Browsing by Author "Blyth, Sarah-Louise"

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    Open Access
    Acceleration of the noise suppression component of the DUCHAMP source-finder
    (2015) Badenhorst, Scott James; Kuttel, Michelle Mary; Blyth, Sarah-Louise
    The next-generation of radio interferometer arrays - the proposed Square Kilometre Array (SKA) and its precursor instruments, The Karoo Array Telescope (MeerKAT) and Australian Square Kilometre Path finder (ASKAP) - will produce radio observation survey data orders of magnitude larger than current sizes. The sheer size of the imaged data produced necessitates fully automated solutions to accurately locate and produce useful scientific data for radio sources which are (for the most part) partially hidden within inherently noisy radio observations (source extraction). Automated extraction solutions exist but are computationally expensive and do not yet scale to the performance required to process large data in practical time-frames. The DUCHAMP software package is one of the most accurate source extraction packages for general (source shape unknown) source finding. DUCHAMP's accuracy is primarily facilitated by the à trous wavelet reconstruction algorithm, a multi-scale smoothing algorithm which suppresses erratic observation noise. This algorithm is the most computationally expensive and memory intensive within DUCHAMP and consequently improvements to it greatly improve overall DUCHAMP performance. We present a high performance, multithreaded implementation of the à trous algorithm with a focus on 'desktop' computing hardware to enable standard researchers to do their own accelerated searches. Our solution consists of three main areas of improvement: single-core optimisation, multi-core parallelism and the efficient out-of-core computation of large data sets with memory management libraries. Efficient out-of-core computation (data partially stored on disk when primary memory resources are exceeded) of the à trous algorithm accounts for 'desktop' computing's limited fast memory resources by mitigating the performance bottleneck associated with frequent secondary storage access. Although this work focuses on 'desktop' hardware, the majority of the improvements developed are general enough to be used within other high performance computing models. Single-core optimisations improved algorithm accuracy by reducing rounding error and achieved a 4X serial performance increase which scales with the filter size used during reconstruction. Multithreading on a quad-core CPU further increased performance of the filtering operations within reconstruction to 22X (performance scaling approximately linear with increased CPU cores) and achieved 13X performance increase overall. All evaluated out-of-core memory management libraries performed poorly with parallelism. Single-threaded memory management partially mitigated the slow disk access bottleneck and achieved a 3.6X increase (uniform for all tested large data sets) for filtering operations and a 1.5X increase overall. Faster secondary storage solutions such as Solid State Drives or RAID arrays are required to process large survey data on 'desktop' hardware in practical time-frames.
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    Deep NIR imaging of galaxy clusters in the Vela supercluster
    (2022) Hatamkhani, Narges; Kraan-Korteweg, Renee Christine; Blyth, Sarah-Louise
    Gravitational forces of large galaxy over densities can perturb the smooth motions expected from the expanding universe, causing peculiar motions. The observed Cosmic Microwave Background (CMB) dipole is the result of the Local Group's peculiar motion, the direction and amplitude of which are still not fully resolved. The newly discovered Vela Supercluster (VSCL, ` = 272. ◦5±20◦ , b = 0◦±10◦ ), an extended structure behind the Zone of Avoidance (ZoA) at cz ∼ 18000 km s−1 , may be a not insignificant contributor to the residual bulk flow that arises beyond cz ∼ 16000 km s−1 . Knowledge of the structure and richness of galaxy clusters within the VSCL will enable us to assess the morphology and mass of this partially obscured supercluster. Compared to the shorter optical wavelengths, near infrared (NIR) observations are less affected by dust obscuration in the ZoA and therewith offer a better tool to probe galaxy clusters in the VSCL, and get a better understanding of this extended supercluster. A series of deep NIR observations of prospective clusters identified in the VSCL were conducted in the J, H and Ks bands with the InfraRed Survey Facility (IRSF) telescope. For 6 (VC02, VC04, VC05, VC08, VC10 and VC11) out of 20 potential clusters a complete set of science quality images were obtained (out to ∼ 70% of the Abell radius) and were fully reduced. I identified galaxy candidates in each cluster using the Source Extractor software, validated them through