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Connelly, Jennifer L. (2012): Optically and x-ray selected galaxy groups at intermediate redshift. Dissertation, LMU München: Fakultät für Physik



In this thesis I present the properties of two samples of galaxy groups containing 39 high quality X-ray selected systems and 38 optically (spectroscopically) selected systems in coincident spatial regions at 0.12<z<0.79. Only nine optical systems are associable with X-ray systems. I discuss the confusion inherent in the matching of both galaxies to extended X-ray emission and of X-ray emission to already identified optical systems. Extensive spectroscopy has been obtained and the resultant redshift catalog and group membership are provided here. X-ray, dynamical, and total stellar masses (integrated down to a limit of 10^10 M⊙) of the groups are also derived and presented. The total mass range of the combined sample is ∼10^12 − 5 × 10 ^14 M⊙. I explore the effects of utilizing different centers and applying three different kinds of radial cut to the systems: a constant cut of 1 Mpc and two r200 cuts (roughly approximating a virial radius), one based on the velocity dispersion of the system and the other on the X-ray emission. I find that an X-ray based r200 tends to result in less scatter in scaling relations and less dynamical complexity of member galaxies as evidenced by results of the Anderson-Darling and Dressler-Shectman tests, indicating that this radius tends to isolate the virialized part of the system. The constant and velocity dispersion based cuts can overestimate membership and can work to inflate velocity dispersion and dynamical and stellar mass. I fit Lx-σ and Mstellar-Lx scaling relations for X-ray and optically selected systems and find they are not dissimilar. The mean fraction of mass found in stars for my systems is ∼0.014 with a logarithmic standard deviation of 0.398 dex. I also define and investigate a sample of groups which are X-ray underluminous given the total group stellar mass. For these systems the fraction of stellar mass contributed by the most massive galaxy is typically lower than that found for the total population of groups. 80% of 15 underluminous groups have less than 40% of their stellar mass in the most massive galaxy which only happens in less than 1% of cases with samples matched in stellar mass, taken from the combined group catalog. These results imply that there may be less Intra-Group Medium contributed from the most massive member in these systems, supporting the model of hierarchical collapse wherein a group begins with a massive galaxy and accretes mass smoothly, resulting in a group with a higher gas fraction and a more massive central galaxy than would result from the merger of roughly equivalent mass ‘subgroups’ (clumps) comprised of similar mass / luminosity galaxies.