The Evolution of Globular-Cluster Systems Formed in Galactic Mergers

星系的形成和演化是当今天体物理学的中心问题。越来越多的证据表明，许多星系都是从较小的星系单元发展而来的，而星系合并可能在决定某些残余物的形状和动力学方面发挥了重要作用，也许还有椭圆星系。然而，这种组合的细节仍然不清楚，并且有几个相互竞争的假说关于椭圆的形成。幸运的是，一些星系合并仍然发生在本地宇宙中，为过去的合并过程提供了宝贵的线索。1992年发现球状星团在富含气体的星系合并过程中大量形成，这为研究球状星团及其宿主星系的形成和演化开辟了新的途径。目前人们感兴趣的问题是：在合并中形成的小叶星团和巨大椭圆星系中的老球状星团之间是否存在进化联系？在合并中形成的球状星团系统是如何演化的？具体来说，它们的光度函数、金属丰度分布和运动学是如何演变的？星系合并是否给星系团系统留下了运动特征（例如，快速的外旋转），可以沿着年龄增长的合并残留物序列追踪？首席研究员Francois Schweizer提出了精心设计的观察性研究来解决这些问题。他指出，尽管近年来人们对正在进行的合并中年轻星团的诞生和椭圆中年老球状星团的性质进行了广泛的研究，但令人惊讶的是，人们对中等年龄的球状星团系统缺乏了解。然而，这些系统对于直接解决上述问题有着巨大的希望。施韦泽和他的合作者是其中之一。首先用哈勃太空望远镜研究合并形成的球状星团，然后用地面光谱观测来确定它们的年龄和化学丰度。他现在将在大约12个星系中寻找中等年龄的球状星团（1 - 7gyr），这些星系被认为是正在进行的合并（例如，NGC 4038/39）和旧椭圆星系之间的过渡天体。他将使用阿特拉斯坎帕纳斯天文台的巴德6.5米望远镜和哈勃太空望远镜上的高级巡天相机来获得这些过渡天体及其球状星团系统的一些最深的图像。对这些图像以及哈勃档案图像的光度分析，将从它们的宽带颜色中得出这些星团系统的年表。同样的深度图像也将用于研究球状星团亮度函数随时间的演变。新生星团系统的光度函数为幂律，而古老星团系统的光度函数则接近对数正态。理论预测，随着球状星团系统年龄的增长，它们的光度函数应该从恒星演化而来。通过低质量星团的优先侵蚀，RST形成第二种形状。将从图像中测量超过预测的周转幅度的深亮度函数，以对该预测进行关键测试。一个平行的多缝光谱学项目，主要是用麦哲伦6.5米望远镜，也用北双子座8米望远镜，将得到一些关键的中等年龄球状星团系统的金属丰度和更准确的年龄。对每个星系典型的40 - 200个星团的径向速度的测量，将允许对同一星系中缺乏金属和富含金属的球状星团的运动学进行比较研究。这将为我们所知的起源提供有价值的线索。第一代和第二代群集亚群。这些观测研究将测试所有提出的椭圆的形成机制，并将特别关注合并形成的第二代球状星团和旧椭圆中富含金属的球状星团之间进化联系的可能证据。这个项目的结果不仅将有助于提高我们对球状星团形成和演化的理解，还将有助于提高我们对星系形成和生长的理解，并最终提高我们对哈勃星系类型形态序列起源的理解

AST 0205994SchweizerThe formation and evolution of galaxies is of central interest in present-day astrophysics. Thereis mounting evidence that many galaxies grew from smaller units, and that galactic mergers mayhave played a major role in determining the shapes and dynamics of certain remnants, and perhapseven of elliptical galaxies. However, the details of this assembly remain unclear, and there areseveral competing hypotheses for the formation of ellipticals. Fortunately, a few galactic mergersstill occur in the local universe and o.er valuable clues to the past assembly process.The 1992 discovery that globular star clusters form in large numbers during mergers of gas-richgalaxies has opened new avenues to studying the formation and evolution of globular clusters andof their host galaxies. Questions of current interest are: Is there an evolutionary link betweenglobular clusters formed in mergers and old globular clusters in giant elliptical galaxies? Howdo systems of globular clusters formed in mergers evolve? Specifically, how do their luminosityfunctions, metallicity distributions, and kinematics evolve? Do galactic mergers imprint clustersystems with kinematic signatures (e.g., fast outer rotation) that can be traced along a sequenceof merger remnants of increasing age?The Principal Investigator, Francois Schweizer, proposes carefully designed observational studiesto address these questions. He points out that-whereas the birth of young clusters in ongoingmergers and the properties of old globular clusters in ellipticals have been studied widely in recentyears-there is a surprising lack of knowledge about globular-cluster systems of intermediate age.Yet, these systems hold great promise for directly addressing the above issues.Dr. Schweizer and collaborators were among the .rst to study globular clusters formed in mergerswith the Hubble Space Telescope and to follow up with ground-based spectroscopic observationsto determine their ages and chemical abundances. He will now search for globular clusters of intermediateage (1 - 7 Gyr) in about a dozen galaxies thought to be transition objects between ongoingmergers (e.g., NGC 4038/39) and old ellipticals. He will use both the Baade 6.5-m telescope atLas Campanas Observatory and the Advanced Survey Camera on the Hubble Space Telescope toobtain some of the deepest images ever taken of these transition objects and their globular-clustersystems. Photometric analysis of these images, as well as of Hubble archival images, will yield thechronology of these cluster systems from their broad-band colors.The same deep images will also be used to study the evolution of the globular-cluster luminosityfunction with time. The luminosity functions of fresh-born cluster systems are power laws,while those of ancient systems are close to lognormal. Theory predicts that as globular-clustersystems age, their luminosity functions should evolve from the .rst to the second shape throughpreferential erosion of low-mass clusters. Deep luminosity functions reaching past the predictedturnover magnitudes will be measured from the images to subject this prediction to a critical test.A parallel program of multi-slit spectroscopy mainly with the Magellan 6.5-m telescopes, butalso with the northern Gemini 8-m telescope, will yield metallicities and more accurate ages for somekey globular-cluster systems of intermediate age. Measurements of radial velocities for typically40 - 200 clusters per galaxy will permit comparative studies of the kinematics of metal-poor andmetal-rich globulars within the same galaxy. This will yield valuable clues about the origins ofwhat are thought to be .rst- and 2nd-generation cluster subpopulations.These observational studies will test all proposed formation mechanisms for ellipticals andwill-specifically-focus on possible evidence for an evolutionary link between 2nd-generation globularsformed in mergers and metal-rich globulars in old ellipticals. The results from this projectwill help improve our understanding not only of globular-cluster formation and evolution, but alsoof galaxy formation and growth and, ultimately, of the origin of Hubble's morphological sequenceof galaxy types.***