Measuring Detailed Abundances of Extragalactic Globular Clusters

密歇根大学的丽贝卡·伯恩斯坦测量银河系外球状星系团的详细丰度我们对正常星系的形成和化学浓缩过程的理解在很大程度上受到来自一个星系的证据的限制：银河系。这一证据在很大程度上体现在古老恒星的详细丰度模式中，这些恒星承载着银河系形成时气体化学的化石记录。即使在另一个正常星系中也不可能获得类似的证据。这很简单，因为距离最近的局部群矮小球状星系之外的老恒星太暗了，无法在丰度分析所需的高信噪比和高光谱分辨率下观察到。与单星不同，用目前的望远镜可以获得距离高达4Mpc的未分辨球状星团的高分辨率光谱。球状星团足够明亮，具有足够低的速度色散，甚至可以在其积分光的光谱中检测到微弱的线条。因为目前的丰度分析方法只适用于单个恒星，所以从未获得过尚未分解的球状星团的详细丰度。一种积分光谱丰度分析的方法将使测量本星系群和遥远地区的大质量星系的详细丰度成为可能。为了发展这项技术，将创建一个银河系和大麦哲伦星云中已分辨球状星团的训练集。在这些训练集星团中，可以获得单星的色等级图和基准金属度，为发展和检验所提出的分析未分辨球状星团的积分光谱的方法提供了丰富的信息。重要的是，训练集包括了广泛年龄和丰度范围的球状星团，因此可以开发必要的策略来处理未分辨球状星团的这种变化。初步结果证明了这种技术的潜力，从NGC 6397和47图卡纳的综合光谱中得出了合理的丰度。但是，此处还将添加一些添加/改进。这些措施包括在合成中加入混合线(包括超精细分裂的影响)，强线的轮廓匹配，以及考虑可能的非局部热力学平衡效应。此外，还将探讨可能影响结果可信度的问题：水平分支形态，通过光谱指示器(例如氢线宽度)可以应用什么约束，对低质量端恒星以及巨星光度函数的敏感性，以及假设的阿尔法元素增强在模拟过程中的重要性。这种测量丰度的方法将使天文学家首次研究银河系以外星系详细的化学富集史。这将极大地增加星系形成理论的观测约束，从而对我们理解星系形成产生广泛的影响。训练集光谱本身还将提供尚不为人所知的丰富的麦哲伦星系团，为线指数系统提供校准库，并帮助天文学家了解球状星团中恒星的初始质量函数和超新星的核合成产额。训练集光谱一旦公布，也将通过现有的数据库提供。大部分资金用于支持使用最先进的观测设施和计算工具的培训和教育研究生。

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.