Browsing by Author "Thwala, Siphiwe Anthony"

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    An unsupervised search of non-thermal diffuse emission in extended sources
    (University of the Witwatersrand, Johannesburg, 2022-01) Thwala, Siphiwe Anthony; Beck, Geoff
    Low-surface brightness diffuse radio emission is a useful probe for studying the connection between non-thermal processes in radio sources and their environment, as well as gaining insights on galaxy formation. The presence of this emission in different astrophysical and cosmological signals is not well understood in the literature due to challenges in detection caused by the low-surface brightness of diffuse objects and their origins, particularly in relation to galaxy feedback and formation. We explored the utility of publicly available data to detect, characterise, and study diffuse emission in extended radio sources at the scales of radio galaxies and galaxy clusters. We performed multiple analyses on extended radio continuum sources found in large publicly available radio surveys. The analyses range from in-depth multi-frequency examinations of individual sources to novel approaches for grouping and classifying radio sources with unsupervised machine learning. The analysis of individual sources brought together a range of datasets and techniques to provide insight about their nature. To group and classify continuum radio sources at scale, a novel unsupervised machine learning approach was designed to combine self-organising maps with convolutional neural networks for automatically detecting and clustering similar sources in radio surveys. To the best of our knowledge, this is the first implementation of an architecture that allows for the training of a machine learning model using multi-frequency and multi-scale radio continuum data cubes, as input, to automatically detect and cluster similar sources in radio surveys. This comes at a time when radio astronomy is undergoing a transformation and data mining methods are critical for optimum scientific utilisation of data from telescopes like MeerKAT, JVLA, MWA, and LOFAR (as well as the upcoming SKA). The different works showcase the most interesting radio sources with diffuse emission observed in these investigations.
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    Search and analysis techniques for extended sources in deep radio surveys
    (2018) Thwala, Siphiwe Anthony
    We produced a search for extended radio sources in the LOFAR Two Meter Survey HETDEX Spring Field, and we were able to identify 17 low-brightness sources in a region of 9:5 8:7 , an area of the sky between RA: 10h37m00s to 11h48m00s and DEC: 44 1900000 to 52 5500000. We focus in this study on the poorly studied radio galaxy PGC2285791 presenting a detailed analysis of its core, of the two extended radio lobes, a spectral index map between 141 and 1400 MHz and its multi-frequency association with an IR and optical counterparts. Our results led to the identi cation of this source with a z = 0:14 extended ( 6 arcmin in the S-N direction) radio galaxy with bubble- like radio lobe structures and a morphological similarity in the extended radio structure of PGC2285791 with the remnant radio galaxy B20924+30, except PGC2285791 has a prominent radio core. Future X-ray and millimeter observations will greatly help in understanding the energetics of the radio lobes by measuring the associated inverse Compton scattering on CMB photon emission.