visual inspection of the RGB composite images and created a catalogue of galaxies with their astrometric and photometric parameters using the IRSF pipeline. There are a total number of 1715 identified galaxies distributed over the six clusters, of which only ∼ 15% were previously known. Investigating the effect of foreground extinction in the region of the observed clusters shows that, the VC02 cluster has the highest foreground extinction while VC04 and VC05 have the lowest (hAKsi = 0. m10, hAKsi = 0. m07 and hAKsi = 0. m06 respectively). I show that the effect of extinction on the isophotal magnitudes is small compared to the foreground extinction and additional extinction-correction is not required. The extinction-corrected completeness magnitude limit for this survey is Mo Ks < −21. m5 which is ∼ 2. m0 deeper than 2MASX. The six observed clusters were analysed in detail out to the cluster centric completeness radius of rc < 1.5 Mpc and Mo Ks < −21. m5. Comparison of the iso-density contour maps and radial density profiles of the VSCL clusters with Ks-band data of well-known clusters (Coma, Norma, 3C129 and Virgo), finds VC04 to be a regular and massive cluster comparable to Coma and Norma (although its velocity dispersion seems rather low for a rich cluster); it is the richest of the six. H I analysis (using MeerKAT-16 data) shows that the spiral galaxies in VC04 are severely H I deficient which is consistent with its richness. VC02 and VC05 are found to be relatively rich clusters while VC08 is rather poor. VC10 has a filament-like structure and is not a cluster. VC11 is an intermediate cluster which contains two major subclusters. It appears that many of the VSCL clusters (VC02, VC04, VC05 and VC11) are not relaxed yet and are still evolving. The Ks-band Luminosity Function (LF) was derived for the VSCL clusters up to Mo Ks < −21. m5 (∼ 2. m5 deeper than M∗ ). I demonstrate that the Red-Sequence (RS) method is a reliable method to measure the LFs of the clusters and then compute the Ks-band LFs of the Coma, Norma and Virgo clusters using the RS method to compare to the LFs of the VSCL clusters. The comparisons show that M∗ derived for the LF of VC04 (M∗ = −24. m70 ± 0.42) agrees well with those of other local clusters. The bright end of the VC04 LF is compatible with that of massive clusters that are dominated by early-type galaxies such as Norma, while the slope (α = −0.89±0.13) is shallower compared to those of younger clusters. The analysis of this to date small sample of the VSCL clusters (6 out of at least 20) shows that the VSCL contains potentially many more rich clusters indicative of it being a rich supercluster.
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    The distance to the Norma cluster and its relation to the great attractor region
    (2015) Mutabazi, Tom; Blyth, Sarah-Louise; Woudt, Patrick A
    The Local Group's peculiar motion with respect to the cosmic microwave background has been measured with high precision and found to be ~627±22 km s-1, but the source of this motion is not clear. Studies to constrain the fractional contribution of the local mass overdensity, particularly in the Great Attractor region and the more distant Shapley Supercluster, remain contradictory. This work is an attempt to disentangle these contributions by measuring a redshift-independent distance to the Norma cluster, which lies at, or close to, the core of the Great Attractor, in order to determine its peculiar velocity. A reliable measurement of the distance and peculiar velocity of Norma will help clarify the velocity flows in the Great Attractor region. The challenge is with observing in the Zone of Avoidance since star crowding and Galactic extinction effects are severe at such low Galactic latitudes. High quality near-infrared images have been used in the photometric analysis for the Norma cluster sample so as to minimise the effect of Galactic extinction. In addition, the imaging cameras used have a small pixel scale resulting in well resolved images for reliable foreground star-subtraction hence accurate photometry. In order to determine the redshift-independent distance, I applied two independent, complemen- tary methods: the Fundamental Plane and the Tully-Fisher relations. The redshift-independent distance and the peculiar velocity of the Norma cluster were measured using 1) the Ks-band Fundamental Plane analysis for 31 galaxies using the 3.6m New Technology Telescope (NTT) at the European Southern Observatory (ESO) 2) the J- and Ks-band Fundamental Plane analysis for 31 galaxies using the Japanese 1.4 m InfraRed Survey Facility (IRSF) at the South African Astronomical Observatory (SAAO) 3) the combined NTT and IRSF Ks-band Fundamental Plane analysis for 41 galaxies 4) the Ks-band Tully-Fisher analysis for 12 galaxies using near-infrared images from the IRSF and HI profiles obtained from the 64-m Parkes radio telescope.
