Young Massive Clusters – Formation Efficiencies and (Initial) Mass Functions

Abstract

Globular clusters are often assumed to be good tracers of major star formation episodes in their host galaxies. While observations over the past 2 decades have confirmed the presence of young objects with globular cluster-like properties in many galaxies, it is still not well understood exactly how the formation efficiency of bound star clusters relative to field stars and their mass spectrum depend on external factors. The cluster initial mass function typically appears to be consistent with a power-law with a slope ~-2, but most attempts to constrain any upper limit on the CIMF have been limited by size-of-sample effects. However, evidence is starting to accumulate for possible truncation of the cluster mass function. It is tentatively suggested that the upper mass limit may currently be at ~10⁵ M_⊙ in the Milky Way disk, while there are indications that it is ~5×10⁵ M_⊙ in M51 and about 2×10⁶ M_⊙ in the Antennae. Some extreme starbursts (e.g. Arp 220, NGC 7252) are (or were) able to form clusters as massive as 10⁷ M_⊙. The overall formation efficiency of star clusters (relative to field stars) in the Galactic bulge may not have been much different from that in the disk today, but was probably significantly higher for metal-poor GCs in halos.