Meerkat polarimetric observations of Pictor A
- Authors: Andati, Lexy Acherwa Livoyi
- Date: 2024-10-11
- Subjects: Polarimetry , MeerKAT , Radio astronomy , Radio galaxies , Cosmic magnetic fields , Pictor A
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466896 , vital:76796 , DOI https://doi.org/10.21504/10962/466896
- Description: Pictor A is one of the brightest and closest radio galaxies in the Southern Hemisphere, offering a unique opportunity for in-depth studies of the astrophysics of radio galaxies and their interactions with their environments. Many multi-wavelength studies of this source have been done. However, the most comprehensive radio frequency study of Pictor A’s morphological components was conducted by Perley et al. (1997) using the Very Large Array (VLA) located in the Northern Hemisphere. To date, that work remains the most detailed study of Pictor A. In this thesis, we conducted a spectropolarimetric study of Pictor A using new L-band data obtained in 2019 from the high-sensitivity MeerKAT telescope, which provides the deepest and most sensitive data of this source at a continuous and finely sampled frequency coverage in the L-band. Thus, due to Pictor A’s proximity, high luminosity, and the MeerKAT’s high sensitivity, the data delivers a unique dataset for our study of the magnetic field structure of Pictor A and allows for a detailed study of the source’s morphological structures. We presented the steps taken during our calibration and data reduction, leading to polarimetryready images. During the first phase of calibration, excision of data corrupted by instrumental effects and radio frequency interference (RFI) resulted in only 50% useable data. Pictor A’s exceptionally bright western hotspot introduced significant artefacts in our images, mitigated in the second calibration phase through direction-dependent calibration. The calibrated data resulted in a multi-frequency synthesis (MFS) Stokes I image of Pictor A at 7.5′′ in resolution with an offsource RMS noise of ∼22 𝜇Jy/beam. The off-source noise in the Stokes Q and U sub-band images ranged between 95 – 278 𝜇Jy/beam and 41 – 233 𝜇Jy/beam, respectively. Additionally, we briefly highlighted the effects of RFI in the L-band on polarimetry, particularly the considerable loss of 𝜆2 coverage of ∼50%. All the calibration recipes used for this work were made available in this thesis. Using Pictor A’s data as a testbed, we introduced a Python-based tool, Smops, developed during the calibration stages of our work. Smops was designed for an intermediate post-processing step. It interpolates input sub-band model FITS images (such as those produced by WSClean) into finely channelized sub-band model FITS images, thereby generating model images at a higher frequency resolution. Smops reduces the need to generate model images with numerous sub-bands, which is computationally intensive and time-consuming. A higher resolution in frequency of the models facilitates more efficient model subtraction during self-calibration. We then presented the total intensity features of Pictor A, which the calibrated data reveals. We confirmed the presence of Pictor A’s radio jet extending from its core to the western hotspot. Notably, this feature, faint and barely visible in previous radio images, is now distinctly observed. The counterjet remains undetectable. Furthermore, we demonstrated the coexistence of radio emission, which is expected to align with previously observed X-ray diffuse emission. This observation confirmed the inverse Compton origin of Pictor A’s lobe emission. Employing the RM-synthesis technique for the spectropolarimetric study of Pictor A, we identified a relatively consistent rotation measure (RM) across its lobes, with an average RM of 48.06 ± 10.19 rad m−2 for the entire source. However, the eastern lobe displayed a wider RM dispersion than the western lobe. Moreover, our study affirmed the depolarisation asymmetry previously observed between the western and eastern lobes of Pictor A, where the eastern lobe exhibited significantly more depolarisation than its western counterpart. Most lines-of-sight across Pictor A displayed single-peaked Faraday spectra, indicating a single Faraday rotating screen. However, we also noted that several lines-of-sight (∼23%) showed more than one Faraday peak. An investigation into the ii possible causes of the multiple observed peaks using QU-fitting suggested that there is a possibility of a Faraday thick structure or multiple Faraday components along these paths. Furthermore, we estimated a Galactic RM contribution towards Pictor A of 23.57 ± 10.87 