Environmental Drivers of Galaxy Evolution

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.