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Optical galaxy cluster detection and weak gravitational lensing derived galaxy cluster mass functions : the development of a new algorithm and its application to the Millennium Simulation, CFHTLS-Deep, and CFHTLS-Wide surveys Milkeraitis, Martha


Hierarchical structure formation and the λCDM model of the Universe reveal that large structures, such as galaxy clusters, form from the aggregation and merging of smaller structures over time. Exploring the morphologies and distribution of galaxy clusters enables the identification of processes that affect this growth, including the significant role of dark matter in structure formation. The particle constituting dark matter remains unknown and current experimental searches for it abound. Weak gravitational lensing (WL) is an observational astronomy technique that can successfully measure dark matter and thus is an important tool in its characterisation, and furthermore, in distinguishing between competing gravity theories. Using WL to measure galaxy cluster mass, including both baryonic matter and dark matter, across a range of redshifts, exposes details of cluster assembly processes. It is imperative to obtain a complete, pure and understood galaxy cluster sample for statistical, accurate investigations of the properties and evolution of all types of galaxy clusters, from which to draw unbiased conclusions about the evolution of the Universe. Cognisant of these factors, this thesis presents a new algorithm, 3D-MF, a three-dimensional matched-filter method, that enables searches for galaxy clusters in large, optical wavelength astronomical surveys. 3D-MF is used herein to successfully discover thousands of galaxy clusters in the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). Subsequently, theoretical mass profiles are used to characterise cluster candidate WL signals, providing mass or velocity dispersions. The CFHTLS-Deep and Wide galaxy cluster catalogues in this thesis represent a new and abundant source of galaxy cluster information, containing ~170 and ~92 galaxy clusters per square degree respectively (with σ > 3.5, and 0.2 ≤ z ≤ 1.0). WL shear catalogues were used to analyse > 15,000 CFHTLS-Wide 3D-MF galaxy clusters, and the average cluster stacks (for clusters 0.2 ≤ z ≤ 0.5) were found to cover a mass range of 10¹³M๏≲ M₂₀₀ ≲ 10¹⁴M๏, having 1D velocity dispersions of σν 200-500 km/s. 3D-MF finds higher redshift clusters down to a lower mass range than other methods; its successes have already led to its implementation in other surveys, as we progress toward a deeper understanding of the Universe.

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