Measuring Detailed Abundances of Extragalactic Globular Clusters

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AST-0507350Rebecca BernsteinUniversity of Michigan Ann ArborMeasuring Detailed Abundances of Extragalactic Globular ClustersOur understanding of the formation and chemical enrichment processes of normal galaxies is largelyconstrained by evidence from one galaxy: the Milky Way. That evidence is largely revealed in the detailedabundance patterns of old stars, which bear the fossil record of the Galaxy's gas chemistry at the time theyformed. It has never been possible to obtain similar evidence in even one other normal galaxy. This is simplybecause old stars beyond the nearest dwarf spheroidal galaxies of the Local Group are too faint to beobserved at the high signal-to-noise and high spectral resolution needed for abundance analysis.Unlike single stars, high-resolution spectra can be obtained of unresolved globular clusters at distances aslarge as 4 Mpc with current telescopes. Globular clusters are bright enough and have low enough velocitydispersions that even weak lines could be detected in spectra of their integrated light. Detailed abundanceshave never been obtained for unresolved globular clusters only because current methods of abundanceanalysis only work for individual stars. A method for abundance analysis of integrated-light spectra willmake it possible to measure detailed abundances in the massive galaxies of the Local Group and wellbeyond.To develop this technique, a training set of resolved globular clusters in the Milky Way and LargeMagellanic Cloud will be created. In these training set clusters, color magnitude diagrams and fiducialmetallicities from single stars can be obtained, supplying a wealth of information for developing and testingthe proposed method for analyzing the integrated light spectra of unresolved globular clusters. Crucially, thetraining set includes globular clusters of a wide range of ages and abundances, so that the necessarystrategies can be developed to deal with such variations in unresolved globular clusters.Preliminary results demonstrate the potential of this technique, with reasonable abundances being derivedfrom the integrated spectra of NGC 6397 and 47 Tucanae. However, a number of additions/improvementswill also be added here. These include incorporation of blended lines in the synthesis (including the effects ofhyperfine splitting), profile matching of strong lines, and consideration of possible non-local thermodynamicequilibrium effects. In addition, issues which may affect the confidence of the results will also be explored:horizontal branch morphology and what constraints can be applied through spectral indicators (e.g.,hydrogen line widths), the sensitivity to the luminosity function of stars at the low mass end, as well as thegiants, and the importance of assumed alpha-element enhancements in the modeling process.This method for measuring abundances will let astronomers study the detailed chemical enrichment historiesof galaxies beyond the Milky Way for the first time. This will dramatically increase the pool of observationalconstraints on galaxy formation theories and therefore have broad impact on our understanding of galaxyformation. The training set spectra themselves will also provide abundances of Magellanic Cloud clusterswhich are not yet well known, provide a calibration library for line index systems, and help astronomersunderstand the initial mass function of stars in globular clusters and the nucleosynthetic yields of supernovae.The training set spectra will also be made available through existing database as soon as they are published.A majority of the funding goes to supporting the training and education graduate students using state of theart observational facilities and computational tools.