Globular Clusters: Diagnostic Indicators of Halo Structure, Galaxy Formation and Interaction Histories

可观测到的宇宙中充满了发光的星系，其中许多比我们自己雄伟的银河系大得多，太阳所在的大螺旋。星系本身可能包含数万亿颗类似太阳的恒星，并呈现出各种各样的形式。它们在空间上不是随机排列的，而是聚集在一起，或者分布在巨大的墙壁和巨大的飘带中，空间尺度几乎无法想象。然而，尽管我们可以辨别并绘制出这些恒星城市的物理分布图，但对它们的起源和形成历史的研究仍然是宇宙学中一个未完全解决的问题。它们是怎么来的？幸运的是，星系本身就包含了它们年轻阶段的重要化石记录--也就是说，在引力的拥抱下，围绕星系在庄严的轨道上运动的丰富、极其美丽的球状星星团。我们的银河系有一百多个这样的星星团;在其他星系，可能有数万个。使用最大的望远镜，我们可以比喻性地“解剖”球状星团，即使是在数千万光年之外的星系中，将其组成恒星发出的组合光传播成彩色光谱。然后，我们分析光谱中的特征，以确定每个星系团围绕其母星系的年龄，化学成分和轨道运动状态。这一知识具有巨大的意义时，再加上事实上，球状星团是已知的最古老的恒星物体在可见的宇宙。因此，它们的性质和动力学告诉我们，超过100亿年前，标志着星系诞生的动荡的形成过程。此外，在数百或数千个星系团中，这些属性的复杂组合和范围也将表明一个星系是否遭受了随后的激烈相互作用-甚至是灾难性的合并-与姐妹星系。有趣的是，对我们银河系未来行为的预测似乎不可避免地会产生与附近仙女座螺旋星系的合并，距离我们只有200万光年。球状星团有能力告诉我们，在星系形成的历史中，这样的事件是多么常见。

The observable universe is bespangled with luminous galaxies, many of them much larger than our own majestic Milky Way, the grand spiral within which the Sun is located. The galaxies themselves may contain literally trillions of Sun-like stars and take on a variety of forms. They are not arranged at random in space, but instead are clustered together or distributed in vast walls and enormous streamers on almost unimaginably large spatial scales. Yet although we can discern and map out the physical distributions of these individual cities of stars, the study of their origin and formation history remains an incompletely resolved question in the science of cosmology. How did they come to be? Fortunately, the galaxies themselves contain important fossil records of their youthful stages -- namely, the rich, supremely beautiful globular star clusters that move in stately orbits around the galaxies, held in a gravitational embrace. Our Milky Way has more than one hundred such star clusters; in other galaxies, there may be tens of thousands. Using the largest telescopes, we can metaphorically 'dissect' the globulars, even in galaxies tens of millions of light years away, spreading the combined light emitted by their constituent stars into a colourful spectrum. We then analyse features in the spectrum to determine the age, chemical composition, and state of orbital motion of each cluster around its parent galaxy. This knowledge takes on enormous significance when coupled with the fact that the globular clusters are the oldest known stellar objects in the visible universe. Their nature and dynamics, therefore, tell us about the turbulent formative processes that marked a galaxy's birth, more than ten billion years ago. Moreover, a complex mix and range of these attributes among the many hundreds or thousands of clusters will also indicate whether a galaxy has suffered a subsequent vigorous interaction -- or even a catastrophic merger -- with a sister galaxy. Interestingly, predictions of the future behaviour of our own Milky Way seem inevitably to yield a merger with the nearby Andromeda spiral galaxy, a mere two million light years away. The globular clusters have the power to tell us how common such events are in galaxy formation histories.