Galaxies, galaxy groups and galaxy clusters are embedded in large dark matter halos. Most galaxies in the local universe are found in the galaxy group environment. Locating the centres of galaxy groups is crucial in order to study their properties such as their halo masses. It is often assumed that the most massive galaxy (or brightest galaxy) resides at the centre of the gravitational potential. With the aim of evaluating the validity of this paradigm in galaxy groups, we used two different methods to probe the centres of galaxy group halos: the weak gravitational lensing and dynamical methods. We use these two methods to determine the best definition of galaxy group centres. Our sample is composed of 49 optically (spectroscopically) selected groups and 36 high quality X-ray-selected groups. In total our sample is composed of 78 distinct groups in the redshift range 0.1 < z < 0.9 from the GEEC sample. Our weak lensing analysis suggests that the weighted centre is a better definition than the most massive galaxy position. We address the question of whether or not the result is significantly different for X-ray and optically selected systems. For optically selected systems, the weighted centre is a significantly better assumption of the group centre than the most massive galaxies position. For the X-ray selected groups, the weighted centre and the most massive galaxy appear to trace the centre equally well, although the best definition is the location of the peak in X-ray emission. We evaluate, for the first time, the impact of dynamically complex groups on weak lensing analysis. Once we removed dynamically complex systems from our sample, the lensing signals for all centre definitions are in better agreement suggesting that groups with large offsets between the centre definitions are unevolved systems. For the dynamical method, velocity dispersion profiles suffer from large uncertainties and, therefore, we are unable to place any constraint on the centre definition from this technique. / Thesis / Master of Science (MSc)

The observable properties of galaxy groups, and especially the thermal and chemical properties of the intragroup medium (IGrM), provide important constraints on the different feedback processes associated with massive galaxy formation and evolution. In this {work}, we present a detailed analysis of the global properties of simulated galaxy groups with X-ray temperatures in the range $0.5 - 2$ keV over the redshift range $0 \leq z \leq 3$. The groups are drawn from a cosmological smoothed particle hydrodynamics simulation that includes a well-constrained prescription for momentum-driven, galactic outflows powered by stars and supernovae but no explicit treatment of AGN feedback. Our aims are (a) to establish a baseline against which we will compare future models; (b) to identify model successes that are genuinely due to stellar/supernovae-powered outflows; and (c) to pinpoint mismatches that not only signal the need for AGN feedback but also constrain the nature of this feedback. We find that even without AGN feedback, our simulation successfully reproduces the observed present-day group properties such as the IGrM mass fraction, the various X-ray luminosity-temperature-entropy scaling relations, as well as both the mass-weighted and the emission-weighted IGrM iron and silicon abundance versus IGrM temperature relationships, for all but the most massive groups. We also show that these trends evolve self-similarly for $z < 1$, in agreement with the observations. In contrast to the usual expectations, we do not see any evidence of the IGrM undergoing catastrophic cooling. And yet, the $z=0$ group stellar mass is a factor of $\sim 2$ too high. Probing further, we find that the latter is due to the build-up of cold gas in the massive galaxies {\it before} they are incorporated inside groups. This not only indicates that another feedback mechanism must activate as soon as the galaxies achieve $M_*\approx$ a few $\times 10^{10}\;\rm{M_{\odot}}$ but that this feedback mechanism must be powerful enough to expel a significant fraction of the halo gas component from the galactic halos. ``Maintenance-mode" AGN feedback of the kind observed in galaxy clusters will not do. At the same time, we find that stellar/supernovae-powered winds are essential for understanding the metal abundances in the IGrM and these results are expected to be relatively insensitive to the addition of AGN feedback. We further examine the detailed distribution of the metals within the groups and their origin. We find that our simulated abundance profiles fit the observational data pretty well except that in the innermost regions, there appears to have an excess of metals in the IGrM, which is attributed to the overproduction of stars in the central galaxies. The fractional contribution of the different types of galaxies varies with radial distances from the group center. While the enrichment in the core regions of the groups is dominated by the central and satellite galaxies, the external galaxies become more important contributors to the metals at $r\simgreat R_{500}$. The IGrM at the groups' outskirts is enriched at comparatively higher redshifts, and by relatively less massive galaxies. / Graduate

In this dissertation, I present my work on the behaviour of different features of radio-loud active galaxies to investigate how energy is transferred from their jets to the environment. To this end, I have studied the knots in the jet in Centaurus A, the radio and X-ray emission from the lobes of the FRII radio galaxies 3C 353 and Pictor A, and the gas properties of a sample of galaxy groups some of which host radio-loud AGN. Using new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years with the Very Large Array and with Chandra, I have measured the X-ray and radio spectral indices, flux density variations, polarisation variations and proper motions of the jet knots. I used these measurements to constrain the likely knot formation mechanisms and particle acceleration processes within this jet and compared my results with the variations detected in the properties of the knots in M87. I found that none of the knots are a result of impulsive particle acceleration and that those knots that are detected in both X-ray and radio are likely due to collisions between the jet and an obstacle, while the radio only knots, the majority of which are moving, are likely due to compressions of the fluid flow. Using six frequencies of new and archival radio data and new XMM-Newton observations of 3C 353, I show that inverse-Compton emission is detected in the lobes of this source. By combining this X-ray emission with the radio synchrotron emission, I have constrained the electron population and the magnetic field energy density in the lobes of this radio galaxy. I have argued that the variations in the X-ray/radio ratio in the brighter lobe requires a changing magnetic field strength. I have extended this work using a statistical analysis of the X-ray and radio emission to show that the observed small-scale variation in surface brightness cannot be reproduced by simple combinations of the electron energy distribution and the magnetic field strength. I therefore suggest that the changes in surface brightness that give rise to the filamentary structures seen in the lobes are probably due to strong spatial variations of the magnetic field strength. Finally, I present a study of galaxy groups observed with XMM-Newton in which I measure temperature and surface brightness profiles to examine whether radio-source heating makes radioloud groups hotter and more luminous than radio-quiet groups. I compare my measurements with previous luminosity-temperature relationships and conclude that there is a significant difference in the gas properties of radio-loud and radio-quiet groups.

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