Browsing by Subject "astronomy"

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    Big Science: SKA and the next phase of astronomy in Africa
    (2013) Horrell, Jasper
    In 2012 the country celebrated the news that South Africa had won a substantial part of the bid to build the most ambitious radio astronomy array in the world - the Square Kilometer Array. This audio lecture explains the purpose and function of this massive investment in international astronomy, and its implications for the future of astronomy in South Africa. This resource is useful for anyone interested in the Square Kilometer Array and astronomy in general.
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    The birth of modern astronomy
    (2013) Wolfe, David
    by Emeritus Professor David Wolfe, University of New Mexico and visiting lecturer, Physics Department, UCT. Professor Wolfe explores the foundations of astronomy, beginning with the ancient Egyptians, Greeks and Babylonians, to the early modern period and the works of Brahe, Kepler, Copernicus and Newton in condifying the mathematical and theoretical models that underpin modern astronomy.
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    Capturing transients: an application of biostatistics to astronomy
    (2022) van Dyk, Anke; Mcbride, Vanessa; Groot, Paul
    Capture-recapture has been identified as a possible use case for estimating the underlying size of astrophysical transient populations. In this work, we present a series of exploratory analyses using capture-recapture methods from biostatistics. In the first of three separate analyses, we reproduce results of Laycock (2017). Strategically sampled X-ray lightcurves of simulated populations of high mass X-ray binaries (HMXBs) are used to probe estimator behaviour and efficiency. Overall, these statistically closed population estimators converge to the input population with increasing number of observations, yet estimator efficiency is shown to be significantly be affected by sampling strategy. I then employ nonstandard estimator models to account for variations in capture probability of individuals within the population, categorised into ‘behavioural', ‘temporal', and ‘heterogeneous' effects. In the second analysis, we present a methodology for closed population capture-recapture analysis using real data from the OGLE-IV XROM survey. The data samples consisted of observations that were grouped into epochs. The large variation in quiescent magnitude of the population creates heterogeneity in the capture probability of sources which requires non-standard modelling. Estimation of population size is therefore limited by the choice of observational magnitude threshold. Bias corrected estimation proves to be potentially useful in this context. In the third and final investigation, we present a ‘robust design' approach with a population of Dwarf Nova located towards and in the Galactic Bulge identified from the OGLE-II, -III, and -IV phases. This approach combines closed and open population practices that allows new individuals identified between the survey phases to be added to the study sample for dynamical estimation. These investigations provide a future course for population size estimation of transients and variable stellar population alongside population synthesis simulations. The generation of capture histories remain non-trivial through the choice of observation grouping, brightness scale, and imposed flux threshold. Further, there remain several unexplored avenues of inquiry and refinement for this methodology pertaining to astronomy using explanatory variables in the modelling. Recommendations are made for further exploration of the topic.
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    Characterisation of small, close-approaching near-earth asteroids
    (2021) Janse van Rensburg, Petronella; Erasmus, Nicolas; Bershady, Matthew A
    Near-Earth Asteroids (NEAs) are a population of asteroids in a steady state, constantly being replenished with asteroids from the main belt. NEAs have orbits that come close to or cross the Earth's orbit and therefore some could have impacting trajectories and pose a threat. Small NEAs (diameter < 300 m) pose a greater threat compared to large NEAs because they are more abundant and can cause significant damage on impact. The characteristics of small NEAs can give an indication of the most likely properties of potential future impactors. Even though in recent years the number of discovery and characterisation programmes of NEAs have increased, the characterisation of the small NEA population still lags behind because they can only be observed with 1-m class telescopes when they pass close to the Earth and become bright enough. Presented here in this MSc thesis are 20 NEAs that were successfully observed and characterised with the South African Astronomical Observatory (SAAO) 40-inch telescope and the Sutherland HighSpeed Optical Camera. Out of the 20 NEAs, 14 had diameters < 300 m (H > 21). Characterisation involved assigning taxonomic probabilities to each