Astronomical observations have revealed that galaxies in clusters — dense associations of hundreds or even thousands of galaxies — are markedly different from their cousins living in a more isolated environment, such as the Milky Way. The colour of cluster galaxies is typically red, rather than blue, they have often ceased forming new stars several billion years ago, and their morphology is mostly round or elliptical, rather than dominated by a thin disk with spiral arms. These differences imply that, somehow, galaxies "know" about the large-scale environment in which they live: their formation and evolution must be markedly different in clusters than in the more typical, less crowded regions of the Universe. However, astronomers have so far only had a sketchy picture of exactly how galaxies interact with their environment, and how this shapes the galaxies that we observe in clusters.
Uncovering these interaction mechanisms is an important fundamental science question, for a number of reasons. Our picture of galaxy formation is necessarily incomplete without an understanding of how it works in the extreme environment of galaxy clusters. Furthermore, galaxy groups harbour as much as one third of all galaxies in the Universe, and the processes governing their evolution are thought to be essentially a milder variant of those operating in clusters. Finally, galaxy clusters have emerged as promising “tools” to study fundamental questions of astronomy and cosmology, such as the nature of dark energy and dark matter. To reach the required precision, these measurements require an accurate understanding of the cluster galaxies and their special evolution.
The ClusterGal project was therefore set up to shed light on cluster galaxies through a new high-resolution cosmological hydrodynamical simulation suite, Hydrangea. These simulations are based on the successful EAGLE project and specifically target massive galaxy clusters and their large-scale filamentary surroundings at the highest resolution currently achievable on such scales. With this setup, we could investigate the formation and evolution of cluster galaxies in a realistic way. This was done by comparing the simulations to state-of-the-art observations, extracting galaxy histories from the simulations, and comparing to the predictions of simple, intuitive models.
