Institution(s): 1. Pontificia Universidad Catolica
Evidence for a new type of star cluster from the dynamical mass scaling relations of compact stellar systems (CSSs) will be presented, based on a study of 125 CSSs around the nearby giant elliptical galaxy NGC 5128, using high-resolution spectra (R=26000) obtained with VLT/FLAMES. All radial velocity (vr) and line-of-sight velocity dispersion (σlos) measurements are performed with the penalized pixel fitting (ppxf) technique. The σlos estimates are corrected to the 2D projected half-light radii, (σ1/2), as well as the cluster cores, (σ0), accounting for observational/aperture effects and are combined with structural parameters in order to derive dynamical half-mass estimates (M1/2), and total dynamical masses (Mdyn), for 116 CSSs around NGC 5128. In total, 93 CSSs have Mdyn measured for the first time along with the corresponding dynamical mass-to-light ratios (M/L)dyn. We find two distinct sequences in the (M/L)dyn-Mdyn plane, which are well fit by power laws of the forms (M/L)dyn ~ M(0.33+/-0.04) and (M/L)dyn ~ M(0.91+/-0.04). The shallower sequence corresponds to the very bright tail of the globular cluster luminosity function (GCLF), with indications for angular momentum content that increases with Mdyn. The steeper relation appears to be populated by a distinct group of objects with significant dark gravitating mass components, such as central massive black holes and/or exotically compact dark matter distributions. This result would suggest that the formation and evolution of these CSSs is markedly different from the classical globular clusters in NGC 5128 despite the fact that they have luminosities similar to the GCLF turn-over magnitude.