency is largely independent of mass in the GMCs.