Demonstrating the Relationship between the Colour and Heavy Element Abundance of Globular Clusters

Abstract

Extragalactic globular clusters are viewed as single points of light, as the density of their stars make them individually indiscernible. The aim of this study was to find a correlation between the integrated colour of globular star clusters and the metallicity of their individual stars. This was accomplished by using the colour magnitudes and metallicities of individual stars from 11 cluster models based on work from Girardi et al. Integrated colour-metallicity diagrams found the relationship between clusters to be an increasing two-step linear function for each of the six colour ratios examined. To validate the findings, the F475W-F814W relationship was applied to integrated colour data of 5557 clusters from the real galaxy NGC 6166. Histograms taken of the colour and metallicity both showed non-gaussian distributions. Bimodal gaussian fits were applied to both and found that each exhibited the presence of subpopulations: one metal-poor (μ = -1.59), and one metal-rich (μ = -0.41). This suggests the occurrence of galactic mergers with NGC 6166 to create the population structure of clusters that is currently seen.