Caught in the act: massive galaxy and cluster formation

With the naked eye we can see billions of stars that make up our Galaxy, the Milky Way. How did the stars get here? How do galaxies form? The Universe is a giant cosmic web, with many filaments of galaxies linking together. Where many filaments meet we find clusters of galaxies, in which a few very massive galaxies shepherd several hundred smaller galaxies. The oldest galaxies are located in these clusters, whilst the younger galaxies generally lie in the filaments. How and why is the Universe like this? My research asks these questions, and looks to massive galaxies, which are ten times more massive than the Milky Way, for answers. According to the most recent computer simulations of our Universe, galaxies formed gradually. The first galaxies formed as clumps of stars within a billion years of the Big Bang. Drawn together by gravity, these small galaxies moved closer; some collided and merged to form larger units. As time passed, clusters of galaxies assembled, and galaxies grew larger as more and more galaxies merged together. However, this theory of 'hierarchical' galaxy formation is not complete. A competing theory argues that massive galaxies formed in single events, when large gas clouds cooled quickly, rapidly forming the majority of the stars we see in these galaxies today. So, did galaxies form gradually, or quickly? When did these galaxies assemble: 1, 5 or 10 billion years ago? Theory and simulations can only take us so far: the answers to these questions lie in observations of the real Universe. Today the Universe is fairly quiet: the current rate at which stars form is the lowest it has been for more than 11 billion years. The most massive galaxies no longer form stars; they are effectively 'dead', destined to burn out slowly as their stars fade and die. One of the most direct ways to see how these galaxies form is to look into the distant past and witness their assembly. I use large modern telescopes, such as the Very Large Telescope in Chile and the Hubble Space Telescope, to observe massive, assembling galaxies and clusters of galaxies when the Universe was only a few billion years old. At this time galaxies were undergoing violent activity. They formed stars at an unprecedented rate, emitting vast quantities of ultraviolet light. Huge jets of particles, moving close to the speed of light, were driven out to large distances by the growing black holes in the centres of these massive galaxies. During my Fellowship I will take images of forming clusters of galaxies in order to understand how these structures came to be. I will map where the young, star forming galaxies are located in the clusters, compared to the older, more massive galaxies, in order to pick out the galaxies which have just been sucked into the clusters. Furthermore, I will count the number of merging galaxies, to see how quickly galaxies can grow in the dense cluster environment. By comparing my observations with computer simulations I will test the models and theories of galaxy formation. Only through comparing theoretical predictions with the real Universe, can we move further towards understanding how galaxies formed.