Galaxy cluster growth as a probe of dark matter

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Dark matter is one of the most mysterious constituents of our Universe, and constitutes up to 80% of its matter content. Most particle physics theories predict dark matter to interact so weakly with standard model particles that it will remain fundamentally undetectable in terrestrial experiments. If correct, dark matter can only be studied where it gathers in sufficient quantities for its gravity to affect things around it we can see. With this PhD project, we will track the behaviour of dark matter in galaxy clusters (the most massive structures in the universe), to constrain its particle nature. Different dark matter models make different testable predictions for the rate at which structures, assemble compared to the standard cold dark matter (CDM). For example, warm dark matter inhibits the initial seeding of dark matter structures in the early Universe, while self-interacting dark matter (SIDM) predicts interactions between dark matter particles at late times that prevent the densest regions from growing. Galaxy clusters are ideal laboratories in which to study its properties because they are still forming through mergers of smaller clusters and galaxy groups, commonly called substructures, and every merger acts like a gigantic particle collider. The decoupling of baryons (stars and gas) and dark matter during mergers in massive substructures provides an important constraint on the non-gravitational forces acting on dark matter particle. If SIDM, we expect variation of a few percent (between 3 and 5%) of substructures' stellar and gas density compared to CDM, as well as frictional forces that cause dark matter to gradually separate from stars and gas. With this project, one will independently 'follow' the dark matter, stellar and gas contents in massive clusters, by mapping their distributions, weighing them, and identifying any differences/similarities (distribution peaks, quantities, etc). For this one will exploit observations obtained by the Hubble Space Telescope in the context of the large treasury programme, BUFFALO (https://buffalo.ipac.caltech.edu/), and follow-up spectroscopy from the largest telescope on Earth (VLT). BUFFALO observations were designed to map clusters' dark matter via the eject of 'gravitational lensing', which distorts and magnifies objects behind the cluster. The results obtained will be interpreted within the theoretical framework of state-of-the-art Durham's simulations of our universe.