Browsing by Author "Frank Bradley"
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- ItemOpen AccessExploring the evolution and hidden large-scale structures of galaxies with MeerKAT HI surveys(2024) Rajohnson, Sambatriniaina Hagiriche Aycha; Kraan-Korteweg, Renee Christine; Frank BradleyBuilding upon previous high-resolution and high-sensitivity H I surveys conducted over narrow angles of the sky, such as the COSMOS H I Large Extragalactic Survey (CHILES), a new era has begun with the emergence of SKA Pathfinders such as MeerKAT and ASKAP. Deep systematic interferometric H I surveys of unprecedented sensitivity and resolution over very wide angles of the sky are now possible. In this thesis, I use the capabilities of MeerKAT to explore H I galaxy scaling relations and investigate concealed large-scale structures (LSS) linked to dynamically significant structures in the Southern Zone of Avoidance (ZOA). To achieve these objectives, I worked on three blind systematic H I surveys conducted with the 4k L-band data from the complete MeerKAT64 array: the MIGHTEE-H I Early Science data, the Vela SARAO MeerKAT Galactic Plane Survey (Vela– SMGPS), and the MeerKAT Vela Supercluster survey (Vela–H I). As a first step, I validated the MIGHTEE-H I Early Science data covering a total of ⇠5 deg2, distributed across three XMMLSS fields (⇠3.5 deg2) and one COSMOS field (⇠1.5 deg2). The spectral line data achieved 1200⇥1000 and 14.500⇥1100 angular resolutions, with an rms sensitivity of 81 and 44 `Jy beam–1 per 44.1 km s–1 channel, for a total observing time of 13 and 17 hours per XMMLSS and COSMOS field, respectively. Following visual source identification and morphological classification, the sample comprised 276 galaxies – 176 spirals, 72 irregulars, 19 mergers, and 9 early-type galaxies. To independently verify the derived H I properties, I cross-referenced fluxes from COSMOS detections with ALFALFA galaxies, revealing agreement for high-signal-to-noise ratio sources. As an application of the data verification process, I constructed the H I size-mass relation, investigating potential systematic eects present in the early data. This relation was derived for the first time from a uniform interferometric sample, reaching an unprecedented redshift range of z ⇠ 0.084. Based on a subsample of 204 galaxies, the relation was found to be consistent with the literature at z ⇠ 0 and indicated that in the absence of significant environmental influences, gas distribution and mean surface mass density within galaxy discs have remained relatively stable over the past one billion years. A 10% intrinsic variation (0.054 ± 0.003 dex) in the H I size at a given H I mass was observed, but no evidence for evolution over the explored redshift range. I performed a detailed census to blindly map signatures of hidden LSS in the Vela region, aiming to narrow down the ZOA in redshift space. This unexplored area hosts the enigmatic Vela Supercluster (VSCL), with its potential merging walls and an inner core obscured in the innermost ZOA. The location of the VSCL is also essential for addressing persistent bulk flow discrepancies. I pursued a two-phase mapping of the VSCL, with a focus on identifying its gas-rich spirals. Firstly, using data from Vela–SMGPS, which surveyed a narrow strip covering 90 deg2 (260 ✓ 290, –2 b 1). I imaged 157 individual pointings with a total observing time of ⇠211 hours. This produced 10 contiguous mosaics with an average rms of 0.39 mJy beam–1 per 44 km s–1 channel and an angular resolution of 3000⇥2700 (±100). Secondly, the MeerKAT Vela–H I survey was designed to explicitly fill the gaps above and below the Galactic Plane (GP) between Vela–SMGPS and prior Vela spectroscopic survey regions (263 ✓ 284, –6.7 b –2, 1 b 6.2). I processed the 667 Nyquist-sampled fields and generated 32 mosaics. With 67 hours of observations over a 242 deg2 area, the survey reached an average rms of 0.74 mJy beam–1 per 44 km