Galaxy groups from observations and simulations

The cold dark matter model has become the leading theoretical paradigm for the formation of structure in the Universe. Together with the theory of cosmic inflation, this model makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. As a result, small structures collapse first and eventually build large structures such as groups and cluster of galaxies. While clusters are among the most massive bound structures in the Universe, groups are more numerous and most of the galaxies reside within galaxy groups. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical models. The current project consists of two parts. In the first part, we investigate the existence and evolution of early-formed fossil galaxy groups, and the development of the luminosity gap between its brightest galaxies. We study the correlation of these properties with the group mass assembly history, by comparing observations to the Millennium simulation of dark matter particles, and the associated semi-analytic catalogues of galaxies, together with the Millennium gas simulation. Fossil Galaxy Groups are believed to be the end result of galaxies merging within a normal galaxy group, leaving behind the X-ray halo characteristic of a group. The sample of fossils in our study are selected according to the useful definition of fossil groups. The luminosity gap statistics in the Millennium Run are compared to the theoretical models. The study of the mass evolution of fossils shows that in comparison to normal groups, fossils are more evolved systems and have assembled their masses at higher redshifts, while normal groups are still evolving. Our work suggests the earlier formation and higher mass concentration of fossil systems. The estimated space densities from the Millennium Run are smaller (sometimes in agreement within the range of errors in the observations) than those obtained from the observations. Furthermore, we study the development of magnitude gap from a general point of view and its correlation with the mass assembly of groups and clusters of galaxies using the same dark matter simulations. The results show that the current definition of fossils, based on the magnitude gap Δm₁₂≤2, does not satisfy the necessity for a group or cluster to be an early formed system. Moreover, the fossil phase (the duration in which the magnitude gap of a galaxy group remains always above a threshold value, i.e. Δm₁₂≤2 is a temporary phase in the life of groups, and most groups would experience such a phase in their lifetime. We revise the current optical definition of fossil groups, by studying the evolution and history of various physical parameters associated with the mass assembly of galaxy groups and clusters. In the second part of this dissertation, we study the optical properties of a sample of 25 optically selected groups from the XMM-IMACS (XI) project. The project aims to improve our knowledge of how the dynamics and properties of group galaxies describe the global characteristics of groups, by using a combination of radio, X-ray, infrared, and optical observations together with the imaging and spectroscopy of the group galaxy population. The observations were performed during three observing run at the Las Campanas observatory. Image processing and precise astrometry was done for spectroscopic follow-up observations. Group virial radii were found by combining the spectroscopic results together with those from the Millennium simulation. Finally we determined the group luminosity functions using the overdensity radii, with the extracted colour-magnitude relation from the spectroscopic observations, and find that the luminosity function of optically selected groups are very similar to that of X-ray selected groups.EThOS - Electronic Theses Online ServiceGBUnited Kingdo