Thesis Topic: Galactic globular clusters as living fossils of the past of our Milky Way

Thesis Supervisors: Michael Hilker and Glenn van de Ven

Abstract:

The predicted hierarchical build-up of galaxies is difficult to unearth as the signatures of merged satellite galaxies quickly become too faint to be detectable. However, the disrupted satellite galaxies leave their globular clusters (GCs) and possibly nuclei as easily observable remnants in nearby galaxies. Unfortunately, which GCs in nearby galaxies are accreted versus born in-situ is hard to tell from their observed colours, positions or velocities. Fortunately, GCs that were accreted together rather well preserve their common orbital energy and angular momentum. This means that GCs and nuclei that once belonged the same satellite galaxy will still clump together in orbital phase-space. Moreover, as the luminosity and metallicity distribution of GCs are mainly set by the mass of the galaxy in which they are born, such a clump will have a particular colour.

Dr. Van de Ven is leading the development of orbit-based dynamical models that will allow the recovery of these coloured clumps for GCs around nearby galaxies. Dr. Hilker is an expert in observational properties GCs and is collecting large datasets of radial velocity samples of extragalactic GCs and of resolved stars in GCs in the Milky Way (MW).

Our own galaxy is a crucial stepping stone toward external galaxies in two ways. First, especially with upcoming astrometry from the Gaia satellite, the orbital parameters of the GCs in the MW are precisely known. Hence, the robustness of the orbital phase-space recovery can be demonstrated. Second, only for GCs in the MW we have access to internal chemo-kinematic measurements of thousands of member stars; radial velocity and chemical abundance measurements from ESO telescopes as well as proper motion measurements from HST and soon from Gaia. These state-of-the-art observations allow us to confirm the origin of MW GCs: in-situ born vs. accreted. This origin will be linked to the luminosity and other global properties of the GCs to provide a classification for the origin of GCs in external galaxies.

In addition, the PhD thesis research will be enable to tackle a range of long-standing open questions about the MW, including:

- What fraction of the GCs in the MW is accreted?

- What kind of satellite galaxies brought in the accreted GCs?

- What fraction of MW halo stars comes from GCs, stripped or from completed disrupted GCs?

- Which part of the MW stellar halo was build-up from the stars of the disrupted satellite galaxies themselves?

- What are the chemo-kinematical properties of genuine MW GCs formed in-situ?

- [your own question!]

The PhD candidate will be part of an international collaboration with members all over Europe, North America, Australia, and in China.

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