The Structure and Environment of Galaxy Clusters in Simulations and Observations

Galaxy clusters are powerful testing grounds for cosmology. They are the largest, most massively bound objects in the Universe and can give us deep insights on how baryons, dark matter, and dark energy impacts on the formation of large scale structure in the cosmos. In this dissertation we study the structure and environment of clusters, how neutrinos impact on cluster masses, and how we may find them. We study cluster profiles on large scales to gain better understanding on the relationship between cosmic web filaments and clusters that reside in the nodes, which gives insight on the evolution of clusters from their environment. For more than two decades, the Navarro, Frenk, and White (NFW) model has stood the test of time; it is used to describe the distribution of mass in galaxy clusters out to their outskirts, beyond which the NFW model is no longer applicable. In this dissertation we assess how well the parameterised Diemer & Kravstov (DK) mass density profile describes the mass distribution of galaxy clusters extracted from cosmological simulations. This is determined from averaged synthetic lensing measurements of the 50 most massive clusters extracted from the OverWhelmingly Large Simulations (Cosmo-OWLS). The characteristics of the data reflect the Weighing the Giants survey and data from the future Large Synoptic Survey Telescope (LSST). In comparison with the NFW model, the DK model is favored by the averaged data, in particular for the LSST data, where the number density of background galaxies is higher. The DK profile depends on the accretion history of clusters which is specified in the current study. Eventually subsamples of galaxy clusters with qualities indicative of disparate accretion histories could be studied. We also study the impact of baryonic processes and massive neutrinos on weak lensing (WL) peak statistics that can be used to constrain cosmological parameters. We use the BAHAMAS suite of cosmological simulations, which self-consistently include baryonic processes and the e↵ect of massive neutrino free-streaming on the evolution of structure formation. We construct synthetic WL catalogues by ray-tracing through light-cones, and use the aperture mass statistic for the analysis. The WL peaks reflect the cumulative signal from massive bound objects and general large-scale structure. We present the first study of WL peaks in simulations that include both baryonic physics and massive neutrinos, so that the uncertainty due to physics beyond the gravity of dark matter can be factored into constraints on cosmological models. Assuming a fiducial model of baryonic physics, we also investigate the correlation between peaks and massive haloes, over a range of neutrino masses. As higher neutrino mass tends to suppress the formation of massive structures in the Universe, the halo mass function and lensing peak counts are therefore modified as a function of neutrino mass. Over most of the S/N . 5, the impact of fiducial baryonic physics is greater (less) than neutrinos for 0.06 and 0.12 (0.24 and 0.48) eV models. Both baryonic physics and massive neutrinos should be accounted for when deriving cosmological parameters from weak lensing observations.