The SAGES project was formed to
Study the Astrophysics of Globular clusters in Extragalactic Systems.
The aim of the project is to investigate the formation and evolution of globular clusters and their host galaxies. This problem is tackled using high resolution imaging from the Hubble Space Telescope, combined with ground-based imaging and multi-object spectroscopy with the Keck Telescopes. This work is supported by NSF grant AST-0206139.
Globular clusters (GCs) are relatively simple stellar populations consisting of 10^4 to 10^6 stars and characterized, to first order, by a single age and chemical composition. 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. Unlike single stars, GCs 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, they are more easily modeled and understood than the unknown mix of stars of different ages and chemical compositions that make up the diffuse stellar population of galaxies.
Recently, two new results have had a major impact on the field of extragalactic GC research. First, young compact objects, apparently 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 this is the case, 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 several bright elliptical galaxies. The color differences might be due to age differences or chemical element abundance differences or some combination of both. It is not clear from the optical colors 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 this question is pivotal to understanding the formation history of galaxies and, in particular, understanding the importance of mergers in forming elliptical galaxies.
A break-through in understanding GC sub-populations can now be achieved with the advent of 10m-class telescopes. Our group is currently undertaking a systematic spectroscopic study with the Keck 10m telescope of extragalactic GC systems in galaxies having a wide range of properties. We plan to complement this by a new approach; infrared imaging. Spectroscopy is the most powerful tool available for accurately determining ages and chemical element abundances, and it is the only way to obtain velocities for exploring kinematic sub-structure in GC systems, but it is prohibitively time-intensive with 4m-class telescopes. Near-infrared observations, combined with existing optical imaging data, are much more sensitive to age and chemical element abundance than optical colors alone. Infrared data will be obtained for large samples of GCs around the selected galaxies - much larger samples than can be studied spectroscopically even with Keck. So far, sub-populations have not been detected in low-luminosity ellipticals. The planned observations will definitively establish their presence or absence, either case having profound implications regarding the origin and evolution of such galaxies. Finally, newly-forming clusters will be studied in selected interacting galaxies to establish whether or not they are the progenitors of ``real'' GCs.
GCs are thought to be good tracers of the major star forming episodes in their host galaxy. The information we will obtain can help answer several important open questions about both GC and galaxy formation and evolution. When did galaxies form? Did all galaxies form at an early epoch? Did isolated galaxies form later than galaxies in clusters (as is expected in the hierarchical clustering paradigm)? Did galaxies experience several major epochs of star formation? Did mergers and interactions play a major role in the history of all galaxies (e.g. are all elliptical galaxies remnants of mergers)? Are the properties of GCs location dependent and, if so, what does that tell us about galaxy formation as a function of location? Why do GC systems around very different galaxies look so much alike?
People Involved
SAGES Publications
Spectral Database
Members Only