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A wideband spectropolarimetry study of the spatially resolved radio galaxies: Cygnus A & Hydra A

- Sebokolodi, Makhuduga Lerato Lydia

Authors: Sebokolodi, Makhuduga Lerato Lydia
Date: 2022-04-04
Subjects: Radio astronomy , Radio galaxies , Faraday effect , Astrophysical spectropolarimetry , Intracluster medium , Cosmic magnetic fields
Language: English
Type: Academic theses , Doctoral theses , text
Identifier: http://hdl.handle.net/10962/454415 , vital:75342 , DOI 10.21504/10962/454415
Description: This study presented results from our deep, wideband, high-spectral and high-spatial-resolution polarisation observations of the two most powerful radio galaxies, namely Cygnus A and Hydra A, with the Jansky Very Large Array (JVLA). The high surface brightness and strong polarisation of these radio sources enabled detailed polarimetric imaging, providing images at 0.75′′ resolution across 2−18 GHz and 2000 independent lines-of-sight across Cygnus A, and images at 1.5′′ (2 − 12 GHz) and 600 lines-of-sight across Hydra A. Our data revealed significant depolarisation and depolarisation structure, as well as deviations from a _2-law. We also found complicated structures in the Faraday spectra ranging from single-peaked to blended/resolved double- and multiple-peaked. The Faraday spectra of Hydra A were more multiple-peaked than Cygnus A. The fractional polarisation increased monotonically with increasing resolution, as expected. However, there were numerous lines-of-sight with complicated behaviour. We also found that the structure and complexity in the depolarisation increased at lower resolutions, suggesting substantial spatial structures across the lobes/tails. We fitted the 0.3′′ (6−18 GHz) and 0.50′′ (6−12 GHz) images of Cygnus A and Hydra A, respectively, with a simple model incorporating random, unresolved fluctuations in the cluster magnetic field to determine the high-resolution, high-frequency properties of the sources and the cluster. We found rotation measures (RM) between −5000 rad m−2 and +6400 rad m−2 across Cygnus A, and −2000 rad m−2 and +11900 rad m−2 across Hydra A, consistent with previous studies. From these derived properties, we generated predicted polarisation images of the sources at lower frequencies (< 6 GHz), convolved to 0.75′′ for Cygnus A and 1.5′′ for Hydra A. The predictions were remarkably consistent with the observed emission in both sources, providing strong support for the depolarisation being a result of unresolved fluctuations in the magnetic fields. We fitted various analytical models to the wideband data. We found that the data for both sources were inconsistent with a wholly mixed gas of thermal and synchrotron gas, particularly for regions withRM > 1000 rad m−2. Instead, the data required a dominant Faraday rotating screen in the foreground of the radio sources. The wideband modelling also showed preference towards models with at least two or more unresolved Faraday rotating patches. Single depolarising models fail to describe the data. This implies the presence of more than one depolarising screen in the vicinity of these sources. The observations were consistent with the lower-frequency depolarisation due to unresolved fluctuations on scales ≳ 300−700 pc in the magnetic field or the electron density superposed on a partially ordered field component. Both the large-scale magnetic fields and unresolved magnetic field fluctuations are external to the radio emission. The magnetic fields around Cygnus A are located in the ambient cluster gas, the shocked gas in the boundary of the lobes or both, while the magnetic fields around Hydra A are most likely located in the ambient cluster gas. , Thesis (PhD) -- Faculty of Science, Physics, 2022
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Date Issued: 2022-04-04

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The State of Social Media Policy Awareness and Compliance at the Nelson Mandela University

- Ngcama, Lwando

Authors: Ngcama, Lwando
Date: 2020-12
Subjects: Social Media , Awareness , Compliance -- Nelson Mandela University
Language: English
Type: Master's theses , Thesis
Identifier: http://hdl.handle.net/10948/49550 , vital:41735
Description: The use of social media in higher education has both benefits and pitfalls. Higher education institutions are concerned about their staff and students’ use of social media and have thus resorted to implementing social media policies and guidelines. The Nelson Mandela University has implemented a social media policy and guidelines to inform and enforce the acceptable use of social media by its staff and students. In order to know whether staff and students of the Nelson Mandela University are aware of and compliant with its policy and guidelines, it is necessary to measure their level of awareness and compliance. Within this context, the primary objective of this study is to describe the state of awareness and compliance of staff and students at the Nelson Mandela University towards its social media policy and social media guidelines. In order to address the primary objective, the uses, benefits and risks of social media in higher education were identified through a literature review. The literature review revealed that social media is used in higher education to support and manage teaching and learning, marketing and crisis management, and for other purposes which may be specific to higher education stakeholders (both staff and students). The risks of social media use in higher education were revealed to be associated with institutional reputation, online content, privacy, intellectual property and the types of media shared by the stakeholders. The role of social media policies and guidelines in higher education was then described, based on a preliminary literature study which reviewed social media policies and guidelines implemented in higher education. Furthermore, the aspects of awareness and compliance towards social media policies and ii guidelines in higher education were also discussed. A general guide for the creation of social media policies and guidelines was identified and used to compare the nature and content of South African public universities’ social media policies and guidelines. Through comparative content analysis, the content of the universities’ social media policy and guidelines was compared, similarities and variances between were drawn, and the role of social media policies and guidelines was further explored. Finally, the knowledge of staff and students at the Nelson Mandela University in respect to the university’s social media policy and guidelines was measured and described through the use of a survey questionnaire and statistical analysis of the data collected. The results of the analysis indicated an overall medium level of awareness for both staff and students, with a mean average score of 3.2; while both groups demonstrated an overall high level of compliance towards the social media policy and guidelines, with a mean average score of 4.3. By addressing the primary objective, the contribution made by this study thus informs the Nelson Mandela University of the extent to which its social media policy and guidelines align with those of other South African public universities, and it indicates where the Nelson Mandela University should look to improve its staff and students’ awareness of and compliance with its social media policy and guidelines. , Thesis (MIT) -- Faculty of Engineering, the Built Environment and Technology, 2020
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Date Issued: 2020-12

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