Uncovering the Mechanisms that Drive Galaxy Formation in Massive Clusters

One of the biggest unsolved problems in extragalactic astrophysics is the origin of the striking difference between isolated (field) galaxies and those living in the crowded environment of galaxy clusters. The former are typically blue, star-forming spiral discs, whereas “red and dead” ellipticals dominate the latter population. A multitude of processes have been suggested as the potential cause of this dichotomy, but the complex non-linear nature of galaxy formation has so far made it impossible to determine the actual physical drivers of galaxy formation in massive clusters. The EAGLE collaboration, led by Leiden Observatory, has recently completed a simulation whose field galaxies achieve an unprecedentedly good match to the real Universe in many key aspects, including their stellar masses, sizes, colours, star formation rates, and atomic hydrogen content. Building on this model, I have led the production of the “Hydrangea” simulations, a large suite of 24 massive galaxy clusters. For the first time, these contain a realistically modelled and well-resolved galaxy population, making them the ideal tool to finally understand the formation of cluster galaxies. The proposed research is centred on the exploitation of this unique opportunity. The accuracy of the simulations will be tested through rigorous comparisons of virtual images to multi-wavelength observational data (X-ray, UV, optical, IR, radio). The physical processes acting in the simulation will then be identified by fitting simple, intuitively understandable models to the Lagrangian history of simulated galaxies and their individual resolution elements. In combination with the EAGLE simulations, this will reveal the impact of environment on galaxy morphology, star formation rates, and gas content, in both the local Universe and at high redshift. The result will be a coherent physical picture of how the formation of galaxies is connected to their large-scale surroundings.