Studies on the formation, evolution, and destruction of massive star clusters

Abstract

This thesis presents the results of mainly observational studies on the formation, evolution, and destruction of massive star clusters. We show, using a variety of observational techniques, that globular clusters which were once thought to only be able to form in the early universe are in fact still forming today. In Chapters 2, 3 and 4 we study the star cluster populations in the spiral galaxy M51 and the dwarf starburst galaxy M82. In particular we show that the formation rate of star clusters can be severely affected by the properties of the host galaxy, e.g. galactic interactions tend to increase the cluster formation rate. In addition, we show that roughly 70% of all clusters which form in a galaxy, will disrupt within the first 10 Myr of their lives. In Chapter 5, we present the first observational evidence that a young cluster population will begin to resemble an old population, through the preferential destruction of lower mass clusters. This finding was the 'missing link' in the evolution of young clusters. The cluster population in the tidal tails of NGC6872 is the topic of Chapter 6, where we show that a whole population of star clusters are presently forming in the tidal debris caused by a galaxy interaction. Through the use of extremely high-resolution spectroscopy and space-based imaging, we show that a star cluster in the galactic merger remnant NGC7252 is as massive as a typical dwarf galaxy in Chapter 7. This result blurs the historical definition of the difference between star clusters and galaxies. Finally, in Chapters 8 and 9 we used a combination of ground based spectroscopy, radio observations, and space-based imaging to study the relatively newly discovered phenomenon of star cluster complexes. In these works, we show that the high star formation rate (per unit area) of these complexes categorizes them as localized starbursts and that the remaining gas within the complexes which was not used during the star formation process is being expelled from the complexes at velocities between 50 and 150 km/s, typical of starbursts in dwarf galaxies. We also show that the complexes themselves are part of larger structures within the galaxies, namely complexes of complexes, which likely reflects the distribution of the progenitor giant molecular clouds (GMCs) which are known to also group together in larger associations. The complexes follow a mass radius relation similar to that of GMCs, unlike that found for individual star clusters (which do not follow any mass-radius relation as shown in Chapter 2). This fact, in addition to the shared density profile of the GMCs and complexes, shows that the complexes retain some of the properties of the progenitor GMCs and reflect the hierarchical formation of structure within galaxies. We end the thesis with a summary of the results, along with ideas for future observations which will allow us to test many of the ideas presented in this thesis.