Schedule Jan 15, 2009
High Mass-to-light Ratios of UCDs - Evidence for Dark Matter?
Holger Baumgardt (Bonn Univ.)

Ultra-compact dwarf galaxies (UCDs) are stellar systems with masses of around 10^7 to 10^8 Msun and half mass radii of 10-100 pc. They have many properties in common with massive globular clusters, however their mass-to-light ratios are on average about twice as large than those of globular clusters at comparable metallicity, and larger than what one would expect for old stellar systems composed out of stars with standard mass functions.

One possible explanation for the high mass-to-light ratios in UCDs would be the existence of dark matter in them, which could have ended up in UCDs if they are the remnant nuclei of tidally stripped dwarf galaxies and dark matter was dragged into these nuclei through e.g. adiabatic gas infall. Tidal stripping has also been suggested as the origin of several massive globular clusters like Omega Cen, which might therefore also have formed with substantial amounts of dark matter.

In my talk I will report results from collisional N-body simulations which study the co-evolution of a system composed out of stars and dark matter. The simulations show that dark matter gets removed from the central regions of such systems due to dynamical friction and mass segregation of stars and that most of the dark matter would remain in the centers of UCDs, making dark matter a viable explanation for their elevated M/L ratios. If at least some globular clusters formed in a way similar to UCDs, substantial amounts of dark matter should reside in their outer parts.

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