Masters Degrees (Applied Mathematics)

Permanent URI for this collectionhttps://hdl.handle.net/10413/7108

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The evolution of galaxies in Sunyaev-Zel’dovich selected galaxy clusters from ACT DR5.
(2023) Ragavan, Damien Cole.; Hilton, Matthew James.
Abstract available in PDF.
Giant radio halos and relics in ACTPol clusters.
(2017) Sikhosana, Sinenhlanhla Precious.; Moodley, Kavilan.; Knowles, Kenda Leigh.
Galaxy clusters are the largest gravitationally-bound structures in the universe. They act as the largest astrophysical laboratories in the universe and are extremely interesting objects to study as they are at crossroads between astrophysics and cosmology. In previous decades the most prominent cluster studies were focused on thermal processes in the intracluster medium (ICM). However, recent studies have shown that non-thermal studies give a different perspective on ICM processes. Giant radio halos and radio relics are examples of this non-thermal diffuse radio emission. Giant radio halos are believed to originate from synchrotron radiation resulting from the re-acceleration of relativistic electrons in the cluster's magnetic field by the turbulent energy following merger activity. Radio relics, another form of non-thermal diffuse radio emission, have been identi ed as possible tracers of merger shock waves. The study of diffuse radio emission has a number of open questions such as; the observed bimodality in the radio power versus X-ray luminosity plot. The bimodality could partly be due to the identi cation of halos and relics in clusters without a well-de ned selection function. In this thesis, we studied giant radio halos and relics in a homogeneous, mass-selected sample of sixteen clusters selected via the Sunyaev- Zel'dovich (SZ) effect by the Atacama Cosmology Telescope (ACT) with polarization sensitive receivers (ACTPol). We carried out a radio wavelength study using data obtained from the Giant Metrewave Radio Telescope (GMRT) for four of these clusters. This subsample of four clusters will be added to the larger sample, eight of which have archival data, and four of which will be proposed for observations in the next GMRT observation cycle. We used the GMRT data at 610 MHz to search for diffuse radio emission in each cluster. We applied various uv-cuts and tapers to isolate the low-resolution emission in the target fi eld. For two of the four observed clusters, we tentatively discovered extended radio emission at a signifi cance level of at least 3o' We then measured radio fluxes for compact sources in the cluster region. We were able to calculate spectral indices for the compact sources that were cross-matched in FIRST.
HI intensity mapping and cross-correlation science with HIRAX.
(2020) Naidoo, Warren.; Moodley, Kavilan.
Abstract available in PDF.
On the physical viability of horizon-free collapse.
(2014) Ntshangase, Mlungisi Alex Doctor.; Govender, Megandren.; Govinder, Keshlan Sathasiva.
The so-called Cosmic Censorship Conjecture has drawn widespread attention amongst astrophysicists and particle physicists. In particular, the end-state of gravitational collapse of a bounded matter distribution is a source of much debate with the discovery of naked singularities resulting from the continued gravitational collapse of reasonable matter distributions. One of the first attempts at investigating the final outcome of gravitational collapse of a stellar object was undertaken by Oppenheimer and Snyder in 1939. Their model was highly idealised and focussed on a dust sphere contracting under its own gravity. With the discovery of the Vaidya solution, it became possible to model stars emitting energy to the exterior spacetime. In this dissipative model, the exterior spacetime is nonempty and the collapsing stellar body is enveloped by a zone of null radiation. The smooth matching of the interior spacetime to the Vaidya exterior was achieved by Santos in 1985. It was then possible to model radiating stars undergoing gravitational collapse. The energy momentum tensor for the interior stellar fluid was modelled on more realistic physics and was extended to include heat flux, neutrino transport, shear, pressure anisotropy, bulk viscosity and the electromagnetic field. It has been shown that the collapse of reasonable matter distributions always lead to the formation of a black hole in the absence of shear or in the case of homogeneous densities. In this study we investigate a radiating stellar model proposed by Banerjee et al (BCD model) in which the horizon is never encountered. The interior matter distribution is that of an imperfect fluid with heat flux and the exterior spacetime is described by the radiating Vaidya metric. Our approach is more general than the one proposed by Banerjee et al as they fix the gravitational potentials for the interior line element by making ad-hoc assumptions. A consequence of their model is that it undergoes horizon–free collapse. We start off with the fact that the horizon never forms throughout the collapse process. This restricts the gravitational behaviour of the model. We utilise the boundary condition to determine the temporal evolution of the model. As a result, we obtain new collapsing models in which the horizon never forms. In order to investigate the physical viability of our generalised BCD model we analyse the luminosity profile and the temperature profiles within the framework of extended irreversible thermodynamics. We highlight interesting physical features of our results.

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Browsing Masters Degrees (Applied Mathematics) by Subject "Astrophysics."