Globular Clusters Systems: Formation Models and Case Studies

Summary

Globular cluster systems represent only a small fraction of the total stellar mass of galaxy halos, but provide unique tracers which can be used to address models of galaxy formation. Several “case studies” of individually important galaxies are presented, in which we look at the characteristics of their globular clusters including the metallicity distributions, specific frequencies, luminosity (mass) distributions, and kinematics. Among these galaxies are the Milky Way, the nearby giant elliptical NGC 5128, the Virgo ellipticals NGC 4472 and M87, and the supergiant cD galaxies at the centers of rich clusters. In each case the possible roles of mergers, small-satellite accretions, and in situ formation in the growth of the galaxy are discussed. We also briefly touch on the connection between the globular clusters and the much more numerous field-halo stars. We conclude that in all formation scenarios, the presence or absence of gas at any stage of the galaxy's evolution plays a crucial role in determining the total cluster population, the number of distinguishable subpopulations, and the metallicity distribution of the clusters.

The analysis of globular cluster systems (GCSs) in other galaxies is starting to fulfil its long-held promise of informing us about galaxy formation in ways that are unique. The more we learn about GCSs, the more we realize that their role in galaxy formation is an intricate and varied process – yet with common themes that apply particularly to the old-halo population that is found in every type of galaxy.