Globular Cluster Systems in Extreme Environments: Origins and History

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Globular clusters are dense, massive, gravitationally bound clusters of stars that are found universally in all large galaxies. They are among the oldest stellar systems, having formed in the first few Gigayears of the universe's history, out of the densest, most massive, and most violent star-forming sites within early galaxies. Although in the present-day universe GCs make up just a percent or less of all stellar mass in galaxies, they play key roles for tracing a huge range of processes in stellar and galactic evolution.  My research proposal aims at three such paths:  (1) In rich clusters of galaxies, globular clusters are also found in the vast space between the galaxies, and the characteristics of these intergalactic GCs trace, in a unique way, the histories of interactions and encounters among the galaxies in the cluster.   To pursue this new frontier I am leading a new observational program with the Hubble Space Telescope and with the Subaru telescope for a comprehensive survey of the intergalactic GC population in the rich Perseus cluster.  (2) The largest numbers of globular clusters are found in the largest galaxies:  the Brightest Cluster Galaxies (BCGs).  These galaxies grow right up to the present day by absorbing the smaller satellite galaxies around them, but the effects of environment on the evolution of these giant galaxies are not completely understood.   Their GC populations hold unique clues to their early history, because GCs have properties that depend strongly on the type of galaxy they originated in.  As part of my ongoing program to study the GC systems in giant galaxies, we will use our new data from the Hubble Space Telescope to systematically survey BCGs over their entire huge range of environments to find out how quickly these galaxies grew, and what types of galaxies contributed to their growth and assembly.  This work will fill in a major piece of the puzzle for the origin of the largest galaxies in the universe. (3) A well known new discovery is that a single globular cluster may have within it multiple stellar populations that all have the same age but different ratios of the light elements.   These are well established by current observations to be present, but it has been difficult to understand how they arose in such small (by galactic standards) stellar systems.  Current ideas for the origin of this effect are controversial.  Building on theoretical work by one of our former PhD students, we will trace the effects of the earliest massive stars in the cluster on the internal abundance mixture of its star forming gas.  This is a promising and exciting new avenue for understanding the origin of multiple populations, based solidly on our rigorous and quantitative theory for star cluster formation.  This work has the potential to shape the community's direction of research into the formation of massive star clusters in the early universe.