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