Filling the gap: a new class of old star cluster?

TL;DR

This study discovers star clusters within a previously identified size-luminosity gap, challenging existing models and suggesting a continuous spectrum of cluster sizes, masses, and densities.

Contribution

It provides the first confirmed evidence of star clusters in the 'avoidance zone', extending known size and mass ranges and questioning current formation theories.

Findings

Discovered star clusters within the size-luminosity gap.

Extended the known range of star cluster sizes and densities.

Linked these clusters to Faint Fuzzies in lenticular galaxies.

Abstract

It is not understood whether long-lived star clusters possess a continuous range of sizes and masses (and hence densities), or if rather, they should be considered as distinct types with different origins. Utilizing the Hubble Space Telescope (HST) to measure sizes, and long exposures on the Keck 10m telescope to obtain distances, we have discovered the first confirmed star clusters that lie within a previously claimed size-luminosity gap dubbed the `avoidance zone' by Hwang et al (2011). The existence of these star clusters extends the range of sizes, masses and densities for star clusters, and argues against current formation models that predict well-defined size-mass relationships (such as stripped nuclei, giant globular clusters or merged star clusters). The red colours of these gap objects suggests that they are not a new class of object but are related to Faint Fuzzies observed in nearby lenticular galaxies. We also report a number of low luminosity UCDs with sizes of up to 50 pc. Future, statistically complete, studies will be encouraged now that it is known that star clusters possess a continuous range of structural properties.