Dark matter in galaxy clusters in the context of LCDM and MOND

The nature of dark matter remains one of the most significant unsolved mysteries in the field of physics. The observations that lead to the dark matter problem are reviewed in Chapter 1 of this thesis. There are many proposed solutions for the various unexplained phenomena. Two major frameworks that continue to be tested today are the lambda cold dark matter cosmological model (LCDM) and Milgromian dynamiccs or MOdified Newtonian Dynamics (MOND). This thesis tests both frameworks using galaxy clusters. Galaxy clusters were chosen as the object of this investigation because they are the largest virialized systems in the Universe, so they are at the intersection of cosmology and astrophysics. In the context of Newtonian dynamics, dark matter halo models with different density profiles were fit to the cluster data. These models were the pseudo-isothermal profile, Burket profile, Lucky-13 profile, NFW profile and the Einasto profile. The simple case of scaling up the baryonic component was also investigated. These models were quantitatively compared and the best fit profile was found to be the pseudo-isothermal profile. This cored profile was favored over the LCDM NFW and Einasto profiles, adding further evidence to the 'cusp-core problem' that has been observed for galaxies. The NFW and Einasto profiles were investigated further using LCDM scaling relations, to varying degrees of success. The observations from the clusters were then fit using MOND. Firstly it was found that additional mass is still needed for MOND in clusters, which is a well known issue. This missing mass was attempted to be modelled in the same way as before using density profiles. The profiles used here were cored density profiles with a converging mass, a density profile proportional to 1/r, and a truncated sphere with constant density. The best fit profile here was the cored profile. In order to determine the nature of this missing mass, a number of correlation tests were applied to the data. An estimation of the possible hydrostatic bias was performed in the context of MOND. The Newtonian models and MOND models were compared using the data unaffected by hydrostatic bias. It was found that the MOND cored density profile was the overall best fit model. This, among other successes for MOND throughout this thesis, as well as some issues arising for LCDM, gives further credence to the theory of MOND. The missing mass needed in MOND in galaxy clusters, however, does not have a clear explanation, but some hints were found that it may be possibly related to the properties related to the properties of the ICM, most notably its temperature.