Globular clusters of the Local Group – statistical classification

Abstract

Aims. To find an objective classification of the globular clusters in our Galaxy, M 31, and LMC. Methods. A new method of Cluster Analysis (CA)was carried out and the set of parameters for this method was selected through an objective process, Principal Component Analysis (PCA). Robustness of the classification was established using bootstrap samples. Results. In every case they exhibit multi-population structure instead of bimodality as is found in many spirals and giant elliptical galaxies. The kinematics of MW and M 31 GCs are examined in support of these sub-populations in the cluster system. It is found that for MW and M 31 GCs a disc, inner halo, and outer halo populations of GCs are more likely to exist than only the disc and halo populations of GCs in MW as concluded by Zinn (1985, ApJ, 293, 424). This supports the existence of three populations more firmly explained by Zinn (1993, Globular Cluster-Galaxy Connection, 38) and Mackey & Gilmore (2004, MNRAS, 355, 504) whereas only two populations are found for LMC GCs. The new multivariate analysis increases the importance of the inclusion of many parameters while at the same time it eliminates less significant parameters and helps to enunciate a unique theory of galaxy formation.