s–1 channel for the beam size of 3800⇥3100 (±300). By focusing on the mostly RFI-free band of 250 < Vhel < 25000 km s–1, I identified 843 and 719 heavily obscured galaxy candidates in Vela–SMGPS and Vela–H I, respectively, most of which were previously unknown. When comparing the distribution to simulations based on the SKA H I-science method which assumes a homogeneous distribution, and examining onsky distributions, a total of six overdensities were observed. With regard to VSCL, the new detections hint at the presence of two wall-like overdensities at 16000 – 19000 km s–1 (W1) and 19000 – 23000 km s–1(W2). These may well intersect within the longitude range 270 ✓ 279 at the lowest latitudes. Other major overdensities include the confirmation of the presence of the Hydra/Antlia wall traversing the GP at ✓ ⇠ 280 ± 2 at Vhel ⇠ 2500 – 4000 km s–1 and the discovery of a 30 long, narrow filament in the GP at ⇠12000 km s–1. By only considering the complete sample from Vela–SMGPS and Vela–H I, I calculated the H I mass function (HIMF) of the two surveys in an attempt to quantify these VSCL overdensities. The overall pattern of the HIMF closely aligns with the ALFALFA HIMF. Additionally, I estimated upper limit overdensity values of 3.5 ± 1.6 and 1.8 ± 2.4 in the inner ZOA for W1 and W2, respectively. The accomplishments of the MIGHTEE-H I Early Science survey, Vela–SMGPS, and Vela–H I oer a promising glimpse into the immense capabilities that await H I science with the forthcoming SKA
- ItemOpen AccessExploring the evolution and hidden large-scale structures of galaxies with MeerKAT HI surveys(2024) Rajohnson, Sambatriniaina Hagiriche Aycha; Kraan-Korteweg, Renee Christine; Frank BradleyBuilding upon previous high-resolution and high-sensitivity H I surveys conducted over narrow angles of the sky, such as the COSMOS H I Large Extragalactic Survey (CHILES), a new era has begun with the emergence of SKA Pathfinders such as MeerKAT and ASKAP. Deep systematic interferometric H I surveys of unprecedented sensitivity and resolution over very wide angles of the sky are now possible. In this thesis, I use the capabilities of MeerKAT to explore H I galaxy scaling relations and investigate concealed large-scale structures (LSS) linked to dynamically significant structures in the Southern Zone of Avoidance (ZOA). To achieve these objectives, I worked on three blind systematic H I surveys conducted with the 4k L-band data from the complete MeerKAT64 array: the MIGHTEE-H I Early Science data, the Vela SARAO MeerKAT Galactic Plane Survey (Vela– SMGPS), and the MeerKAT Vela Supercluster survey (Vela–H I). As a first step, I validated the MIGHTEE-H I Early Science data covering a total of ⇠5 deg2, distributed across three XMMLSS fields (⇠3.5 deg2) and one COSMOS field (⇠1.5 deg2). The spectral line data achieved 1200⇥1000 and 14.500⇥1100 angular resolutions, with an rms sensitivity of 81 and 44 `Jy beam–1 per 44.1 km s–1 channel, for a total observing time of 13 and 17 hours per XMMLSS and COSMOS field, respectively. Following visual source identification and morphological classification, the sample comprised 276 galaxies – 176 spirals, 72 irregulars, 19 mergers, and 9 early-type galaxies. To independently verify the derived H I properties, I cross-referenced fluxes from COSMOS detections with ALFALFA galaxies, revealing agreement for high-signal-to-noise ratio sources. As an application of the data verification process, I constructed the H I size-mass relation, investigating potential systematic eects present in the early data. This relation was derived for the first time from a uniform interferometric sample, reaching an unprecedented redshift range of z ⇠ 0.084. Based on a subsample of 204 galaxies, the relation was found to be consistent with the literature at z ⇠ 0 and indicated that in the absence of significant environmental influences, gas distribution and mean surface mass density within galaxy discs have remained