rad m−2. We concluded that while our Galaxy may contribute to the mean RM for this source, it cannot explain smallscale fluctuations, which suggests that some fraction of the observed rotation measures could result from some inter-galactic medium, X-ray gas near the shock boundary region (the sheath), or other unknown intervening material. We introduced Scrappy, a Python-based tool tailored for processing lines-of-sight data. Scrappy yields RM-synthesis diagnostic data products such as the data associated with each line-of-sight, and their corresponding plots in 𝜙-space (e.g. cleaned and dirty Faraday spectra and RMTF), and 𝜆2-space (e.g. the fractional polarisation, and Stokes Q and U ). Scrappy further avails a Bash-based pipeline, showrunner.sh, that processes input sub-band Stokes images, automatically selects usable sub-bands, stacks images into Stokes cubes, generates lines-of-sight, processes their corresponding data, and produces diagnostic plots. Additionally, it creates per-pixel maps of fractional polarisation, RM, polarisation angle, peak FDF, and linear polarised intensity. The pipeline ensures reproducibility. To visualise the diagnostic plots from Scrappy, we developed PolarVis, a simple web-based tool that enables the visualisation of diagnostic plots associated with each available line-of-sight, thus facilitating the quick exploration of interesting lines-of-sight in regions across this source. This tool facilitates the visualisation of polarisation behaviour for specific lines-of-sight, enabling quick identification of interesting regions of the source. Furthermore, its interactivity promotes the exploration of line-of-sight data. Availing data to the public with this tool permits validation or comparison of results from varying techniques, hence fostering a sense of transparency. As a result, the 2389 lines-of-sight of Pictor A are presented using PolarVis and are available at https://pica.ratt.center. , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Andati, Lexy Acherwa Livoyi
- Date: 2024-10-11
- Subjects: Polarimetry , MeerKAT , Radio astronomy , Radio galaxies , Cosmic magnetic fields , Pictor A
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466896 , vital:76796 , DOI https://doi.org/10.21504/10962/466896
- Description: Pictor A is one of the brightest and closest radio galaxies in the Southern Hemisphere, offering a unique opportunity for in-depth studies of the astrophysics of radio galaxies and their interactions with their environments. Many multi-wavelength studies of this source have been done. However, the most comprehensive radio frequency study of Pictor A’s morphological components was conducted by Perley et al. (1997) using the Very Large Array (VLA) located in the Northern Hemisphere. To date, that work remains the most detailed study of Pictor A. In this thesis, we conducted a spectropolarimetric study of Pictor A using new L-band data obtained in 2019 from the high-sensitivity MeerKAT telescope, which provides the deepest and most sensitive data of this source at a continuous and finely sampled frequency coverage in the L-band. Thus, due to Pictor A’s proximity, high luminosity, and the MeerKAT’s high sensitivity, the data delivers a unique dataset for our study of the magnetic field structure of Pictor A and allows for a detailed study of the source’s morphological structures. We presented the steps taken during our calibration and data reduction, leading to polarimetryready images. During the first phase of calibration, excision of data corrupted by instrumental effects and radio frequency interference (RFI) resulted in only 50% useable data. Pictor A’s exceptionally bright western hotspot introduced significant artefacts in our images, mitigated in the second calibration phase through direction-dependent calibration. The calibrated data resulted in a multi-frequency synthesis (MFS) Stokes I image of Pictor A at 7.5′′ in resolution with an offsource RMS noise of ∼22 𝜇Jy/beam. The off-source noise in the Stokes Q and U sub-band images ranged between 95 – 278 𝜇Jy/beam and 41 – 233 𝜇Jy/beam, respectively. Additionally, we briefly highlighted the effects of RFI in the L-band on polarimetry, particularly the considerable loss of 𝜆2 coverage of ∼50%. All the calibration recipes used for this work were made available in this thesis. Using Pictor A’s data as a testbed, we introduced a Python-based tool, Smops, developed during the calibration stages of our work. Smops was designed for an intermediate post-processing step. It interpolates input sub-band model FITS images (such as those produced by WSClean) into finely