NEA based on spectra from the Bus-DeMeo classification scheme and thereby inferring its most probable composition, as well as using a Lomb-Scargle periodogram to extract the rotation period from multi-band photometry. The taxonomic probabilities were determined with the colours g0−r 0 and r0−i 0 , in combination with a machine learning (ML) algorithm trained on synthetic colours from observed spectra obtained from literature. The taxonomies considered were the S-, C-, and X-complexes, and the D-, Q-, and V-types. In this thesis, the taxonomic probabilities are reported for all of the targets. A distinct taxonomic class was assigned to 15 NEAs that had a probability >50% in a specific taxonomy. New taxonomic classes are reported for 11 of the targets. A notable result of this study is the confirmation of the prediction that the most common meteorite, ordinary chondrites, are due to S-complex and Q-type asteroids. The fraction of meteorite falls due to ordinary chondrites are similar to the combined fraction of Scomplex and Q-type asteroids in this study (∼80%). This confirmation was only possible by including the Q-type asteroids in the classification and being able to differentiate between the C-complex and Q-type asteroids with two colours and a ML approach. A rotation period was extracted for nine NEAs that were observed for long enough to resolve a light curve period. The remaining targets had only partial or flat light curves and no period could be resolved from the periodogram. Reported here are also three small NEAs with H > 22 magnitude which were found to have rotation periods smaller than the 2.2 hour spin barrier and could be rigid pieces of rock instead of rubble piles.
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    Characterising star forming and luminous infrared galaxies with the Southern African Large Telescope (SALT)
    (2018) Ramphul, Rajin Anand; Vaisanen, Petri; van Der Heyden, Kurt
    Context: Stellar population modelling is a popular technique that has been extensively applied to main sequence galaxies. Yet starburst galaxies and Luminous InfraRed Galaxies (LIRGs) have, so far, not been studied as much using the method. LIRGs in the local universe are known to be highly interacting galaxies with strong star formation in obscured environments. Still, LIRGs also have diversity in terms of morphology and mode and location of star formation. Aim: This thesis investigates the stellar population properties of a group of 52 starbursts and luminous infrared galaxies (LIRGs) in the local universe that forms part of the SUperNovae and starBurst in the InfraReD (SUNBIRD) survey. The galaxies in a distance range of 3.5 < Dl < 280 Mpc and infrared luminosity of 10.30 < LIR < 11.91 L were observed with the Southern African Large Telescope in long-slit spectroscopy mode. Method: The stellar populations of the galaxies are derived by fitting Bruzual & Charlot (2003) templates to the reduced spectra using STARLIGHT software with a Monte Carlo method implemented to recover uncertainties on age, metallicity and extinction. The derived stellar population models are then subtracted from the observed spectra to produce emission spectra from which emission line fluxes are measured. Both integrated spectra and spatially resolved apertures are extracted to be analysed in this work. Results: The light-weighted and mass weighted age of the sample is found to be 160 Myr and 7.2 Gyr respectively. The star formation history of the sample shows a rise of activity in the past ∼ 50 Myr and with a jump of an order of magnitude in the past 3 Myr. Analysis of the stellar metallicity hints at inflow of pristine gas, which decreases the observed metallicity content as well as ignites SF-activity. Analysis of the oxygen abundances shows that while LIRGs and SF galaxies are under abundant, their under-abundance may have previously been over-estimated as compared to main sequence galaxies. The radial age profile of the sample is flat, similar to that of late-type Sd galaxies. Interaction is found to cause a drop in the age of apertures although the post-merging stages shows continued star forming activity in the nuclear region. The stellar metallicity gradient is found to be −0.029 ± 0.018 dex/kpc, comparable to Sb or Sbc galaxies favouring an inside-out formation scenario for the galaxies. As interaction stage increases, both age and metallicity gradients are seen to get flatter, eventually getting slightly positive. The more active interaction stages are HII driven, while isolated and post merging stages shows higher AGN activity. The current work offers an update on the abundances of IR dominated galaxies from the previous work done by Rupke et al. (2008). The formation scenario of our LIRGs in the local universe is shown to be in line with the scenario put forward by Hopkins et al. (2008). Future works with medium resolution spectra acquired during the course of this thesis should allow for detection of gas inflows and better constrain the different ionising mechanisms involved at different interaction stages.
