Exploring the formation histories of galaxies - globular clusters and beyond

Abstract

One of the most struggling questions in todays astronomy is the question for the formation processes and evolutionary histories of galaxies. In order to decide between different scenarious it is necessary to get reliable information about both the star formation history (SFH) and the chemical enrichment history of a galaxy.Unfortunately, most galaxies are observable only in integrated light, so that SFH determinations using the colour magnitude diagram (CMD) approach, are only possible for a very limited sample of nearby galaxies. As I show in this thesis, both photometric spectral energy distributions (SEDs) and spectra obtained for the integrated light of a galaxy can give only very vague hints about the distribution of SF over the lifetime of a galaxy. Only for a short timescale of 1 to 4 Gyrs some information can be recovered.However, constraining the age and metallicity distributions of star cluster (SC) and globular cluster systems (GCSs) can provide important clues to the formation and evolutionary history of their parent galaxies. Within the framework of this thesis, I have developed new tools for the analysis of GCSs which make use of all information available for a given set of SC observations:I present a large grid of evolutionary synthesis models for simple stellar populations, including SEDs as well as 25 absorption-line indices following the Lick/IDS system, and I present a tool for interpreting index observations using these models, the Lick Index Analysis Tool. This tool determines age and metallicity of SCs, including their respective $\pm 1 \sigma$ uncertainties, using all, or any subset of, measured indices. Testing the tool against index measurements from various authors for Galactic GCs, which have reliable age and metallicity determinations from CMD analyses in the literature, yields very good agreement. Additionally, I present a new method for the combined analysis of broad-band SEDs and spectral indices, PRODUCT, which successfully allow to constrain ages and metallicities of individual GCs even in cases when poor datasets are available only.Applications of the models and analysis tools to the GCS of the large elliptical galaxy NGC 5128, using a dataset consisting of both broad-band photometry and Lick indices for an unprecedentedly large sample of GCs show that, despite the relatively poor quality of these data which do not allow to constrain ages and metallicities with reasonable precision by analysing each dataset alone, a combined analysis using the analysis tool PRODUCT allows independent determinations of ages and metallicities for a large fraction of the analysed sample with unprecedented precision. In addition to the cluster populations well known in the literature (old GCs from low to high metallicities, and young metal-rich GCs), I found a population of intermediate age, metal-poor GCs in NGC 5128 which has not been described in the literature before.