Observational Evidence for a Dark Side to NGC5128's Globular Cluster
  System
  We present a study of the dynamical properties of 125 compact stellar systems
(CSSs) in the nearby giant elliptical galaxy NGC5128, using high-resolution
spectra (R 26,000) obtained with VLT/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 (sigma_los)
measurements are performed with the penalized pixel fitting (ppxf) technique,
which provided sigma_ppxf estimates for 115 targets. The sigma_ppxf estimates
are corrected to the 2D projected half-light radii, sigma_{1/2}, as well as the
cluster cores, sigma_0, 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 NGC5128'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
(Upsilon_V^dyn). We find two distinct sequences in the Upsilon_V^dyn - M_dyn
plane, which are well approximated by power laws of the forms Upsilon_V^dyn
M_dyn^0.33+\-0.04 and Upsilon_V^dyn - M_dyn^0.79+\-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 which 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
NGC5128 and the Local Group, despite the fact that these clusters have
luminosities similar to the GCLF turn-over magnitude. We include a thorough
discussion of myriad factors potentially influencing our measurements.
