Revealing the drivers of galaxy formation in the densest cosmic environments

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One of the most perplexing aspects of our Universe is the huge variety of galaxy shapes and colours, from blue (star-forming) spiral discs to red (no longer star-forming) ellipticals. These differences are thought to be due to changing in- and outflows of gas (the "baryon cycle"), a process that can be investigated particularly cleanly when galaxies fall towards densely inhabited cosmic environments such as galaxy clusters. But we do not understand well how and why these changes occur, especially in the early Universe and for low-mass "dwarf" galaxies, and which other processes might be important, especially tidal stripping of stars into the diffuse "intra-cluster light".I will solve this problem by creating state-of-the-art defining computer simulations of galaxy clusters to interpret major new observations with ground- and space-based telescopes, including by the recently launched James Webb Space Telescope and with the new WEAVE spectrograph on the William Herschel Telescope. Exploiting extensive development work by myself and others, these simulations will realistically model the star forming gas in cluster galaxies for the first time. Spanning the full range from small "groups" to the most massive observed clusters, they will also include close analogues of four well-studied clusters in the local Universe.From these simulations, I will create synthetic observations, mimicking what they would look like in different wavelengths from X-rays through optical to radio. Comparing these with real observations, I will perform stringent tests of the simulation model and our understanding of galaxy formation encoded within it. In parallel, I will analyse the simulation outputs in detail, tracking individual galaxies through time to identify which physical processes emerge from the imposed laws of fundamental physics. In combination, these two approaches will solve the mystery of how galaxies form and evolve in the most extreme cosmic environments, and what they can tell us about the role of gas in- and outflows for galaxy formation in general.