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) ∼ M^-1.5 in the mass range 10⁶ < M/M_⊙ < 10⁸, 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 10⁵ M_⊙ come from tidally disrupted dwarf galaxies, with the most massive cluster formed having a typical mass of a few times 10⁶ M_⊙, like ω Centauri. For a galaxy cluster with a mass 7 × 10¹³ M_⊙, similar to Fornax, we predict ∼19 UCDs more massive than 2 × 10⁶ M_⊙ and ∼9 UCDs more massive than 10⁷ M_⊙ within a projected distance of 300 kpc come from tidally stripped dwarf galaxies. The observed number of UCDs are ∼200 and 23, respectively. We conclude that most UCDs in galaxy clusters are probably simply the high-mass end of the GC mass function.