Formation and evolution of star clusters and their host galaxies

Abstract

The vast majority of galaxies contains large populations of stellar clusters, which are bound groups of a few tens to millions of stars. A cluster is formed from a single giant molecular cloud and therefore its stars share the same age and chemical composition. The formation and evolution of star clusters is influenced by local processes as well as mechanisms acting on (extra)galactic scales. A thorough understanding of star cluster formation and evolution allows us to probe the process of star formation and to use the star cluster population as a fossil record to trace the evolution of the host galaxy. This work concerns a theoretical, numerical and interpretative study of the formation and evolution of (populations of) star clusters in the context of their galactic environment. It covers the internal dynamics of stellar clusters during their formation and subsequent evolution, and also connects their evolution to galaxy-scale events such as galaxy mergers. The key question that is asked throughout is how star clusters dynamically respond to their environment, and what the macroscopic implications are for a population of star clusters. The first part of this work treats the formation of stellar subclusters – the first stage of what could potentially become a (population of) star cluster(s). It is found that stellar subclusters are formed close to dynamical equilibrium when ignoring the gravitational potential of the primordial gas, implying that stellar structure can survive gas expulsion by supernovae. The second part discusses the internal dynamics and tidal disruption of star clusters, during which low-mass stars are preferentially ejected. This influences the photometric properties of clusters. The mass-to-light ratio is found to increase with cluster mass and luminosity, which is used to explain the properties of Galactic globular clusters. It is also shown how the conversion of the globular cluster luminosity function to a cluster mass function is affected. A new analytic, physical model for the evolution of the stellar mass function within star clusters is presented, which reproduces the results from numerical simulations of dissolving clusters and explains the observed stellar mass functions of a set of globular clusters. In the third and final part, the properties of star cluster populations and the evolutionary histories of their host galaxies are connected. Using a new numerical simulation for the co-evolution of galaxies and their star cluster populations, it is shown that the rate of cluster disruption is governed by galactic processes, and that the variation of the disruption rate in time and space leaves a clear imprint on the properties of the star cluster population. Galaxy mergers are found to yield a net destruction of star clusters due to the high gas densities and corresponding increase of the disruption rate, despite the large burst of cluster formation induced by the galaxy interaction. A model for the Antennae galaxies and their cluster population shows excellent agreement and illustrates that it is essential to account for the complexity of the galactic environment when tracing the history of a galaxy using its star cluster population.