relatively stable over the past one billion years. A 10% intrinsic variation (0.054 ± 0.003 dex) in the H I size at a given H I mass was observed, but no evidence for evolution over the explored redshift range. I performed a detailed census to blindly map signatures of hidden LSS in the Vela region, aiming to narrow down the ZOA in redshift space. This unexplored area hosts the enigmatic Vela Supercluster (VSCL), with its potential merging walls and an inner core obscured in the innermost ZOA. The location of the VSCL is also essential for addressing persistent bulk flow discrepancies. I pursued a two-phase mapping of the VSCL, with a focus on identifying its gas-rich spirals. Firstly, using data from Vela–SMGPS, which surveyed a narrow strip covering 90 deg2 (260 ✓ 290, –2 b 1). I imaged 157 individual pointings with a total observing time of ⇠211 hours. This produced 10 contiguous mosaics with an average rms of 0.39 mJy beam–1 per 44 km s–1 channel and an angular resolution of 3000⇥2700 (±100). Secondly, the MeerKAT Vela–H I survey was designed to explicitly fill the gaps above and below the Galactic Plane (GP) between Vela–SMGPS and prior Vela spectroscopic survey regions (263 ✓ 284, –6.7 b –2, 1 b 6.2). I processed the 667 Nyquist-sampled fields and generated 32 mosaics. With 67 hours of observations over a 242 deg2 area, the survey reached an average rms of 0.74 mJy beam–1 per 44 km s–1 channel for the beam size of 3800⇥3100 (±300). By focusing on the mostly RFI-free band of 250 < Vhel < 25000 km s–1, I identified 843 and 719 heavily obscured galaxy candidates in Vela–SMGPS and Vela–H I, respectively, most of which were previously unknown. When comparing the distribution to simulations based on the SKA H I-science method which assumes a homogeneous distribution, and examining onsky distributions, a total of six overdensities were observed. With regard to VSCL, the new detections hint at the presence of two wall-like overdensities at 16000 – 19000 km s–1 (W1) and 19000 – 23000 km s–1(W2). These may well intersect within the longitude range 270 ✓ 279 at the lowest latitudes. Other major overdensities include the confirmation of the presence of the Hydra/Antlia wall traversing the GP at ✓ ⇠ 280 ± 2 at Vhel ⇠ 2500 – 4000 km s–1 and the discovery of a 30 long, narrow filament in the GP at ⇠12000 km s–1. By only considering the complete sample from Vela–SMGPS and Vela–H I, I calculated the H I mass function (HIMF) of the two surveys in an attempt to quantify these VSCL overdensities. The overall pattern of the HIMF closely aligns with the ALFALFA HIMF. Additionally, I estimated upper limit overdensity values of 3.5 ± 1.6 and 1.8 ± 2.4 in the inner ZOA for W1 and W2, respectively. The accomplishments of the MIGHTEE-H I Early Science survey, Vela–SMGPS, and Vela–H I oer a promising glimpse into the immense capabilities that await H I science with the forthcoming SKA
- ItemOpen AccessExploring the evolution and hidden large-scale structures of galaxies with MeerKAT HI surveys(2024) Rajohnson, Sambatriniaina Hagiriche Aycha; Kraan-Korteweg, Renee Christine; Frank BradleyBuilding upon previous high-resolution and high-sensitivity H I surveys conducted over narrow angles of the sky, such as the COSMOS H I Large Extragalactic Survey (CHILES), a new era has begun with the emergence of SKA Pathfinders such as MeerKAT and ASKAP. Deep systematic interferometric H I surveys of unprecedented sensitivity and resolution over very wide angles of the sky are now possible. In this thesis, I use the capabilities of MeerKAT to explore H I galaxy scaling relations and investigate concealed large-scale structures (LSS) linked to dynamically significant structures in the Southern Zone of Avoidance (ZOA). To achieve these objectives, I worked on three blind systematic H I surveys conducted with the 4k L-band data from the complete MeerKAT64 array: the MIGHTEE-H I Early Science data, the Vela SARAO MeerKAT Galactic Plane