The inner workings of compact stellar systems:

With the group of Holger Baumgardt and Pavel Kroupa, we investigate possible explanations for two striking observational findings among compact stellar systems:

1. The rise of dynamical mass-to-light ratios between the regime of globular clusters (GCs) and UCDs (Hasegan et al. 2005, Dabringhausen et al. 2008, Mieske et al. 2008).

(Mieske et al. 2008)


Is the observed rise of M/L caused by dark matter in UCDs?

We have performed N-body simulations to investigate the co-evolution of dark matter and stars in compact stellar systems (Baumgardt \& Mieske 2008). They show that the observed rise of M/L ratio for UCDs is consistent with a scenario where compact stellar systems in general are formed with a significant amount of dark matter, which is subsequently removed from the lower mass systems due to dynamical evolution.

Is the rise due to a variation in the IMF in UCDs?

We have shown that a bottom-heavy IMF in UCDs may be detected by measuring the absorption strength in the CO-band region (2.3m).

Together with Jörg Dabringhausen and Pavel Kroupa, we are investigating how to detect a top-heavy IMF in UCDs, a possibility proposed by Dabringhausen, Baumgardt & Kroupa (2009). We recently published the results of a first statistical study, which indeed suggests that the elevated M/L ratios in UCDs are due to a top-heavy IMF (Dabringhausen et al. 2012)

2. The colour-magnitude relation of metal-poor globular clusters (Harris et al. 2006, Mieske et al. 2006 and 2010, Strader et al. 2006), the so-called 'blue tilt'.

(Mieske et al. 2006)

Is the observed tilt due to field-star capture?

In Mieske & Baumgardt (2007) we have shown that field star capture is much too inefficient to account for the blue tilt, and cannot be responsible for formation of a significant, metal-rich sub-component in star clusters.

Self-enrichment scenarios may offer a plausible explanation.