Globular cluster systems and their implications of galaxy formation

Abstract

In this thesis the spectroscopic and photometric results of the Galactic and extragalactic globular clusters are presented. And their implications on galaxy formation have been investigated. Integrated spectroscopy with a high resolution of 24 Galactic globular clusters were obtained using the Isacc Newton Telescope, from which 21 line indices were carefully measured and calibrated. By comparing with recent simple stellar population models (Thomas et al. 2003 and Lee & Worthey 2005), it is confirmed that Galactic globular clusters are old (~10- 12 Gyr). There is a discrepancy between the two models in low metallicity and the line indices measured have a better fit to Lee & Worthey (2005) model. These is a significant effect of blue horizontal branch stars on the Balmer absorption lines in integrated spectra, which could cause globular clusters to be underestimated their age. Shell in elliptical galaxies are probably signature of recent galaxy merger/interaction. Properties of globular cluster systems (GCSs) in six shell galaxies have been examined and whether this signature can be seen in GCSs have been investigated. The GCSs in shell galaxies are found not to differ noticeably from those in normal elliptical galaxies in the sense and blue subpopulations are consistent with previous studies. This result is contradictory to results by Sikkema et al. (2006), who have found possible young globular clusters in the two galaxies. Using the Advanced Camera for Survey on the Hubble Space telescope, 10 low density early-type galaxies were observed and properties of their GCSs have been investigated to constrain galaxy formation history depending on environments. By comparing results from the Virgo Cluster Survey as a high-density counterpart, both similarities and differences between the two density groups were found. General behaviour of colour distributions of GCSs found in cluster environment are also seen in those in field galaxies; mean colour gets redder as increasing with galaxy luminosity, less bimodal colour distributions are detected in lower galaxy luminosity, and there is a correlation between red colour peak and host galaxy luminosity. However, mean colours of GCS in low-density appear to be slightly bluer than those in high-density at a given galaxy luminosity, which implies that GCS in field environments is either less metal-rich or younger than those in cluster environments. More diverse shape of colour distributions are found in dense region than in low-density region, which would reflects more complicated galaxy formation history in dense region. In spite of finding environmental this effect on galaxy formation, this effect is so subtle that galaxy (final) mass is still a dominant factor to determine galaxy formation and stellar populations in there.