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Star Formation Efficiencies and Star Cluster Formation

Starbursts produce large numbers of Young Star Clusters (YSCs). Multi-color photometry in combination with a dedicated SED analysis tool allows to derive ages, metallicities, ${\rm E_{B-V}}$, and masses including $1 σ$ uncertainties for individual clusters and, hence, mass functions for YSC systems. The mass function, known to be Gaussian for old Globular Cluster (GC) systems, is still controversial for YSC systems. GC formation is expected in massive gas-rich spiral -- spiral mergers because of their high global star formation efficiencies and observed in $\geq 1$ Gyr old merger remnants. Yet it has not been possible to identify young GCs among YSC populations. We suggest a compactness parameters involving masses and half-light radii of YSCs to investigate if young GCs are formed in starbursts and if the ratio of young GCs to more loosely bound star clusters depends on galaxy type, mass, burst strength, etc.