Contribution of stripped nuclear clusters to globular cluster and ultracompact dwarf galaxy populations

We use the Millennium II cosmological simulation combined with the semi-analytic galaxy formation model of Guo et al. to predict the contribution of galactic nuclei formed by the tidal stripping of nucleated dwarf galaxies to globular cluster (GC) and ultracompact dwarf galaxy (UCD) populations of galaxies. We follow the merger trees of galaxies in clusters back in time and determine the absolute number and stellar masses of disrupted galaxies. We assume that at all times nuclei have a distribution in nucleus-to-galaxy mass and nucleation fraction of galaxies similar to that observed in the present day Universe. Our results show stripped nuclei follow a mass function N(M) similar to M-1.5 in the mass range 10(6) < M/M-circle dot < 10(8), significantly flatter than found for globular clusters. The contribution of stripped nuclei will therefore be most important among high-mass GCs and UCDs. For the Milky Way we predict between one and three star clusters more massive than 10(5) M-circle dot come from tidally disrupted dwarf galaxies, with the most massive cluster formed having a typical mass of a few times 10(6) M-circle dot, like omega Centauri. For a galaxy cluster with a mass 7 x 10(13) M-circle dot, similar to Fornax, we predict similar to 19 UCDs more massive than 2 x 10(6) M-circle dot and similar to 9 UCDs more massive than 10(7) M-circle dot within a projected distance of 300 kpc come from tidally stripped dwarf galaxies. The observed number of UCDs are similar to 200 and 23, respectively. We conclude that most UCDs in galaxy clusters are probably simply the high-mass end of the GC mass function.