The high-mass end of the globular cluster mass function


Thesis Supervisor: Michael Hilker




Basically every galaxy of a stellar mass larger than 10^9 solar masses contains a system of old globular clusters (GCs). The luminosity function of the globular clusters always seems to have the same shape independent of the mass of the host galaxy and the total number of globular clusters. The shape of the GC luminosity function can be approximated by a Gaussian that peaks at a characteristic luminosity/mass. When translated into a mass spectrum the slope of the mass function above this characteristic mass is well fit by a power law with exponent -1.8. The power law, however, breaks down and gets steeper at a GC mass of about 10^6 solar masses. This mass coincides with the transition region between GCs and the so-called ultra-compact dwarf galaxies (UCDs). Although very similar to GCs in many respects, UCDs show two main differences to them: they follow a mass-size relation and show an elevated mass-to-light ratio in comparison to GCs of the same metallicity.

It was proposed that UCDs originate from the amalgamation of massive star cluster complexes in merging galaxies. Thus, there might be connection between the formation of UCDs and the kink in the star cluster mass function. Nearby galaxy clusters are known to harbour very rich globular cluster systems around the central giant ellipticals, and are especially rich in UCDs. One idea is that the GC/UCD mass function is produced by individual starforming galaxies or galaxy groups that fall into a galaxy cluster. Modelling the evolution of the galaxy cluster assembly over a Hubble time
can show whether this idea holds.

This PhD project aims at studying in detail the high-mass end of the globular cluster mass function in different environments. The globular cluster data are partially in hand (from deep imaging of the Hudra I and Centaurus cluster), more data shall be gathered from the literature or through new observational campaigns. The PhD candidate is also encouraged to participate in the theoretical modelling of the assembly scenario of globular cluster systems. This part will be done in collaboration with Prof. Dr. Pavel Kroupa (AIfA, Bonn, Germany) who is an expert in the field of star clusters, dwarf galaxies and dynamical modelling.

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