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]

In my research on UCDs, one of my aims is to explain the elevated M/L ratios of UCDs in the context of star formation and structure assembly. The results up to now seem to indicate that the elevated M/L ratios of UCDs are of baryonic origin rather than due to dark matter (Frank et al. 2011, Dabringhausen et al. 2012). I have furthermore led searches for UCDs in various galaxy clusters, and am involved in a number of proposals to tackle both the internal and external properties of UCDs.

Together with Dr. Michael Hilker, between November 2007 and November 2009 I co-supervised the PhD thesis of Ingo Misgeld at ESO Garching, which deals with both canonical dwarf galaxies and the search for UCDs in galaxy clusters. Between December 2008 and December 2010, I furthermore co-supervised the PhD thesis of Matthias Frank at ESO Garching, focusing on internal properties of individual UCDs. These two thesis works have led to important new papers, on the specific frequency of UCDs and on the spatially resolved internal dynamics of a massive UCD.