Observational evidence for a dark side to NGC 5128's globular cluster system

We present a study of the dynamical properties of 125 compact stellar systems (CSSs) in the nearby giant elliptical galaxy NGC 5128, using high-resolution spectra (R ≈ 26, 000) obtained with Very Large Telescope/FLAMES. Our results provide evidence for a new type of star cluster, based on the CSS dynamical mass scaling relations. All radial velocity (v_r) and line-of-sight velocity dispersion (σ_los) measurements are performed with the penalized pixel fitting (ppxf ) technique, which provided σ_ppxf estimates for 115 targets. The σ_ppxf estimates are corrected to the 2D projected half-light radii, σ_1/2, as well as the cluster cores, σ₀, accounting for observational/aperture effects and are combined with structural parameters, from high spatial resolution imaging, in order to derive total dynamical masses ($M_{dyn}$) for 112 members of NGC 5128's star cluster system. In total, 89 CSSs have dynamical masses measured for the first time along with the corresponding dynamical mass-to-light ratios (${{\rm Y}}_V^{dyn}$). We find two distinct sequences in the ${{\rm Y}}_V^{dyn}$–$M_{dyn}$ plane, which are well approximated by power laws of the forms ${{\rm Y}}_V^{dyn}\propto M_{dyn}^{0.33\pm 0.04}$ and ${{\rm Y}}_V^{dyn}\propto M_{dyn}^{0.79\pm 0.04}$. The shallower sequence corresponds to the very bright tail of the globular cluster luminosity function (GCLF), while the steeper relation appears to be populated by a distinct group of objects that require significant dark gravitating components such as central massive black holes and/or exotically concentrated dark matter distributions. This result would suggest that the formation and evolution of these CSSs are markedly different from the "classical" globular clusters in NGC 5128 and the Local Group, despite the fact that these clusters have luminosities similar to the GCLF turnover magnitude. We include a thorough discussion of myriad factors potentially influencing our measurements.