channelized sub-band model FITS images, thereby generating model images at a higher frequency resolution. Smops reduces the need to generate model images with numerous sub-bands, which is computationally intensive and time-consuming. A higher resolution in frequency of the models facilitates more efficient model subtraction during self-calibration. We then presented the total intensity features of Pictor A, which the calibrated data reveals. We confirmed the presence of Pictor A’s radio jet extending from its core to the western hotspot. Notably, this feature, faint and barely visible in previous radio images, is now distinctly observed. The counterjet remains undetectable. Furthermore, we demonstrated the coexistence of radio emission, which is expected to align with previously observed X-ray diffuse emission. This observation confirmed the inverse Compton origin of Pictor A’s lobe emission. Employing the RM-synthesis technique for the spectropolarimetric study of Pictor A, we identified a relatively consistent rotation measure (RM) across its lobes, with an average RM of 48.06 ± 10.19 rad m−2 for the entire source. However, the eastern lobe displayed a wider RM dispersion than the western lobe. Moreover, our study affirmed the depolarisation asymmetry previously observed between the western and eastern lobes of Pictor A, where the eastern lobe exhibited significantly more depolarisation than its western counterpart. Most lines-of-sight across Pictor A displayed single-peaked Faraday spectra, indicating a single Faraday rotating screen. However, we also noted that several lines-of-sight (∼23%) showed more than one Faraday peak. An investigation into the ii possible causes of the multiple observed peaks using QU-fitting suggested that there is a possibility of a Faraday thick structure or multiple Faraday components along these paths. Furthermore, we estimated a Galactic RM contribution towards Pictor A of 23.57 ± 10.87 rad m−2. We concluded that while our Galaxy may contribute to the mean RM for this source, it cannot explain smallscale fluctuations, which suggests that some fraction of the observed rotation measures could result from some inter-galactic medium, X-ray gas near the shock boundary region (the sheath), or other unknown intervening material. We introduced Scrappy, a Python-based tool tailored for processing lines-of-sight data. Scrappy yields RM-synthesis diagnostic data products such as the data associated with each line-of-sight, and their corresponding plots in 𝜙-space (e.g. cleaned and dirty Faraday spectra and RMTF), and 𝜆2-space (e.g. the fractional polarisation, and Stokes Q and U ). Scrappy further avails a Bash-based pipeline, showrunner.sh, that processes input sub-band Stokes images, automatically selects usable sub-bands, stacks images into Stokes cubes, generates lines-of-sight, processes their corresponding data, and produces diagnostic plots. Additionally, it creates per-pixel maps of fractional polarisation, RM, polarisation angle, peak FDF, and linear polarised intensity. The pipeline ensures reproducibility. To visualise the diagnostic plots from Scrappy, we developed PolarVis, a simple web-based tool that enables the visualisation of diagnostic plots associated with each available line-of-sight, thus facilitating the quick exploration of interesting lines-of-sight in regions across this source. This tool facilitates the visualisation of polarisation behaviour for specific lines-of-sight, enabling quick identification of interesting regions of the source. Furthermore, its interactivity promotes the exploration of line-of-sight data. Availing data to the public with this tool permits validation or comparison of results from varying techniques, hence fostering a sense of transparency. As a result, the 2389 lines-of-sight of Pictor A are presented using PolarVis and are available at https://pica.ratt.center. , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2024
- Full Text:
- Date Issued: 2024-10-11
Studying the brightest radio sources in the southern sky
- Authors: Sejake, Precious Katlego
- Date: 2022-04-06
- Subjects: Galaxies Formation , Galaxies Evolution , Active galaxies , Radio galaxies , Radio sources (Astronomy) , Southern sky (Astronomy)
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/455350 , vital:75423