Survey (Vela– SMGPS), and the MeerKAT Vela Supercluster survey (Vela–H I). As a first step, I validated the MIGHTEE-H I Early Science data covering a total of ⇠5 deg2, distributed across three XMMLSS fields (⇠3.5 deg2) and one COSMOS field (⇠1.5 deg2). The spectral line data achieved 1200⇥1000 and 14.500⇥1100 angular resolutions, with an rms sensitivity of 81 and 44 `Jy beam–1 per 44.1 km s–1 channel, for a total observing time of 13 and 17 hours per XMMLSS and COSMOS field, respectively. Following visual source identification and morphological classification, the sample comprised 276 galaxies – 176 spirals, 72 irregulars, 19 mergers, and 9 early-type galaxies. To independently verify the derived H I properties, I cross-referenced fluxes from COSMOS detections with ALFALFA galaxies, revealing agreement for high-signal-to-noise ratio sources. As an application of the data verification process, I constructed the H I size-mass relation, investigating potential systematic eects present in the early data. This relation was derived for the first time from a uniform interferometric sample, reaching an unprecedented redshift range of z ⇠ 0.084. Based on a subsample of 204 galaxies, the relation was found to be consistent with the literature at z ⇠ 0 and indicated that in the absence of significant environmental influences, gas distribution and mean surface mass density within galaxy discs have remained relatively stable over the past one billion years. A 10% intrinsic variation (0.054 ± 0.003 dex) in the H I size at a given H I mass was observed, but no evidence for evolution over the explored redshift range. I performed a detailed census to blindly map signatures of hidden LSS in the Vela region, aiming to narrow down the ZOA in redshift space. This unexplored area hosts the enigmatic Vela Supercluster (VSCL), with its potential merging walls and an inner core obscured in the innermost ZOA. The location of the VSCL is also essential for addressing persistent bulk flow discrepancies. I pursued a two-phase mapping of the VSCL, with a focus on identifying its gas-rich spirals. Firstly, using data from Vela–SMGPS, which surveyed a narrow strip covering 90 deg2 (260 ✓ 290, –2 b 1). I imaged 157 individual pointings with a total observing time of ⇠211 hours. This produced 10 contiguous mosaics with an average rms of 0.39 mJy beam–1 per 44 km s–1 channel and an angular resolution of 3000⇥2700 (±100). Secondly, the MeerKAT Vela–H I survey was designed to explicitly fill the gaps above and below the Galactic Plane (GP) between Vela–SMGPS and prior Vela spectroscopic survey regions (263 ✓ 284, –6.7 b –2, 1 b 6.2). I processed the 667 Nyquist-sampled fields and generated 32 mosaics. With 67 hours of observations over a 242 deg2 area, the survey reached an average rms of 0.74 mJy beam–1 per 44 km s–1 channel for the beam size of 3800⇥3100 (±300). By focusing on the mostly RFI-free band of 250 < Vhel < 25000 km s–1, I identified 843 and 719 heavily obscured galaxy candidates in Vela–SMGPS and Vela–H I, respectively, most of which were previously unknown. When comparing the distribution to simulations based on the SKA H I-science method which assumes a homogeneous distribution, and examining onsky distributions, a total of six overdensities were observed. With regard to VSCL, the new detections hint at the presence of two wall-like overdensities at 16000 – 19000 km s–1 (W1) and 19000 – 23000 km s–1(W2). These may well intersect within the longitude range 270 ✓ 279 at the lowest latitudes. Other major overdensities include the confirmation of the presence of the Hydra/Antlia wall traversing the GP at ✓ ⇠ 280 ± 2 at Vhel ⇠ 2500 – 4000 km s–1 and the discovery of a 30 long, narrow filament in the GP at ⇠12000 km s–1. By only considering the complete sample from Vela–SMGPS and Vela–H I, I calculated the H I mass function (HIMF) of the two surveys in an attempt to quantify these VSCL overdensities. The overall pattern of the HIMF closely aligns with the ALFALFA HIMF. Additionally, I estimated upper limit overdensity values of 3.5 ± 1.6 and 1.8 ± 2.4 in the inner ZOA for W1 and W2, respectively. The accomplishments of the MIGHTEE-H I Early Science survey, Vela–SMGPS, and Vela–H I oer a promising glimpse into the immense capabilities that await H I science with the forthcoming SKA
