Globular clusters as probes of the evolutionary histories of E/S0 galaxies

Abstract

Globular clusters (GCs) are gravitationally bound collections of millions of stars formed in a very short period of time from the same material. Located in galaxies, they serve as "fossil records" providing an easy diagnostic for determining the epoch and metallicity of major star formation events that shape the evolution of their parent galaxies. In this thesis an observational study of GCs in over a dozen galaxies is carried out where ages and metallicities are derived through optical and near-infrared photometry. A relation between the ages of the GC systems and galaxy morphology is found. While E galaxies have genuinely old GCs (~10 billion years), S0’s host on average younger clusters. This relation is found to be stronger for the metal-poor population if compared to the metal-rich one. The GC system of NGC4365, one of the GC systems that has claims of containing a large fraction of intermediate-age clusters (~2-8 Gyrs) is found to be as old as the GC systems of NGC4486 and NGC4649. When studied through spectroscopy the GC system of NGC4365 is found to be ~10 billion years old. The well known colour bimodality feature, generally interpreted as metallicity bimodality and indicative of different epochs/mechanisms of GC system formation/assembly is studied through optical/near infrared photometry for the first time in a large number of galaxies. For some galaxies, the optical/near-infrared colour distributions are found to be much less likely bimodal than they are in the commonly used optical colours. This suggests that colour bimodality is not necessarily indicative of metallicity bimodality. For some GC systems non-linearities in the colour metallicity relation caused by the horizontal branch morphology might be the cause of the optical colour bimodality.