Observational Dynamics of Low-Mass Stellar Systems - The Mass Content of Ultra-Compact Dwarf Galaxies and Diffuse Globular Clusters
Using spectroscopy and imaging obtained with ground-based telescopes and the Hubble Space Telescope (HST), we study the dynamics of ultra-compact dwarf galaxies (UCDs) and diffuse Galactic globular clusters (GCs). We present, for the first time, the spatially resolved kinematics of a UCD. We obtain a velocity dispersion profile fully consistent with a constant mass-to-light ratio (M/L) that in turn agrees well with its stellar population M/L. We find no evidence for an extended dark matter halo nor for a massive central black hole. While this does not exclude a galaxian origin for the UCD, we conclude that its internal kinematics are fully consistent with it being a massive star cluster. Results from the gravity-sensitive 2.3micron CO index of UCDs do not support a scenario explaining their high M/L with an overabundance of low-mass stars. A comparison the photometric and dynamical mass of the GC Palomar 4 yields good agreement with Newtonian dynamics and adds to the evidence against Modified Newtonian dynamics (MOND). We find mass segregation in Palomar 4 and Palomar 14 that may be primordial given their long relaxation times, or that suggests that these clusters were once significantly more compact.