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

点击翻译按钮获取中文摘要

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.