Extragalactic globular cluster systems as dynamical probes of galaxy formation and interaction

我们目前对星系的本质和起源的理解，很大程度上依赖于我们对丰富的、极其美丽的球状星团的仔细分析。球状星团包含已知最古老的恒星，其中许多可以追溯到星系本身形成的时候。因此，它们是星系历史的理想“化石示踪剂”。一个典型的星团包含成千上万颗类似太阳的恒星，它们相互相互吸引，就像一群发光的蜜蜂。因此，单个球状星团是超亮的，可以在遥远的星系中进行研究。（事实上，许多目标星系的距离都在数千万光年之外。）球状星团绕其母星系运行的时间尺度为数千万年或数亿年，它们的轨道性质和化学成分可以用地面上或地球轨道上最大的反射望远镜进行光谱研究。在早期的宇宙中，后来形成恒星的原始气体几乎完全由最轻的元素组成：氢和氦。我们研究了自宇宙早期以来球状星团内的恒星在组成上发生了多大变化的光谱证据，并试图理解这与它们在母星系周围运动的动态状态之间的关系。近年来，相对年轻的球状星团的发现使人们对整个星系之间的相互作用和合并如何在星系的诞生和演化中发挥重要作用有了新的认识。例如，我们银河系的命运似乎不可避免地包括与附近的仙女座螺旋星系的合并——这是未来10亿年后的事件，可能预示着大量丰富的星团的诞生。

Much of our current understanding of the nature and origin of galaxies depends on our careful analyses of the rich, supremely beautiful globular star clusters. The globular clusters contain the oldest known stars, and many of them date from the time of the formation of the galaxies themselves.  They are thus ideal 'fossil tracers' of a galaxy's  history.  A typical cluster contains hundreds of thousands of sun-like stars held in a mutual gravitational embrace, like a swarm of luminous bees.  Individual globular clusters are consequently ultra-luminous, and can be studied in remote galaxies.  (Indeed, many of the target galaxies are at distances of tens of millions of light years.)  Globular clusters orbit their parent galaxies on timescales of tens or hundreds of millions of years, and their orbital properties and chemical compositions can be studied spectroscopically, using the largest reflecting telescopes on the ground or in Earth orbit. In the early universe, the primordial gas out of which the stars would later form consisted almost entirely of the lightest elements: hydrogen and helium. We investigate the spectroscopic evidence for how much the stars within the globular clusters have changed in composition since the early stages of the universe, and seek to understand how that is related to their dynamical states of motions around the parent galaxies.    In recent years, discoveries of relatively young globular clusters have led to a fresh understanding of how interactions and mergers between entire galaxies can play an important role in galaxy birth and evolution.  The fate of our own Milky Way galaxy, for example, seems inevitably to include a merger with the nearby Andromeda spiral galaxy -- an event a billion years in the future which may be heralded by the spawning of large numbers of rich star clusters.