Constraints on Upper Cutoffs in the Mass Functions of Young Star Clusters

We test claims that the power-law mass functions of young star clusters (ages ≲ few × 10⁸ yr) have physical upper cutoffs at {M}_* ∼ {10}⁵ {M}_⊙ . Specifically, we perform maximum likelihood fits of the Schechter function, \psi (M)={dN}/{dM}\propto {M}^β \exp (-M/{M}_* ), to the observed cluster masses in eight well-studied galaxies (LMC, SMC, NGC 4214, NGC 4449, M83, M51, Antennae, and NGC 3256). In most cases, we find that a wide range of cutoff masses is permitted ({10}⁵ {M}_⊙ ≲ {M}_* < ∞ ). We find a weak detection at {M}_* ∼ {10}⁵ {M}_⊙ in one case (M51) and strong evidence against this value in two cases. However, when we include realistic errors in cluster masses in our analysis, the constraints on M _* become weaker and there are no significant detections (even for M51). Our data are generally consistent with much larger cutoffs, at {M}_* ∼ {few}× {10}⁶ {M}_⊙ . This is the predicted cutoff from dynamical models in which old globular clusters and young clusters observed today formed by similar physical processes with similar initial mass functions.