The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database ⋆

Context. The internal dynamics of ultra-compact dwarf galaxies (UCDs) has attracted increasing attention, with most of the UCDs studied to date located in the Virgo cluster. Aims. Our aim is to perform a comprehensive census of the internal dynamics of UCDs in the Fornax cluster, and to shed light on the nature of the interface between star clusters and galaxies. Methods. We obtained high-resolution spectra of 23 Fornax UCDs with –$10.4>M_V>-13.5$ mag ($10^6<{ M/M_{\sun}<10^8}$), using FLAMES/Giraffe at the VLT. This is the largest homogeneous data set of UCD internal dynamics assembled to date. We derive dynamical $M/L$ ratios for 15 UCDs covered by HST imaging. Results. In the MV–σ plane, UCDs with $M_V<-12$ mag are consistent with the extrapolated Faber-Jackson relation for luminous elliptical galaxies, while most of the fainter UCDs are closer to the extrapolated globular cluster (GC) relation. At a given metallicity, Fornax UCDs have, on average, $M/L$ ratios lower by 30–40% than Virgo UCDs, suggesting possible differences in age or dark matter content between Fornax and Virgo UCDs. For our sample of Fornax UCDs we find no significant correlation between $M/L$ ratio and mass. We combine our data with available $M/L$ ratio measurements of compact stellar systems with $10^4<{ M/M_{\sun}}<10^8$ M, and normalise all $M/L$ estimates to solar metallicity. We find that UCDs ($M \gtrsim 2 \times 10^6~M_{\sun}$) have $M/L$ ratios twice as large as GCs ($M\lesssim 2 \times 10^6~M_{\sun}$). We argue that dynamical evolution has probably had only a small effect on the current $M/L$ ratios of objects in the combined sample, implying that stellar population models tend to under-predict dynamical $M/L$ ratios of UCDs and over-predict those of GCs. Considering the scaling relations of stellar spheroids, we find that UCDs align well along the “Fundamental Manifold”. UCDs can be considered the small-scale end of the galaxy sequence in this context. The alignment for UCDs is especially clear for $r_{\rm e} \gtrsim 7$ pc, which corresponds to dynamical relaxation times that exceed a Hubble time. In contrast, globular clusters exhibit a broader scatter and do not appear to align along the manifold. Conclusions. We argue that UCDs are the smallest dynamically un-relaxed stellar systems, with $M\gtrsim 2 \times 10^6~M_{\sun}$ and $7\lesssim{r_{\rm e}/{\rm pc}} \lesssim 100$. Future studies should aim at explaining the elevated $M/L$ ratios of UCDs and the environmental dependence of their properties.

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