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    Exploring HI asymmetries in real and simulated galaxies
    (2021) Hank, Nadine A N; Blyth, Sarah-Louise; Deg, Nathan
    In the ΛCDM model of the Universe, galaxy-galaxy interactions and mergers are considered key drivers in their evolution. These dynamical events lead to peculiar and disturbed morphologies, which can be studied using morphometric statistics. The rotational 2D asymmetry parameter, adopted from optical studies, has recently been used to quantify asymmetries in the H i images of galaxies. This is useful since H i disks typically extend further than the stellar disks and are more sensitive to distortions from interactions. Asymmetries can also manifest in the 1D spectral domain, distorting the shape of the global H i profiles of galaxies. The shape of this profile is determined predominantly by the kinematics of the galaxy, and the H i spatial distribution to a lesser extent. By using archival H i data and simulations, we have begun investigating the systematics and uncertainties of using the 1D and 2D asymmetry parameters for merger studies. In this thesis, we present a new 1D measure of lopsidedness and examine the applicability of two different 2D asymmetry measures. We investigate the evolution of 2D asymmetry of the gas and stellar distributions in a simulated major merger event and demonstrate that the gas distribution registers the interaction before the stellar distribution is affected. We also find that the outer asymmetry of both distributions is considerably higher postmerger, whereas the intensity-weighted asymmetry returns to pre-merger values. We then explore how well the 1D and 2D parameters agree with visual classifications of asymmetry for a sample of 115 WHISP galaxies and observe that the 1D folding difference lopsidedness and the 2D intensity-weighted asymmetry parameters compare well with the visual classification of asymmetries in the H i profiles and images respectively. We examine the relationship between the 1D and 2D asymmetries in WHISP and find that the 1D folding difference lopsidedness and the 2D intensity-weighted asymmetry yield the strongest linear correlation between spectral and morphological asymmetries, with r = 0.53 after inclination cuts have been applied. Lastly, we investigate the location of interacting galaxies in asymmetry parameter space and find that the joint use of 1D and 2D parameters can separate most interacting galaxies from the non-interacting sample.
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    Jet study in ultra-relativistic heavy-ion collisions with the ALICE detectors at the LHC
    (2004) Blyth, Sarah-Louise
    This thesis presents first results on jet reconstruction in simulated Pb+Pb collisions using the ALICE detectors and a UAl-based cone jet finding algorithm which has been modified and optimised to reconstruct high-PT jets on an event-by-event basis. Optimisation of the algorithm parameters and methods used to suppress the large background energy contribution while maximising the algorithm efficiency, are discussed and the resulting jet energy and direction resolutions are presented.
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    RFI monitoring for the MeerKAT Radio Telescope
    (2015) Schollar, Christopher; Blyth, Sarah-Louise; Kuttel, Michelle Mary; Schroeder, Anja
    South Africa is currently building MeerKAT, a 64 dish radio telescope array, as a pre-cursor for the proposed Square Kilometre Array (SKA). Both telescopes will be located at a remote site in the Karoo with a low level of Radio Frequency Interference (RFI). It is important to maintain a low level of RFI to ensure that MeerKAT has an unobstructed view of the universe across its bandwidth. The only way to effectively manage the environment is with a record of RFI around the telescope. The RFI management team on the MeerKAT site has multiple tools for monitoring RFI. There is a 7 dish radio telescope array called KAT7 which is used for bi-weekly RFI scans on the horizon. The team has two RFI trailers which provide a mobile spectrum and transient measurement system. They also have commercial handheld spectrum analysers. Most of these tools are only used sporadically during RFI measurement campaigns. None of the tools provided a continuous record of the environment and none of them perform automatic RFI detection. Here we design and implement an automatic, continuous RFI monitoring solution for MeerKAT. The monitor consists of an auxiliary antenna on site which continuously captures and stores radio spectra. The statistics of the spectra describe the radio frequency environment and identify potential RFI sources. All of the stored RFI data is accessible over the web. Users can view the data using interactive visualisations or download the raw data. The monitor thus provides a continuous record of the RF environment, automatically detects RFI and makes this information easily accessible. This RFI monitor functioned successfully for over a year with minimal human intervention. The monitor assisted RFI management on site during RFI campaigns. The data has proved to be accurate, the RFI detection algorithm shown to be effective and the web visualisations have been tested by MeerKAT engineers and astronomers and proven to be useful. The monitor represents a clear improvement over previous monitoring solutions used by MeerKAT and is an effective site management tool.
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    The effect of the cosmic web on galaxy evolution in RESOLVE-A
    (2021) Hoosain, Munira; Blyth, Sarah-Louise; Skelton, R.E.
    Galaxy environment plays a significant role in galaxy evolution. While most work has focused on the effect of cluster and group environment, large scale structures, such as filaments and voids, may provide additional contributions. Recently, various authors have found correlations between galaxy morphology, stellar mass and colour, and the distance to cosmic web filaments in both simulations and data. However, the effect of filaments on the gas supply of galaxies is still under investigation due to conflicting results. I use data from the Resolved Spectroscopy of a Local Volume (RESOLVE) survey, which is a low-redshift galaxy census complete down to log M?/M= 8.9, to study the relationship between galaxy properties and the cosmic web. I use the Discrete Persistence Structures Extractor (DisPerSE), a topology-based software package, to map filaments in the RESOLVEA field. This work shows that galaxies in RESOLVE-A have, higher stellar masses close to filaments. When accounting for the additional effect of groups, I find no variation in the colour of galaxies with respect to their distance to filament. Low-mass (log M?/M< 9.7) galaxies increase in gas fraction with increasing distance to filament, which may indicate that low-mass galaxies lose gas as they enter filaments.
