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

在可观测的宇宙中，闪烁着明亮的星系，其中许多比我们雄伟的银河系要大得多，太阳所在的银河系是一个巨大的螺旋形星系。星系本身可能包含上万亿颗类似太阳的恒星，并呈现出各种各样的形式。它们不是在空间上随意排列，而是在几乎难以想象的大空间尺度上聚集在一起或分布在巨大的墙壁和巨大的流线中。然而，尽管我们能够辨别和绘制出这些单个恒星城市的物理分布，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。它们是如何形成的？幸运的是，这些星系本身包含了它们年轻时期的重要化石记录——也就是说，在引力的拥抱下，丰富、极其美丽的球状星团在星系周围以庄严的轨道运行。我们的银河系有100多个这样的星团；在其他星系中，可能有数以万计。使用最大的望远镜，我们可以“解剖”球状星团，甚至在数千万光年之外的星系中，将其组成恒星发出的联合光传播成彩色光谱。然后，我们分析光谱中的特征，以确定每个星团围绕其母星系的年龄、化学成分和轨道运动状态。考虑到球状星团是可见宇宙中已知最古老的恒星天体，这些知识就具有了巨大的意义。因此，它们的性质和动力学告诉我们100多亿年前标志着星系诞生的动荡形成过程。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与姐妹星系的剧烈相互作用，甚至是灾难性的合并。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的仙女座螺旋星系合并的结果，仙女座螺旋星系距离我们只有200万光年。球状星团有能力告诉我们这类事件在星系形成历史中有多常见。

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.