[Show abstract][Hide abstract] ABSTRACT: The last fifteen years have seen the discovery of new types of low-mass
stellar systems that bridge the gap between the once well-separated regimes of
galaxies and of star clusters. Whether such objects are considered galaxies
depends also on the definition of the term "galaxy", and several possible
criteria are based on their internal dynamics (e.g. the common concept that
galaxies contain dark matter). Moreover, studying the internal dynamics of
low-mass stellar systems may also help understand their origin and evolutionary
history. The focus of this paper is on two classes of stellar systems at the
interface between star clusters and dwarf galaxies: ultra-compact dwarf
galaxies (UCDs) and diffuse Galactic globular clusters (GCs). A review of our
current knowledge on the properties of UCDs is provided and dynamical
considerations applying to diffuse GCs are introduced. In the following, recent
observational results on the internal dynamics of individual UCDs and diffuse
Galactic globular clusters are presented.
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