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    Using HI stacking to study galaxy properties in the nearby universe (Looking for needles in the HI-stack)
    (2016) Healy, Julia L; Blyth, Sarah-Louise; Elson, E C
    Neutral atomic hydrogen (Hi) is the raw fuel from which the star-forming molecular gas forms and is therefore an important tracer of galaxy evolution. Due to the intrinsic faintness of the Hi emission line (observed at rest at 21 cm), galaxies beyond a few hundred megaparsecs are difficult to observe directly with current radio telescopes. However, in the next year, MeerKAT and other SKA pathfinder telescopes will begin operating and enable deeper, large surveys (e.g. LADUMA) of neutral gas in galaxies. Hi Stacking is an observational technique that will be highly exploited to learn about the Hi content of galaxies that are not directly detected. Stacking involves combining the Hi spectra of all the galaxies in a distant sample, thereby generating a high signal-to-noise measure of their average Hi content. This work presents a new Python-based package capable of stacking Hi galaxy spectra. This package will be used to stack the Hi spectra of high-redshift galaxies observed with the MeerKAT telescope. In this work the package is applied to a sample of galaxies observed as part of the Nançay Interstellar Baryon Legacy Extragalactic Survey (NIBLES, van Driel et al. (2016)) to learn more about the gas properties of galaxies in the local universe. Using the stacking technique, we are able to recover the average Hi mass of different galaxy populations for which there was no Hi directly detected. In order to obtain the average gas properties that best represent the overall galaxy populations, we also stack both the Hi detected and non-detected spectra from the NIBLES survey. We find that our gas fraction vs. stellar mass distribution results agree well with previous stacking experiments (Brown et al., 2015; Catinella et al., 2010; Fabello et al., 2011a) and the NIBLES sample enables us to probe an order of magnitude lower in stellar mass. We find a dependence on the underlying stellar mass distribution for our gas fraction vs. NUV - r colour, especially when comparing to Brown et al. (2015) and Fabello et al. (2011a).
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    Using the ¢-meson to probe the medium created in Au+Au collisons at RHIC
    (2007) Blyth, Sarah-Louise
    [pg 76 is missing] This thesis presents measurements of ¢-meson production and elliptic flow, 1,2, with the aim of probing the characteristics of the medium created in ultra-relativistic nucleus-nucleus collisions. Using the decay channel q;-+ K+ K-, high statistics measurements have been made for ¢-meson production at mid-rapidity in Au+Au collisions at ,.,jsim = 200 GeV using the STAR detector at RHIC. The ¢-meson invariant yields both integrated, and as a function of PT, are presented for a wide range of collision centralities. The transverse momentum distributions exhibit an evolution in shape with decreasing collision centrality from exponential-like to power-law-like, indicating a variation in the relative contributions from competing particle production mechanisms to ¢ production as a function of collision centrality. Measurements of the nuclear modification factor, Rep, for 0-5% central relative to 40-60% and 60-80% central collisions show that binary-scaled ¢-meson production is suppressed in central compared to more peripheral collisions. The baryon-meson scaling of Rep observed for other identified particles is also observed for the ¢-meson through its similarity to the K� Rep for 0-5%/40-60% ratio. The scaling appears to break down in the 0- 5%/60-80% case. The centrality dependence of the N ( 0) / N ( ¢) ratios are also presented. The central ratio is consistent up to Pr ;S 4 Ge V / c with expectations from a model in which multistrange hadrons are produced predominantly by recombination of thermal s quarks. In addition, the ¢-meson elliptic flow, v2 (pr ), has been measured in Au+Au collisions at ,.,fsim = 62.4 GeV and 200 GeV and is found to be similar for the two collision energies. The significant v2 for the ¢-meson, comparable to that of other particles, indicates significant rescattering of the medium constituents. In minimum bias collisions at 200 GeV, for PT < 2 GeV /c, the ¢-meson v2 is consistent with a mass ordering expected from hydrodynamics while for PT > 2 Ge V / c it is consistent with meson-scaling. This is strong evidence for partonic collectivity of the medium created in the collisions. First measurements of the centrality dependence of the ¢-meson integrated elliptic flow scaled by the eccentricity of the nuclear overlap region, (v2) / Epart, show an increasing trend with centrality and may indicate greater collectivity of the medium created in central compared to peripheral Au+Au collisions.