- ItemOpen AccessFirst MeerKAT HI survey results mapping large scale structures hidden behind the Milky Way out to z = 0.08(2023) Steyn, Nadia; Kraan-Korteweg, Renee Christine; Frank BradleySKA pathfinders (e.g., ASKAP and MeerKAT) are performing large HI surveys, surpassing previous-generation radio surveys in angular resolution, volume, and depth — going nearly two orders of magnitude deeper. The vast amount of data being produced by these telescopes creates a need for new tools and techniques, such as automated source-extraction. One particular science goal is to trace the large scale structure of galaxies in the local Universe behind the Zone of Avoidance (ZoA). This is difficult to do in the optical and infrared bands because of the thick dust and stellar crowding along the plane of the Milky Way. HI surveys are ideal because the 21 cm spectral line emission of neutral hydrogen (HI) atoms is unaffected by these features. This dissertation presents two HI surveys in the ZoA: the MeerKAT16 Early Science project, conducted before the completion of the full array, with 16-dish subarrays and the ROACH-32k correlator; and later, the Galactic Plane Legacy Survey (GPS), a larger MeerKAT survey project utilising 60-dish subarrays and the SKARAB-4k correlator. The motivation of the MeerKAT16 survey was two-fold: serving as a pilot project for the envisioned large MeerKAT64 Vela Supercluster (VSCL) survey; and a test-bed for optimising galaxy identification processes using automated pipelines. The survey aimed to map the rich galaxy cluster VC04, embedded in one of the prospective VSCL walls. We searched two mosaic cubes for HI sources by means of a deep visual search, and then using the Source Finding Application (SoFiA; version 1.3.2). The purpose of using both methods was to understand and optimise the SoFiA pipeline on real data, which is important for the development of source finding strategies for future large surveys. We catalogued 119 reliable galaxy detections (and an additional 37 candidates with lower certainty) within the early science data. SoFiA found galaxies all the way at the VSCL distance (V(hel) ≈ 18 000 km/s), where we detected hints of two walls, and HI deficiency in the centre of VC04. GPS surveyed a long narrow strip (|b| ≤ 2°) along the Galactic plane, with the aim of penetrating the most obscured part of the ZoA. A segment of the data spanning 302° ≤ l ≤ 332° was reduced, imaged, and analysed as part of this project. This region is interesting because it encompasses the Great Attractor (GA) — a massive overdensity highly influential to the local flow of galaxies. Aided by the newer version of SoFiA (v2.3.1), we found 477 galaxy candidates, most of which are new discoveries, and performed an in-depth comparison against the largest systematic HI survey covering the southern ZoA up to this point — the Parkes HIZOA survey. Additionally, we compared our results to simulations that follow the method used for the SKA HI science case. The GPS redshift distribution reveals a striking overdensity at the GA distance (V(hel) ≈ 4000 km/s), inconsistent with a uniform galaxy distribution, whereas the high-redshift end is more underdense than predicted by the simulation. These deep interferometric HI surveys provided a new glimpse of highly interesting structures crossing the Galactic plane, demonstrating that large scale structure can be mapped even in the deepest part of the ZoA. The success of MeerKAT16 and GPS is a preview of the HI science potential of the upcoming SKA, which will surpass MeerKAT in depth, sensitivity and resolution.