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    Deconstructing the WISE nearby galaxy population
    (2019) August, Tamlyn; Jarrett, Thomas; Cluver, Michelle
    This thesis aims to test the reliability of the Wide-Field Infrared Space Explorer (WISE) star formation indicators centred at 12 and 22 µm. To accomplish this, the total infrared (TIR) luminosity was used as a calibrator to understand the behaviour of the two indicators. Our sample consists of Spitzer galaxies from the SINGS and KINGFISH surveys, with a select few excluded. The TIR calibration also necessitates that most of our galaxies have FIR data from Herschel. The photometric analysis done on the raw WISE and Spitzer images of our galaxies is explained, including how the isophotes and background sky level were determined for each band/image. Once the reliability of the WISE indicators was established, new star formation rate relations were formulated. The new relations are comparable to previous relations in the literature, but improve on them in stellar mass-, metallicity-, and luminosity range. We also find that metallicity of the galaxies has minimal effect on our relations. In our comparison of emission from PAH molecules, the two bands in question, IRAC4 and W 3 are strongly correlated. This may suggest that the W3 band is dominated by emission from the 11.3 µm PAH molecule, or it might be an effect of the band overlap between IRAC4 and W3.
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    Deep HI observations of nearby late-type galaxies
    (2019) Sorgho, Amidou; Carignan, Claude
    This thesis makes use of sensitive Hi observations to map the distribution and kinematics of the low column density neutral hydrogen in late-type nearby mostly isolated galaxies and in the nearby M81 group. Using the KAT-7, GBT and MeerKAT AR1 telescopes, we present in the second chapter a survey of the Hi in a sample of twenty nearby, mostly isolated galaxies down to low column density levels. This provided a new Hi view of some of these galaxies, which allowed to derive their kinematics out to unprecedented extents. Despite the short spacings of KAT-7 and MeerKAT AR1, and the large size of the single-dish GBT that make these telescopes ideal for detecting faint structures, the observations revealed no clear detection of low column density Hi clouds down to a typical sensitivity of ∼2.2 × 1018 cm−2 that could be associated to gas accretion in the observed galaxies. However, we do not discard the existence of such structures that, we note, could be in the form of discrete clouds smaller than the beam size of the telescopes. In the third chapter, we use the DRAO telescope to perform a sensitive survey of the Hi in a 5 ◦ × 5 ◦ area of the M81 group. Similarly to previous observations, we find that the three major and interacting galaxies of the group – M81, M82 and NGC 3077 – are connected through Hi bridges and intergalactic Hi clouds. One of the major findings of the survey is the more complete map of the western Hi arm connecting the three galaxies to the dwarf galaxy NGC 2976. These observations offer enough resolution to map the structure of the arm, and reveal a complex of small clouds filling the space between the arm and the Hi forming “main body” of the interacting galaxies. Using a tilted-ring model, we also construct a large-scale rotation curve of the system formed by the interacting galaxies. Consistently with the large-scale velocity field, we observe a flat trend for the rotation velocity of the system from 20 kpc out to 80 kpc, well beyond the outskirts of the M81 disk, although with asymmetries like a wiggle at the vicinity of M82. The fourth chapter focuses on a subset of the M81 survey containing the dwarf galaxy IC 2574 and the Hi complex HIJASS J1021+68. In this chapter we perform a thorough analysis of the distribution and kinematics of the Hi in the two systems, and thanks to the high sensitivity we are able to detect a substantially large amount of low column density Hi around IC 2574, in the form of an Hi envelope, and in two large concentrations around the galaxy. We find evidence that HIJASS J1021+68 – which is found to be connected to IC 2574 through a filament of discrete clouds – is not a dark galaxy as previously suggested, but is instead a complex of clouds either stripped from, or falling onto the primordial Hi envelope of IC 2574. The kinematical analysis of IC 2574 using a 3D tilted-ring model brings us to derive its rotation curve out to a larger extent than previous works and allows us to constrain its Dark Matter halo parameters, which we find consistent with the literature. Overall, the different results presented in this work prove that the Hi content of local galaxies is higher than what current observations reveal, and new sensitive telescopes such as MeerKAT and the upcoming SKA will unveil a new Hi view of galaxies.
