Protoclusters: cradles of galaxy formation

The greatest mass extinction event in history occurred approximately 3 billion years after the Big Bang. Up to this point the majority of galaxies across the Universe were forming young stars at prodigious rates. These galaxies were drawn together by their mutual gravity, and in the densest regions hundreds to thousands of galaxies grouped together to form galaxy clusters. Around 10 billion years ago the galaxies in these young clusters stopped forming new stars, and now clusters are the graveyards of our Universe, containing thousands of "dead" galaxies.

From observations of the present-day Universe we know these things must have occurred, but we know very little about this period in our Universe's history. When and where did the first galaxies form? How quickly did these galaxies congregate together in clusters? And what triggered the end of star formation? Did the galaxies simply run out of new material from which to make stars, or did the supermassive black holes at the centres of these galaxies expel whatever material remained? Perhaps the most interesting question is: why did large numbers of galaxies, millions of light-years apart, undergo the same fate at approximately the same time?

The answers to these questions are essential to understanding the Universe in which we live, and they form the main strands of my proposed research. I will tackle these questions by observing young clusters of galaxies at different ages using the world's most powerful space and ground-based telescopes. By studying galaxies that are at different stages of development, I will map out their evolution from when they formed their stars to the mass extinction event and beyond, determining exactly when and why star formation ceased.

The first major technical challenge lies in simply identifying the galaxies that belong to the young clusters, as our view is dominated by the more numerous fore- and background galaxies. I have developed a new technique to identify cluster galaxies that has several advantages over existing methods. My new method is more efficient than existing tools, it identifies all types of cluster galaxies, and it successfully removes a larger fraction of fore- and background galaxies. Applying my technique will allow me to tackle problems that have puzzled astronomers for decades.