A formation scenario for Ultra-compact Dwarf Galaxies

Supervisor: Gerhard Hensler and Glenn van de Ven (Department of Astrophysics) in collaboration with Michael Hilker (ESO, Garching), Berhand Baumschlager (Univ. of Oslo), and Sabine Thater (Department of Astrophysics) 

Funding Situation: potentially via a VISESS PhD fellowship

Project outline: Over the past two decades a new class of stellar systems has been detected in galaxy clusters and galaxy groups. They appear as satellites of major massive elliptical galaxies and because of their compactness are called "ultra-compact dwarf galaxies" (UCDs). Since the first detection their origin is heavily debated. Their properties in terms of mass, size, age, luminosity, and UCDs being pure old stellar systems, make them bridge the gap between Globular Clusters (GCs) and compact ellipticals galaxies. However, unlike GCs, they follow the galaxy size-luminosity relation. Further constraints to differentiate between different formation scenarios come from the high metallicity of UCDs (from 1/10 to super-solar), their high and even super-solar -element abundance, and the observation that only some reach solar [a/Fe].

Looking now at the host galaxies of UCDs, we infer that these massive elliptical galaxies underwent major mergers with tidal tails serving as formation sites of GCs and tidal-tail dwarf galaxies (TDGs). We recently investigated the evolution of these TDG candidates by numerical chemo-dynamical simulations of mass concentrations within tidal arms (Baumschlager et al., 2019, MNRAS,483). We investigated their long-term survival as normal dwarf ellipticals, but found from the high star-formation rates, compactness, massive star-cluster formation, and chemical evolution of these TDG candidates a promising path toward UCDs. The proposed PhD project will, for the first time, apply an advanced numerical chemo-dynamical scheme to propagate TDG models until they orbit the host galaxy as gas-free stellar systems. The element abundances and stellar dynamics of these stellar systems will then be compared with the wealth of existing UCD observations to find out whether this provides a third and possibly dominating origin scenario of UCDs.