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    Deep HI observations of nearby late-type galaxies
    (2019) Sorgho, Amidou; Carignan, Claude
    This thesis makes use of sensitive Hi observations to map the distribution and kinematics of the low column density neutral hydrogen in late-type nearby mostly isolated galaxies and in the nearby M81 group. Using the KAT-7, GBT and MeerKAT AR1 telescopes, we present in the second chapter a survey of the Hi in a sample of twenty nearby, mostly isolated galaxies down to low column density levels. This provided a new Hi view of some of these galaxies, which allowed to derive their kinematics out to unprecedented extents. Despite the short spacings of KAT-7 and MeerKAT AR1, and the large size of the single-dish GBT that make these telescopes ideal for detecting faint structures, the observations revealed no clear detection of low column density Hi clouds down to a typical sensitivity of ∼2.2 × 1018 cm−2 that could be associated to gas accretion in the observed galaxies. However, we do not discard the existence of such structures that, we note, could be in the form of discrete clouds smaller than the beam size of the telescopes. In the third chapter, we use the DRAO telescope to perform a sensitive survey of the Hi in a 5 ◦ × 5 ◦ area of the M81 group. Similarly to previous observations, we find that the three major and interacting galaxies of the group – M81, M82 and NGC 3077 – are connected through Hi bridges and intergalactic Hi clouds. One of the major findings of the survey is the more complete map of the western Hi arm connecting the three galaxies to the dwarf galaxy NGC 2976. These observations offer enough resolution to map the structure of the arm, and reveal a complex of small clouds filling the space between the arm and the Hi forming “main body” of the interacting galaxies. Using a tilted-ring model, we also construct a large-scale rotation curve of the system formed by the interacting galaxies. Consistently with the large-scale velocity field, we observe a flat trend for the rotation velocity of the system from 20 kpc out to 80 kpc, well beyond the outskirts of the M81 disk, although with asymmetries like a wiggle at the vicinity of M82. The fourth chapter focuses on a subset of the M81 survey containing the dwarf galaxy IC 2574 and the Hi complex HIJASS J1021+68. In this chapter we perform a thorough analysis of the distribution and kinematics of the Hi in the two systems, and thanks to the high sensitivity we are able to detect a substantially large amount of low column density Hi around IC 2574, in the form of an Hi envelope, and in two large concentrations around the galaxy. We find evidence that HIJASS J1021+68 – which is found to be connected to IC 2574 through a filament of discrete clouds – is not a dark galaxy as previously suggested, but is instead a complex of clouds either stripped from, or falling onto the primordial Hi envelope of IC 2574. The kinematical analysis of IC 2574 using a 3D tilted-ring model brings us to derive its rotation curve out to a larger extent than previous works and allows us to constrain its Dark Matter halo parameters, which we find consistent with the literature. Overall, the different results presented in this work prove that the Hi content of local galaxies is higher than what current observations reveal, and new sensitive telescopes such as MeerKAT and the upcoming SKA will unveil a new Hi view of galaxies.
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    Dispersion measure variations in pulsar observations with LOFAR
    (2019) Ibrahim, Abubakr; Serylak, Maciej; Mohamed, Shazrene
    I present an analysis of the dispersion measure (DM) variations for 68 pulsars. The observations were taken using six International LOFAR Stations in Europe over the period of 3.5 years (between June 2014 and November 2017) at the centre frequency of 150 MHz with 80 MHz of bandwidth. During this time each pulsar was observed on weekly basis resulting in an average of 160 observations per source. I show that, the variations of the DM measurements show various trends along the span of the observation: increasing or decreasing, and in some cases more changes from one trend to another. I perform the structure function analysis for each of observed pulsar included in the study, in order to check if the DM variations follow the Kolmogorov power spectrum which describes the turbulence structure of the interstellar medium (ISM). I find that for a number of pulsars results show consistency with the Kolmogorov distribution (e.g. PSRs J1913−0440 and J2157+4017) while other sources show significant difference (e.g. PSRs J0108+6608 and J0614+2229). I also obtain the DM derivatives (i.e. dDM/dt) for each pulsar, in order to examine the correlation between the DM and its derivative. The result of this correlation shows a best-fit with a square-root dependence of 0.6±0.2, which is comparable with the result that was previously obtained by Hobbs et al. (2004), who shows a dependence of square-root between the DM and its derivative; with a gradient of 0.57±0.09. Also, one of the major results of this study that, thanks to the timing analysis, allowed me to produce a new timing solution for three pulsars: PSRs J0613+3731, J0815+4611 and J1740+27. This study concludes in that: i) the DM variations can be used to understand the general properties of the ISM ii) the low-frequency observations can enable us to study the dispersion effect on pulsar signals, which can be very useful for the effort of the pulsar timing array (PTA) project iii) IISM studies using pulsar timing is a powerful technique requiring careful approach to data reduction and analysis due to characteristic of the pulsars.
