Environmental Drivers of Galaxy Evolution

纵观历史，我们对宇宙的看法是通过观察那些在黑暗中发光的部分形成的。我们看到了种类繁多的星系：一些是充满恒星、气体和尘埃的巨大螺旋盘；另一些是古老恒星的集合，它们以橄榄球的形状捆绑在一起。虽然螺旋星系经常被年轻炽热恒星的诞生点亮，但更老、更红的椭圆星系已经把年轻的恒星形成暴发抛在了身后。我们还观察到，引力可以将数十个甚至数百个星系聚集在一起，形成我们称为星系团的结构。在宇宙中这些拥挤的区域，星系像一群蜜蜂一样围绕着彼此旋转，通过它们的相互引力将它们聚集在一起。然而，最近几年，我们已经开始收集证据，证明我们所看到的发光的恒星、气体和星系只占存在的总质量的一小部分。事实上，我们知道宇宙的大部分是由一种神秘的“暗物质”组成的，这是我们的标准望远镜看不到的。然而，我们知道它在那里是因为它对我们可以看到的东西施加的引力。我的研究目标是准确地追踪这种暗物质是如何以及在哪里传播到整个宇宙的，并发现它是如何与扎根于其中的星系联系在一起的。为了揭示宇宙的黑暗面，我使用了一种名为“引力透镜”的强大工具。引力透镜的工作原理来自爱因斯坦的广义相对论，即大质量物体扭曲了它们周围的时空。这意味着万物--甚至光--都能感受到重力的引力。通过测量光线绕着一个大质量物体传播时的扭曲或弯曲程度，我们实际上可以称量这个物体的重量……即使我们不能直接看到它。我在这里提出的大部分研究都集中在宇宙中一个特别有趣的区域，即A901/902超星系团。这是一个跨越1500多万光年的巨大结构。我计划利用世界各地和太空中的主要望远镜的观测，一层一层地仔细地拆解这个超星系团：哈勃太空望远镜拍摄的图像来确定星系的位置和外观，引力透镜来绘制构成超星系团骨架的暗物质图，X射线卫星来追踪位于星系团核心的炽热X射线气体储存库。我的目标是了解这些成分是如何相互作用的，特别是星系是如何受到居住在那里的影响的。星系会因为生活在如此拥挤的环境中而在其一生中进化和改变吗？我们知道，富星系团中的星系通常看起来与我们在太空中孤立区域看到的星系截然不同。正如大城市的生活可能与农村的生活非常不同一样，星系团的生活节奏在很大程度上要快得多，也更暴力。进入这种环境的星系可能会受到许多环境影响的影响，这些影响可能会剥夺它们的燃料供应(这意味着它们将无法在未来形成更多的恒星)并改变它们的形状。这可以通过几种方式发生：通过星系穿过巨大的中心热气库，通过暗物质的引力，或者通过高速碰撞或与其他星系的近距离接触。我们现在不知道的是，当星系被吸收成星系团时，这些物理过程中的哪些(如果有的话)负责转化它们。这就是为什么我正在进行的观测是重要的：有了它们，我能够详细地观察超星系团中最暗的星系，并试图准确地追踪这些变化发生的时间和方式。把所有的拼图拼凑在一起将有助于我们理解星系是如何在宇宙中形成和演化的，以及它们是如何形成我们今天观察到的令人惊叹的结构的。

Throughout history, our view of the Universe has been formed by observations made of those parts that glow in the dark. We see wonderfully diverse types of galaxies: some are grand spiral disks full of stars, gas, and dust; others are collections of old stars bound together in a shape resembling a rugby ball. While spiral galaxies are often lit up by the birth of hot young stars, the older, redder elliptical galaxies have left their youthful bursts of star formation well behind them. We also observe that the force of gravity can draw galaxies together in groups of tens or even hundreds to form structures we call galaxy clusters. In these crowded regions of the Universe, galaxies orbit about each other like a swarm of bees, held together by their mutual gravitational pull. In recent years, however, we have begun to gather proof that the glowing stars, gas and galaxies that we see represent only a small fraction of the total mass present. In reality, we know that most of the Universe is composed of a mysterious `dark matter' that is invisible to our standard telescopes. However, we know it is there because of the gravitational pull it exerts on things that we can see. The goal of my research is to trace exactly how and where this dark matter is spread throughout the Universe, and to discover how it is linked to the galaxies that are rooted in it. To reveal the dark side of the Universe I use a powerful tool called 'gravitational lensing'. Gravitational lensing works on the principle from Einstein's theory of General Relativity that massive objects warp the spacetime around them. This means that everything -- even light -- feels the pull of gravity. By measuring how much light is distorted or bent as it passes around a massive object, we can actually weigh that object...even if we can't see it directly. Much of the research I am proposing here focuses on a particularly interesting region of the Universe known as the A901/902 Supercluster. This is an enormous structure spanning more than 15 million lightyears. I plan to use observations from major telescopes around the world and in space to carefully peel apart this supercluster layer by layer: images from the Hubble Space Telescope to determine where the galaxies are and what they look like, gravitational lensing to map the dark matter that forms the skeleton of the supercluster, and X-ray satellites to trace the glowing reservoirs of hot X-ray gas that lie in the cluster cores. My aim is to understand how these ingredients interact, and in particular how the galaxies are affected by living there. Do the galaxies evolve and change over their lifetime because they live in such crowded conditions? We know that the galaxies in rich clusters generally look quite different from those we see in isolated regions of space. Just as life in the big city can be very different from rural life, the pace of life in a galaxy cluster is for the most part much faster and more violent. Galaxies entering this environment might fall prey to a number of environmental effects that could be strip them of their fuel supply (meaning they will be unable to form more stars in the future) and change their shapes. This could happen in several ways: by galaxies plunging through large central reservoirs of hot gas, by the gravitational force of the dark matter, or by high speed collisions or close encounters with other galaxies. What we don't know right now is which, if any, of these physical processes are responsible for transforming galaxies as they are absorbed into clusters. That's why the observations I am making are important: with them, I am able to look in detail at even the faintest galaxies in the supercluster and try to trace exactly when and how these transformations occur. Putting all the pieces of the puzzle together will help us to understand how galaxies form and evolve in the Universe, and how they come to form the amazing structures we observe today.