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 ultracompact 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 x 10(5) and 2.5 x 10(7) L (circle dot)), 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 +/- 0.31, and a second population of similar to 20 GCs with elevated mean M/L (dyn) = 2.68 +/- 0.22. We show that black holes with masses similar to 4%-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, 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.

Automated summary

The central regions of tidally disrupted galaxies can survive as ultracompact dwarfs (UCDs) that hide among the luminous globular clusters (GCs) in the halo of massive galaxies. High-resolution spectroscopic observations of luminous GC candidates in NGC 5128/Centaurus A reveal new measurements and confirm the presence of overmassive black holes that contribute to elevated mass-to-light ratios. Bimodality is observed in the distribution of mass-to-light ratios, with a population of normal GCs and a population of GCs with elevated ratios. One extreme outlier, VHH81-01, is identified as a strong candidate for a tidally stripped nucleus.