Galaxy clusters are the most recent of cosmological structures to have formed by the present time in the currently favoured hierarchical scenario of structure formation and are widely regarded as powerful probes of cosmology and galaxy formation physics alike. Over the past few years, it became increasingly clear that precision cluster cosmology requires the development of detailed, realistic theoretical models of galaxy clusters and the confrontation of synthetic surveys generated using these

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... odels with observations. This motivates a campaign of large cosmological hydrodynamicical simulations, with plausible 'sub-grid' prescriptions for the relevant galaxy formation physics. This thesis presents a new suite of large-volume cosmological hydrodynamical simulations called cosmo-OWLS. They form an extension to the OverWhelmingly Large Simulations (OWLS) project, and have been designed to help improve our understanding of cluster astrophysics and non-linear structure formation, which are now the limiting systematic errors when using clusters as cosmological probes. Starting from identical initial conditions in either the Planck or WMAP7 cosmologies, the most important 'sub-grid' physics, including feedback from supernovae and active galactic nuclei (AGN), has been systematically varied. Via the production of synthetic surveys of the simulations and comparisons with observations, the realism of these state-ofthe-art models was explored. At the same time, the simulations were shown to provide a valuable tool for interpreting the observational data, as well as powerful means for testing commonly-employed methods for estimating, for example, cluster masses and determining survey selection functions, which are crucial for cluster cosmology. The properties of the simulated galaxy groups and clusters were first compared to v a wide range of observational data, such as X-ray luminosity and temperature, gas mass fractions, entropy and density profiles, Sunyaev-Zel'dovich flux, I-band massto-light ratio, dominance of the brightest cluster galaxy, and central massive black hole (BH) masses, by producing synthetic observations and mimicking observational analysis techniques. These comparisons demonstrated that some AGN feedback models can produce a realistic population of galaxy groups and clusters, broadly reproducing both the median trend and, for the first time, the scatter in physical properties over approximately two decades in mass (10 13 M M 500 10 15 M ) and 1.5 decades in radius (0.05 r/r 500 1.5). However, in other models, the AGN feedback is too violent (even though they reproduce the observed BH scaling relations), implying calibration of the models is required. The production of realistic populations of simulated groups and clusters, as well as models that bracket the observations, opens the door to the creation of synthetic surveys for assisting the astrophysical and cosmological interpretation of cluster surveys, as well as quantifying the impact of selection effects. A study of the scatter and evolution of the hot gas properties of the populations of galaxy groups and clusters, such as X-ray luminosity and temperature, gas mass and Sunyaev-Zel'dovich flux, as a function of the important non-gravitational physics of galaxy formation was then conducted. The median relations and the scatter about them are reasonably well-modelled by evolving broken power-laws. The non-radiative model and the model that neglects AGN feedback are consistent with having selfsimilar mass slopes, whereas the mass slopes of the AGN feedback models deviate significantly from the self-similar expectation. Self-similar evolution, which is widely adopted in current cosmological studies, was also found to break down when efficient feedback is included. The log-normal scatter varies mildly with mass, is relatively insensitive to non-gravitational physics, but shows a moderately strong decreasing trend with increasing redshift. The X-ray luminosity has a significantly larger scatter than all the other hot gas proxies examined. It is thus the poorest one, while the 'best' one is the mean X-ray temperature. Synthetic Sunyaev-Zel'dovich observations, generated using a 'multi-purpose' light cone software package developed during the thesis, were used to check the veracity of vi Publications In the course of completing the work presented in this thesis, the following papers have been submitted for publication in a refereed journal: 1. Amandine M. C. Le Brun, Ian G. McCarthy, Joop Schaye and Trevor J. Ponman, Towards a realistic population of simulated galaxy groups and clusters, 2014, MNRAS, 441, 1270. 2. Ian G. McCarthy, Amandine M. C. Le Brun, Joop Schaye and Gilbert P. Holder, The thermal Sunyaev-Zel'dovich effect power spectrum in light of Planck, 2014, MNRAS, 440, 3645. 3. Marco Velliscig, Marcel P. van Daalen, Joop Schaye, Ian G. McCarthy, Marcello Cacciato, Amandine M. C. Le Brun and Claudio Dalla Vecchia, The impact of galaxy formation on the total mass, profiles and abundance of haloes, 2014, MNRAS, 442, 2641. 4. Amandine M. C. Le Brun et al., How well can we recover the Sunyaev-Zel'dovich flux of low-mass haloes? The role of source confusion and deviations from the universal pressure profile, to be submitted to MNRAS. 5. Amandine M. C. Le Brun et al., Scatter and evolution of the hot gas properties of a realistic population of simulated galaxy clusters, to be submitted to MNRAS. AMANDINE MARIE CAMILLE LE BRUN AUGUST 26, 2014 viii Acknowledgements First and foremost, my warmest thanks go to my supervisor, Ian McCarthy, for rescuing my desperate situation at the end of my Master by providing me with an extremely interesting fully-funded PhD, for always taking the time to answer my often really naive questions, helping me finding my way out of blind alleys, trying to prevent me from overcommitting myself and for being so understanding during testing times. I also would like to thank my second supervisor, Chris Collins, for always taking the time to answer my questions and to discuss my ongoing work, as well as (in alphabetical order) for all their help, for listening to my frequent complaints and for moral support when things were not ix going so well (which happened really often), as well as all the postgraduate students from both the University of Birmingham and the ARI for providing (most of the time) welcome distractions.