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    Evidence for a clumpy, rotating gas disk in a submillimeter galaxy at z = 4
    (2012) Hodge, J A; Carilli, C L; Walter, F; de Blok, W J G; Riechers, D; Daddi, E; Lentati, L
    We present Karl G. Jansky Very Large Array observations of the CO(2-1) emission in the z = 4.05 submillimeter galaxy (SMG) GN20. These high-resolution data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr after the big bang. The data reveal a clumpy, extended gas reservoir, 14 {+-} 4 kpc in diameter, in unprecedented detail. A dynamical analysis shows that the data are consistent with a rotating disk of total dynamical mass 5.4 {+-} 2.4 Multiplication-Sign 10{sup 11} M {sub Sun }. We use this dynamical mass estimate to constrain the CO-to-H{sub 2} mass conversion factor ({alpha}{sub CO}), finding {alpha}{sub CO} = 1.1 {+-} 0.6 M {sub Sun }(K km s{sup -1} pc{sup 2}){sup -1}. We identify five distinct molecular gas clumps in the disk of GN20 with masses a few percent of the total gas mass, brightness temperatures of 16-31K, and surface densities of >3200-4500 Multiplication-Sign ({alpha}{sub CO}/0.8) M {sub Sun} pc{sup -2}. Virial mass estimates indicate they could be self-gravitating, and we constrain their CO-to-H{sub 2} mass conversion factor to be <0.2-0.7 M {sub Sun }(K km s{sup -1} pc{sup 2}){sup -1}. A multiwavelength comparison demonstrates that the molecular gas is concentrated in a region of the galaxy that is heavily obscured in the rest-frame UV/optical. We investigate the spatially resolved gas excitation and find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with star formation occurring over a large portion of the disk. We discuss the implications of our results in the context of different fueling scenarios for SMGs.
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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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    From here to the edge of the observable universe
    (2012) Catchpole, Robin
    Lecture series presented by Dr Robin Catchpole, Institute of Astronomy, Cambridge, formerly Senior Astronomer at the Royal Observatory, Greenwich. This series of audio lectures is for people interested in astronomy who wish to learn more about the observable universe.
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    Full Mueller imaging: direction dependent corrections in polarimetric radio imaging
    (2018) Jagannathan, Preshanth; Taylor, Andrew Russell
    Magnetic fields pervade the universe, spanning a multitude of scales from the dipolar field on Earth, to the largest gravitationally bound structures such as galaxy clusters [1]. The magnetic fields play a vital role in the evolution of these astronomical systems. In addition to the multitude of scales, magnetic fields are present in different astronomical systems of varying strengths. The strongest observed astronomical magnetic fields are in neutron stars with a field strength of ≈ 1015 G [2], far higher than any man-made fields till date. In stark contrast magnetic fields in the interstellar medium while ubiquitous are only a few µG in field strength. Many fundamental processes in astrophysics have magnetism at their heart, be it cosmic ray particle acceleration, star formation, or the launch of radio galaxy jets, pulsars, etc. One key fundamental process that allows us to detect and characterize cosmic magnetic fields with radio astronomy is the polarization of synchrotron radiation. Synchrotron radiation is intrinsically polarized broadband continuum radiation emitted by relativistic charged particles accelerated by the presence of magnetic fields. The emissivity of the synchrotron radiation is tied to the magnetic field strength B and the spectral index α (defined such that the flux density S ∝ ν −α ) such that ε ∝ B 1+α .
