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

Mieske, S., Hilker, M., Jordan, A., Infante, L., Kissler-Patig, M., Rejkuba, M., Richtler, T., Cote, P., Baumgardt, H., West, M. J., Ferrarese, L. and Peng, E. W. (2008) The nature of UCDs: Internal dynamics from an expanded sample and homogeneous database. Astronomy and Astrophysics, 487 3: 921-937. doi:10.1051/0004-6361:200810077


Author Mieske, S.
Hilker, M.
Jordan, A.
Infante, L.
Kissler-Patig, M.
Rejkuba, M.
Richtler, T.
Cote, P.
Baumgardt, H.
West, M. J.
Ferrarese, L.
Peng, E. W.
Title The nature of UCDs: Internal dynamics from an expanded sample and homogeneous database
Journal name Astronomy and Astrophysics   Check publisher's open access policy
ISSN 0004-6361
1432-0746
Publication date 2008-09
Sub-type Article (original research)
DOI 10.1051/0004-6361:200810077
Open Access Status DOI
Volume 487
Issue 3
Start page 921
End page 937
Total pages 17
Place of publication Les Ulis, France
Publisher E D P Sciences
Language eng
Formatted 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 > MV > −13.5 mag (106 < M/M < 108), 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 MV < −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 104 < M/M < 108 M,
and normalise all M/L estimates to solar metallicity. We find that UCDs (M≳ 2 × 106 M) have M/L ratios twice as large as GCs
(M≲ 2 × 106 M ). 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
re≳ 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≳ 2×106 M and 7≲ re/pc ≲ 100.
Future studies should aim at explaining the elevated M/L ratios of UCDs and the environmental dependence of their properties.
Keyword Galaxies : clusters : individual : Fornax
Galaxies : dwarf
Galaxies : fundamental parameters
Galaxies : nuclei galaxies : star clusters
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: School of Mathematics and Physics
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