Globular Cluster Sub-Populations: Understanding Galaxy Formation

brodieglobular clusters （GCs）是相对简单的恒星群，由1万到100万颗恒星组成，其特征在一级上是单一的年龄和化学成分（金属丰度）。它们被认为是宇宙中最古老的辐射天体，它们通常在所有形态类型的星系周围大量运行。作为星系形成的早期环境的化石遗迹，它们是星系形成和演化过程的有力探测器。GCs似乎是宿主星系中主要恒星形成时期的良好示踪剂，与单一恒星不同，它们可以在我们的本星系群之外很远的地方被观察到，为不同类型星系的早期历史提供线索。因为它们是相对简单的恒星群，所以GCs比由不同年龄和金属丰度的未知恒星组成的星系分散恒星群更容易建模和理解。近十年来，有两项研究成果对河外气相色谱研究领域产生了重大影响。首先，在相互作用的星系（两个星系正处于合并的阵痛中）中发现了年轻的大质量星团（ymc），可能是在形成过程中捕获的gc。这表明主要星系的相互作用诱导了新星团的形成。如果这些天体真的是“原球状星团”，那么在一次合并事件之后，应该会出现两个球状星团；来自原始星系的旧种群和新种群实际上是在星系-星系相互作用中形成的。其次，光学颜色揭示了在大多数（但不是所有）星系的GC系统中存在蓝色和红色亚群的证据。颜色的差异是由于金属丰度的不同，也可能是年龄的不同，两者的组合还有待确定。尚不清楚其中一个亚种群是在合并事件中产生的，还是这些亚种群可能是由于多个时代或形成机制造成的。然而，回答这些问题对于理解星系的形成历史至关重要。在目前一个非常成功的项目的基础上，这个项目的目标是由加州大学圣克鲁斯分校的Jean Brodie博士领导，现在通过确定大样本河外GC的确切属性，如年龄、元素丰度和运动学，来了解GC亚种群。这些属性是区分各种GC和星系形成场景的关键。对恒星形成星系中年轻星团的观察提供了一个有趣的机会，可以获得第一手的了解早期宇宙中gc形成的过程，但首先有必要确定ymc是否会存活很长一段时间，因此可以正确地将其视为“年轻的球状星团”。具体来说，在12到18个星系中，每个星系中~50个gc的高信噪比光谱将得到准确的个体年龄和金属丰度。样本涵盖了广泛的星系质量、形态和环境。大量（100个）星系团的运动学信息将得到，近红外（NIR）测光法将提供一些更粗糙的，但基本上与年龄无关的，每个星系中多达1000个gc的金属丰度估计。在附近的恒星形成星系中，将使用10到15个ymc的高色散光谱来确定它们的病毒质量，并探索恒星初始质量函数的普遍性。对4到6个螺旋的近红外成像调查将探测到数百个处于形成早期阶段的ymc。数据将从几个地面设施获得，包括凯克10米望远镜，配备了新的、强大的多路复用光谱仪、高分辨率光学和近红外光谱仪，以及MMT和麦哲伦望远镜，配备了宽视场近红外成像仪。数据量将是巨大的，学生和博士后将在研究的各个阶段发挥主要作用，从观察，数据减少和分析，到发表

AST-0206139BrodieGlobular 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 ofwhich 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, unlikesingle stars, they can be observed far beyond our Local Group of galaxies, providing clues about theearly 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 andmetallicities 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, havebeen detected in interacting galaxies (two galaxies in the throes of merging together). This suggeststhat 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 tounderstanding 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.***