- Description: Active Galactic Nuclei (AGN) are among the most remarkable and powerful extragalactic radio sources in the Universe. The study of AGN enables us to understand better the critical mechanisms leading to the launch of radio jets, and its link to the central engine. Radio jets are thought to impact their host galaxy by promoting or suppressing star formation. By studying AGN, we can better understand their formation, evolution, and environment. The host galaxy cross-identification is a crucial step to be carried out to build a multi-wavelength analysis of powerful radio sources; AGN. The GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) 4Jy (G4Jy) Sample comprises 1,863 of the powerful radio sources in the southern sky. However, 140 sources from the G4Jy Sample were followed-up with the Open Time on MeerKAT. Of these 140 sources, 126 had an ambiguous host galaxy, and 13 had an identified host galaxy; however, there were some discrepancies in the literature concerning the host galaxy. The host-galaxy identification of these sources is limited by the poor resolution of radio data at 25" to 45". This study aims to assess the radio morphology of these 140 sources and identify their host galaxy using the ⇠ 7” resolution images from MeerKAT in conjunction with datasets at other wavelengths. This analysis is carried out by visually inspecting the overlays. The overlays comprise radio contours from 150 MHz, 200 MHz, 843/1400 MHz and 1300 MHz overlaid on the mid-infrared image (3.4 μm). The MeerKAT images reveal sources with various radio morphologies. While most of the sources have radio morphology of typical symmetric lobes, 10 radio sources have head-tail morphology, 14 are wide-angle tail (WAT), and 5 have X-, S- /Z-shaped morphology. Overall, we find host galaxies for 70% of the sources in the sample, with the remainder comprising sources with ambiguous host galaxy (20.7%) and sources with a faint mid-infrared host galaxy (9.3%). These results highlight the importance of angular resolution and sensitivity for morphological classification and host galaxy cross-identification. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2022
- Full Text:
- Date Issued: 2022-04-06
- Authors: Sejake, Precious Katlego
- Date: 2022-04-06
- Subjects: Galaxies Formation , Galaxies Evolution , Active galaxies , Radio galaxies , Radio sources (Astronomy) , Southern sky (Astronomy)
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/455350 , vital:75423
- Description: Active Galactic Nuclei (AGN) are among the most remarkable and powerful extragalactic radio sources in the Universe. The study of AGN enables us to understand better the critical mechanisms leading to the launch of radio jets, and its link to the central engine. Radio jets are thought to impact their host galaxy by promoting or suppressing star formation. By studying AGN, we can better understand their formation, evolution, and environment. The host galaxy cross-identification is a crucial step to be carried out to build a multi-wavelength analysis of powerful radio sources; AGN. The GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) 4Jy (G4Jy) Sample comprises 1,863 of the powerful radio sources in the southern sky. However, 140 sources from the G4Jy Sample were followed-up with the Open Time on MeerKAT. Of these 140 sources, 126 had an ambiguous host galaxy, and 13 had an identified host galaxy; however, there were some discrepancies in the literature concerning the host galaxy. The host-galaxy identification of these sources is limited by the poor resolution of radio data at 25" to 45". This study aims to assess the radio morphology of these 140 sources and identify their host galaxy using the ⇠ 7” resolution images from MeerKAT in conjunction with datasets at other wavelengths. This analysis is carried out by visually inspecting the overlays. The overlays comprise radio contours from 150 MHz, 200 MHz, 843/1400 MHz and 1300 MHz overlaid on the mid-infrared image (3.4 μm). The MeerKAT images reveal sources with various radio morphologies. While most of the sources have radio morphology of typical symmetric lobes, 10 radio sources have head-tail morphology, 14 are wide-angle tail (WAT), and 5 have X-, S- /Z-shaped morphology. Overall, we find host galaxies for 70% of the sources in the sample, with the remainder comprising sources with ambiguous host galaxy (20.7%) and sources with a faint mid-infrared host galaxy (9.3%). These results highlight the importance of angular resolution and sensitivity for morphological classification and host galaxy cross-identification. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2022
- Full Text:
- Date Issued: 2022-04-06
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
- Full Text:
- Date Issued: 2022-04-04
- 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
- Full Text:
- Date Issued: 2022-04-04
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