EPSAM

Stellar Dynamics

Lead Supervisor:  Dr James Reeves

Old globular star clusters are found by the hundreds and thousands in the haloes of all large galaxies. A key property of these globular cluster systems is their mass function (the number of clusters with a given mass), which is found in most galaxies to have a peak at a cluster mass of order 100,000 solar masses. This contrasts sharply with the mass functions of very young star clusters, which form in great numbers in starburst and merging galaxies at the present day, and where the numbers of clusters increase steadily towards cluster masses as low as about 1000 solar masses. While the mass of the peak in the globular cluster mass function has long been thought to be a universal quantity, recent work has shown that, in our Milky Way Galaxy, the peak mass in fact depends strongly on the internal densities of the globulars themselves, increasing by an order of magnitude from the lowest-density clusters to the highest-density clusters. It has been argued that this is the result of selective destruction, over 10-billion year timescales, of low-mass globular clusters from a mass function that originally had no peak but rose towards arbitrarily low masses as in young star-cluster systems today. The main cluster destruction mechanism is expected to be evaporation driven by internal, two-body relaxation in tidally limited clusters. The aim of this project is first to check whether the main observational result (the dependence of the peak of the mass function on internal cluster density) holds generally for globular clusters in galaxies other than the Milky Way. This will make use of a published catalogue of the masses and densities of more than ten thousand globular clusters in scores of elliptical galaxies in the Virgo Cluster. Models based on the evaporation-driven depletion of a cluster system with a mass function rising to low cluster masses will be fitted to these data. Following this, the models will be adapted to also describe the dynamical evolution of the size distributions of globular clusters, and ultimately the evolution of a full suite of globular cluster structural correlations (the so-called ``fundamental plane''). At every stage, direct comparisons will be made with the size distributions and fundamental-plane correlations observed in real globular-cluster systems. Particular attention will be paid to the implications in the models for the expected size distributions and structural correlations of newly formed globular cluster systems. These will be compared to observations of the young star cluster systems in local starbursts and galaxy mergers. Ultimately, the goal is to provide a comprehensive theoretical framework that quantifies the role of dynamical cluster evolution in establishing connections between all of the main properties of young star-cluster systems forming today and the globular-cluster systems that formed in protogalaxies.