The AIMSS Project - II. Dynamical-to-stellar mass ratios across the star cluster-galaxy divide

Duncan A. Forbes*, Mark A. Norris, Jay Strader, Aaron J. Romanowsky, Vincenzo Pota, Sheila J. Kannappan, Jean P. Brodie, Avon Huxor

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

24 Citations (Scopus)


The previously clear division between small galaxies and massive star clusters is now occupied by objects called ultra-compact dwarfs (UCDs) and compact ellipticals (cEs). Here we combine a sample of UCDs and cEs with velocity dispersions from the AIMSS project with literature data to explore their dynamical-to-stellar mass ratios.We confirm that the mass ratios of many UCDs in the stellar mass range 106-109 M{N-ary circled dot operator} are systematically higher than those for globular clusters which have mass ratios near unity. However, at the very highest masses in our sample, i.e. 109-1010 M{N-ary circled dot operator} , we find that cE galaxies also have mass ratios of close to unity, indicating their central regions are mostly composed of stars. Suggested explanations for the elevated mass ratios of UCDs have included a variable IMF, a central black hole, and the presence of dark matter. Here we present another possible explanation, i.e. tidal stripping. Under various assumptions, we find that the apparent variation in the mass ratio with stellar mass and stellar density can be qualitatively reproduced by published tidal stripping simulations of a dwarf elliptical galaxy. In the early stages of the stripping process the galaxy is unlikely to be in virial equilibrium. At late stages, the final remnant resembles the properties of 107 M{N-ary circled dot operator} UCDs. Finally, we discuss the need for more detailed realistic modelling of tidal stripping over a wider range of parameter space, and observations to further test the stripping hypothesis.

Original languageEnglish
Pages (from-to)2993-3003
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 1 Jan 2014


  • Dwarf - galaxies
  • Evolution - galaxies
  • Galaxies
  • General
  • Kinematics and dynamics -galaxies
  • Star clusters


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