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    Galactic and extragalactic distance scales: Some South African contributions
    (2005) Feast, Michael
    The work of South African astronomers in establishing and refining methods of measuring the distances of stars and galaxies is reviewed.
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    Improved cosmological constraints from a joint analysis of the SDSS-II and SNLS supernova samples
    (2014) Betoule, M; Kessler, R; Guy, J; Mosher, J; Hardin, D; Biswas, R; Astier, P; El-Hage, P; Konig, M; Kuhlmann, S; Marriner, J; Pain, R; Regnault, N; Balland, C; Bassett, B A; Brown, P J; Campbell, H; Carlberg, R G; Cellier-Holzem, F; Cinabro, D; Conley, A; D’Andrea, C B; DePoy, D L; Doi, M; Ellis, R S; Fabbro, S; Filippenko, A V; Foley, R J; Frieman, J A; Fouchez, D; Galbany, L; Goobar, A
    Aims. We present cosmological constraints from a joint analysis of type Ia supernova (SN Ia) observations obtained by the SDSS-II and SNLS collaborations. The dataset includes several low-redshift samples (z < 0:1), all three seasons from the SDSS-II (0:05 < z < 0:4), and three years from SNLS (0:2 < z < 1), and it totals 740 spectroscopically confirmed type Ia supernovae with high-quality light curves. Methods. We followed the methods and assumptions of the SNLS three-year data analysis except for the following important improvements: 1) the addition of the full SDSS-II spectroscopically-confirmed SN Ia sample in both the training of the SALT2 light-curve model and in the Hubble diagram analysis (374 SNe); 2) intercalibration of the SNLS and SDSS surveys and reduced systematic uncertainties in the photometric calibration, performed blindly with respect to the cosmology analysis; and 3) a thorough investigation of systematic errors associated with the SALT2 modeling of SN Ia light curves. Results. We produce recalibrated SN Ia light curves and associated distances for the SDSS-II and SNLS samples. The large SDSS-II sample provides an eective, independent, low-z anchor for the Hubble diagram and reduces the systematic error from calibration systematics in the low-z SN sample. For a flat CDM cosmology, we find m = 0:295 0:034 (stat+sys), a value consistent with the most recent cosmic microwave background (CMB) measurement from the Planck and WMAP experiments. Our result is 1:8 (stat+sys) dierent than the previously published result of SNLS three-year data. The change is due primarily to improvements in the SNLS photometric calibration. When combined with CMB constraints, we measure a constant dark-energy equation of state parameter w = ?1:018 0:057 (stat+sys) for a flat universe. Adding baryon acoustic oscillation distance measurements gives similar constraints: w = ?1:0270:055. Our supernova measurements provide the most stringent constraints to date on the nature of dark energy.
