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

Abstract

We compare the mass functions of young star clusters (ages ≤ 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, $\psi (M)\,={dN}/{dM}\propto {M}^{\beta }\exp (-M/{M}_{\ast })$ , 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}_{* }\to \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 relatively small range of indices β_PL = -2.3 ± 0.3 for our GMC sample and β_PL = -2.0 ± 0.3 for the cluster sample. This result, that ${\beta }_{\mathrm{Clusters}}\approx {\beta }_{\mathrm{GMC}}\approx -2$ , is consistent with theoretical predictions for cluster formation and suggests that the star formation efficiency is largely independent of mass in the GMCs.