A population of luminous globular clusters and stripped nuclei with
  elevated mass to light ratios around NGC 5128
  The dense central regions of tidally disrupted galaxies can survive as
ultra-compact dwarfs (UCDs) that hide among the luminous globular clusters
(GCs) in the halo of massive galaxies. An exciting confirmation of this model
is the detection of overmassive black holes in the centers of some UCDs, which
also lead to elevated dynamical mass-to-light ratios ($M/L_{dyn}$). Here we
present new high-resolution spectroscopic observations of 321 luminous GC
candidates in the massive galaxy NGC 5128/Centaurus A. Using these data we
confirm 27 new luminous GCs, and measure velocity dispersions for 57 luminous
GCs (with $g$-band luminosities between $2.5 \times 10^5$ and $2.5 \times 10^7
L_{\odot}$), of which 48 are new measurements. Combining these data with size
measurements from Gaia, we determine the $M/L_{dyn}$ for all 57 luminous GCs.
We see a clear bimodality in the $M/L_{dyn}$ distribution, with a population of
normal GCs with mean $M/L_{dyn}=1.51\pm0.31$, and a second population of
$\sim$20 GCs with elevated mean $M/L_{dyn}=2.68\pm0.22$. We show that black
holes with masses $\sim4$-18 % of the luminous GCs can explain the elevated
mass-to-light ratios. Hence, it is plausible that the NGC 5128 sources with
elevated $M/L_{dyn}$ are mostly stripped galaxy nuclei that contain massive
central black holes, though future high spatial resolution observations are
necessary to confirm this hypothesis for individual sources. We also present a
detailed discussion of an extreme outlier, \textit{VHH81-01}, one of the
largest and most massive GC in NGC 5128, making it an exceptionally strong
candidate to be a tidally stripped nucleus.
