Galaxies in Clusters: the Observational Characteristics of Bow-Shocks, Wakes and Tails

The dynamical signatures of the interaction between galaxies in clusters and the intracluster medium (ICM) can potentially yield significant information about the structure and dynamical history of clusters. To develop our understanding of this phenomenon we present results from numerical modelling of the galaxy/ICM interaction, as the galaxy moves through the cluster. The simulations have been performed for a broad range, of ICM temperatures (kT = 1,4 and 8 keV), representative of poor clusters or groups through to rich clusters. There are several dynamical features that can be identified in these simulations; for supersonic galaxy motion, a leading bow-shock is present, and also a weak gravitationally focussed wake or tail behind the galaxy (analogous to Bondi-Hoyle accretion). For galaxies with higher mass-replenishment rates and a denser interstellar medium (ISM), the dominant feature is a dense ram-pressure stripped tail. In line with other simulations, we find that the ICM/galaxy ISM interaction can result in complex time- dependent dynamics, with ram-pressure stripping occurring in an episodic manner. In order to facilitate this comparison between the observational consequences of dynamical studies and X-ray observations we have calculated synthetic X-ray flux and hardness maps from these simulations. These calculations predict that the ram-pressure stripped tail will usually be the most visible feature, though in nearby galaxies the bow-shock preceding the galaxy should also be apparent in deeper X-ray observations. We briefly discuss these results and compare with X-ray observations of galaxies where there is evidence of such interactions.Comment: 14 pages, 8 diagrams, MNRAS (in press

Simulations of the Effects of Stripping and Accretion on Galaxy Haloes in Clusters

We present results from a series of hydrodynamic simulations investigating ram pressure stripping of galactic haloes as the host galaxy falls radially into a cluster. We perform a parameter study comprising of variations in initial gas content, gas injection rate (via stellar mass loss processes), galaxy mass and amplitude of infall. From the simulation results we track variations in both physical quantities (e.g. gas mass) and directly observable quantities (e.g. X-ray luminosities). The luminosity of the galaxy's X-ray halo is found to compare favourably with the observationally determined correlation with optical blue band luminosity (L_X:L_B) relation. Factors affecting the X-ray luminosity are explored and it is found that the gas injection rate is a dominant factor in determining the integrated luminosity. Observational properties of the material stripped from the galaxy, which forms an X-ray wake, are investigated and it is found that wakes are most visible around galaxies with a substantial initial gas content, during their first passage though the cluster. We define a statistical skewness measure which may be used to determine the direction of motion of a galaxy using X-ray observations. Structures formed in these simulations are similar to the cold fronts seen in observation of cluster mergers where a sharp increase in surface brightness is accompanied by a transition to a cooler region.Comment: Accepted for publication in MNRAS. 19 pages, 21 figure

The comparative impact of acquisitions on the financial performance of acquiring companies across market segments of the JSE

There is broad agreement in the literature that in general, mergers and acquisitions in both the short and long run are largely zero or negative, net present value exercises for the shareholders of the acquiring company. At the same time it is believed that significant returns are realised by the shareholders of the target companies. Despite this broad agreement and due to the complexity of the open market, there are still a large number of variables which can and could possibly account for many of the exceptions which have been highlighted in a range of studies. The aim of this study was to investigate whether there is any evidence to suggest that merger and acquisition activity in the different market sectors of the JSE are either, more or less successful than the average. A total of 82 transactions were identified as meeting the strict requirements of the methodology. These included representatives from five sectors Basic Industries, Consumer Goods&Services, Financials, Information Technology and Resources. Event study methodology was used to investigate the abnormal returns of acquiring companies before and after the announcement of the event. It was established that in all but one of the market sectors there was statistically significant evidence that merger and acquisition transactions in different market sectors are either better or worse than the average at creating value for the acquiring company shareholders.Dissertation (MBA)--University of Pretoria, 2010.Gordon Institute of Business Science (GIBS)unrestricte

Role of Charged Gauge Fields in Generating Magnetic Seed Fields in Bubble Collisions during the Cosmological Electroweak Phase Transition

We calculate the magnetic field generated during bubble collisions in a first-order electroweak phase transition that may occur for some choices of parameters in the minimal supersymmetric Standard Model. We show that for sufficiently gentle collisions, where the Higgs field is relatively unperturbed in the bubble overlap region, the equations of motion can be linearized so that in the absence of fermions the charged W fields are the source of the electromagnetic current for generating the seed fields. Solutions of the equations of motion for the charged gauge fields and Maxwell's equations for the magnetic field in O(1,2) space-time symmetry are expressed in closed form by applying boundary conditions at the time of collision. Our results indicate that the magnetic fields generated by charged $W^{\pm}$ fields in the collision are comparable to those found in previous work. The magnetic fields so produced could seed galactic and extra-galactic magnetic fields observed today.Comment: 15 Pages, 7 Figure

Covering California's Kids: The Impact of Healthy Kids on Access, Health Status and Costs

Summarizes how the implementation of CHIs designed to improve children's access to primary care helped reduce preventable hospitalizations among lower-income children. Includes policy implications

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