Journal Article

High mass-to-light ratios of ultra-compact dwarf galaxies – evidence for dark matter?

H. Baumgardt and S. Mieske

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 391, issue 2, pages 942-948
Published in print December 2008 | ISSN: 0035-8711
Published online November 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13949.x
High mass-to-light ratios of ultra-compact dwarf galaxies – evidence for dark matter?

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Ultra-compact dwarf galaxies (UCDs) are stellar systems with masses of around 107 to 108M and half-mass radii of 10–100 pc. They have some properties in common with massive globular clusters, however dynamical mass estimates have shown that UCDs have mass-to-light ratios which are on average about twice as large than those of globular clusters at comparable metallicity, and tend to be larger than what one would expect for old stellar systems composed out of stars with standard mass functions.

One possible explanation for elevated high mass-to-light ratios in UCDs is the existence of a substantial amount of dark matter, 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 prior to tidal stripping through, for example, adiabatic gas infall. Tidal stripping of dwarf galaxies has also been suggested as the origin of several massive globular clusters like Omega Cen, in which case one should expect that globular clusters also form with substantial amounts of dark matter in them.

In this paper, we present collisional N-body simulations which study the co-evolution of a system composed out of stars and dark matter. We find that the dark matter gets removed from the central regions of such systems due to dynamical friction and mass segregation of stars. The friction time-scale is significantly shorter than a Hubble time for typical globular clusters, while most UCDs have friction times much longer than a Hubble time. Therefore, a significant dark matter fraction remains within the half-mass radius of present-day UCDs, making dark matter a viable explanation for the elevated M/L ratios of UCDs. If at least some globular clusters formed in a way similar to UCDs, we predict a substantial amount of dark matter in their outer parts.

Keywords: stellar dynamics; methods: N-body simulations; galaxies: star clusters

Journal Article.  5125 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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