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: http://www.eso.org/~smieske/ucds.htm
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Operations staff astronomer at the VLT
Ultra-compact dwarf galaxies (UCDs):
UCDs are a new class of stellar systems discovered during the last decade in the core regions of nearby galaxy cluster. See also this recent press release.
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UCDs are characterised by old stellar populations, are larger, brighter and more massive than the biggest Milky Way globular clusters (GCs), but at the same time significantly more compact than typical dwarf galaxies of comparable luminosity.
There are several hypotheses for their origin: 1. UCDs are remnant nuclei of tidally stripped dwarf galaxies (Bassino et al. 1994; Bekki et al. 2003). 2. UCDs are merged stellar super-clusters (e.g. Fellhauer & Kroupa 2002). 3. UCDs are genuine compact dwarf galaxies formed in the smallest peaks of primordial dark matter fluctuations (Drinkwater et al. 2004).
Recent studies have shown that UCDs have dynamical M/L ratios twice as large as GCs at comparable metallicity, and that for many UCDs, M/L is above the expectations for canonical stellar mass functions (Dabringhausen et al. 2008; Mieske & Kroupa 2008; Mieske et al. 2008). This suggests that either, UCDs harbour stellar populations with extreme mass functions, or that UCDs correspond to small-scale concentrations of dark matter. Given the high dark matter densities implied, this would open new avenues towards studying the clustering properties of dark matter particles (Gilmore et al. 2007).
Comparison of UCD (bottom) with a normal dwarf galaxy (top) [courtesy M. Hilker]