Globular Cluster Sub-Populations: Understanding Galaxy Formation

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

AST-0206139 Brodie Globular clusters (GCs) are relatively simple stellar populations consisting of 10,000 to 1,000,000 stars and characterized, to first order, by a single age and chemical composition (metallicity). They are thought to be the oldest radiant objects in the universe, and they orbit, usually in large numbers, around galaxies of all morphological types. As fossil remnants of the early environments out of which galaxies formed they are powerful probes of the processes of galaxy formation and evolution. GCs appear to be good tracers of the major star forming episodes in their host galaxies and, unlike single stars, they can be observed far beyond our Local Group of galaxies, providing clues about the early histories of different types of galaxies. Because they are relatively simple stellar populations, GCs are more easily modeled and understood than the unknown mix of stars of different ages and metallicities that make up the diffuse stellar population of galaxies. In the last decade two results have had a major impact on the field of extragalactic GC research. First, young massive clusters (YMCs), perhaps GCs caught in the very process of forming, have been detected in interacting galaxies (two galaxies in the throes of merging together). This suggests that major galaxy interactions induce the formation of new star clusters. If these objects are really "protoglobulars", two populations of GCs should be present after a merger event; the old population from the progenitor galaxies and the new population actually formed in the galaxy-galaxy interaction. Second, optical colors have revealed evidence for blue and red sub-populations in the GC systems of most, but not all galaxies. The color differences are due to metallicity and probably age differences, in combinations still to be determined. It is not yet clear whether one of the sub-populations was produced in a merger event or whether the sub-populations might be due to multiple epochs or mechanisms of formation. However, answering these questions is pivotal to understanding the formation history of galaxies. Building on a very successful current program, the objectives of this project, lead by Dr. Jean Brodie at the University of California at Santa Cruz, are now to understand GC sub-populations by determining exact properties such as ages, element abundances and kinematics for large samples of extragalactic GCs. These properties are the key to discriminating between various GC and galaxy formation scenarios. Observations of young clusters in star-forming galaxies offer an intriguing opportunity to gain first-hand insight into the processes by which GCs formed in the early universe but it is first necessary to establish whether YMCs will survive for long periods of time and can thus be rightly regarded as "young globulars". Specifically, high signal-to-noise spectra of ~50 GCs in each of 12 to18 galaxies will yield accurate individual ages and metallicities. The sample covers a wide range of galaxy masses, morphologies and environments. Kinematic information will be obtained for larger numbers (> 100) of clusters and near-infrared (NIR) photometry will provide somewhat cruder, but essentially age-independent, metallicity estimates for up to 1000 GCs in each galaxy. High-dispersion spectroscopy for 10 to 15 YMCs in nearby star-forming galaxies will be used to determine their virial masses and explore the universality of the stellar initial mass function. A NIR imaging survey in 4 to 6 spirals will detect hundreds of YMCs in their earliest stages of formation. The data will be obtained from several ground-based facilities, including the Keck 10m telescopes equipped with new, powerful multiplexing spectrographs, and high-resolution optical and NIR spectrographs, and the MMT and Magellan telescopes with their wide-field NIR imagers. The data volumes will be large and students and post-docs will play a major role in all stages of the research from observations, data reduction and analysis, to publication. ***