The nature of UCDs: Internal dynamics from an expanded sample and homogeneous database

Abstract

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 M_V-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}<10⁸ M, and normalise all M/L estimates to solar metallicity. We find that UCDs (M ≳ 2 × 10^6 M_sun) have M/L ratios twice as large as GCs (M≲ 2 × 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_e ≳ 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≳ 2 × 10^6 M_sun and 7≲{r_e/pc} ≲ 100. Future studies should aim at explaining the elevated M/L ratios of UCDs and the environmental dependence of their properties.

Based on observations obtained in service mode at the ESO Paranal Observatory with the VLT (programme 078.B-0496).

Table 5 is only available in electronic form at http://www.aanda.org