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    Meshless hydrodynamic simulations of young supernova remnants
    (2020) Mogawana, Orapeleng; Mohamed, Shazrene; van der Heyden, K
    The majority of massive stars end their lives by ejecting their outer envelopes in a corecollapse supernova explosion. The collision of their ejecta with the surrounding circumstellar medium results in the formation of supernova remnants that have been detected at all wavelengths, from radio to gamma-rays. For several dozen supernova remnants, very-long-baseline radio interferometers have spatially resolved the interaction region and directly measured the expansion rates of the shocked gas; many show evidence of the interaction of supernova ejecta with the dense slow winds characteristic of the red supergiant progenitors. Understanding the dynamics and morphology of the interaction region, particularly in young supernova remnants leads to estimates of the total mass of the circumstellar medium, as well as its density distribution around the star given the value of the wind velocity. Here we studied the interaction of the supernova ejecta with different circumstellar environments to investigate the hydrodynamic evolution of young supernova remnants in the SedovTaylor phase. We used the massively parallel, multi-physics magneto-hydrodynamics (MHD) and gravity code, GIZMO, for our simulations. We chose GIZMO for its flexibility in allowing the user to choose different methods to solve the fluid equations, i.e., new Lagrangian Godunovtype schemes, e.g., Meshless Finite Volume (MFV) and Meshless Finite Mass (MFM), as well as various flavors of smoothed particle hydrodynamics (SPH), or Eulerian fixed-grid schemes. Since the majority of previous studies used the latter, we focused on an extensive comparison of all the meshless methods in solving the Sedov-Taylor blastwave test, a problem for which there is an exact solution. For our given compute resources, we found the parameters (e.g., smoothing length, number of neighbours, artificial viscosity, and particle resolution) for each meshless method that gave the best agreement with the exact solution. We then carried out 2D and 3D simulations of the hydrodynamic interaction of the supernova ejecta with varying density profiles assumed for the circumstellar medium, namely: a 1/r 2 density profile, for a typical, spherically symmetric red supergiant stellar wind, and an axisymmetric torus profile, inspired by the observation of a dense, dusty torus of the circumstellar material around the red supergiant, WOH G64 (Ohnaka et al., 2008). Radially assembled Hierarchical Equal Area isoLatitude Pixelization (HEALPix) shells were used to set-up the initial density and velocity profiles for the ejecta, which is marked by a flat inner core and a steeply declining outer edge. The Weighted Voronoi Tessellation code was used to produce the 1/r 2 and axisymmetric torus density distributions. We showed that the growth of Richtmyer-Meshkov instabilities in the 2D and 3D 1/r 2 profiles are visible as early as 20 yrs into the evolution of the remnant and become increasingly unstable up to 100 yr. While 2D simulations of 1/r 2 profiles show the presence of the Richtmyer-Meshkov instabilities in the hot shell of a contact discontinuity, in 3D we see large bubbles and filamentary structure of the instabilities. Our results for the numerical approaches to simulating the systems for the 1/r 2 density cases were broadly consistent with previous studies in the literature where stationary grids were used. Two scenarios with different torus-cavity density contrasts were considered in which we found that the instability rolls along the half-opening angle takes ∼ 40 yr to develop in the axisymmetric torus with smooth density drop, whereas the axisymmetric torus with steep density drop does not develop any instability rolls up to the end of the simulation. We concluded with a discussion of the implications of our models for the morphology of supernova remnants and their expected levels of multi-wavelength emission.
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    Probing student engagement with size and distance in introductory astronomy
    (2022) Makwela, Tshiamiso Neo; Allie, Saalih; Taylor, Dale; Blyth, Sarah
    Astronomy Education Research has shown that students have many challenges when it comes to understanding key concepts in Astronomy. Amongst these is a poor understanding of astronomical scales. Recently for example, both sizes and distances have been shown to present similar difficulties to students in both South Africa and Norway. It is difficult to attribute the findings simply to inadequate teaching due to the significant differences between the two countries with regard to language, culture, and the type of science teaching. It has, therefore, been suggested that since astronomical sizes and distances are beyond immediate human experience the explanation might in fact lie at a deeper cognitive level. The present thesis is aimed at exploring the link between astronomical sizes and distances as well as cognition. Part I The thesis focuses on investigating students' understanding of sizes and distances in astronomy. This was done by probing student notions of astronomical scales, using the size and distance questions from the Introductory Astronomy Questionnaire (IAQ), the instrument which led to the original findings noted previously. These questions were administered before and after a specially structured teaching intervention on sizes and distances. The results of this study in 2018 were found to be (a) in agreement with similar studies previously reported in South Africa and Norway, namely, that both sizes and distances in astronomy were poorly understood in both contexts and (b) that the teaching intervention was least effective for distances. Based on the findings above, the focus of the thesis shifted to a more fine-grained investigation of how students conceived of distances, as they increased from "human scale" to "beyond human scale". The study was carried out using the Grounded Theory Method (GTM). Data were generated by prompting written explanations from introductory astronomy students on how they engaged with three distances two of which may be considered to be within human experience while the third lies beyond the realm of direct experience. The distances used were 7 metres, 100 kilometres and the distance to the moon. The second distance was partly informed by the idea that we often communicate large distances to each other in terms of time. In addition, the framing of the questions excluded the possibility of visual explanations. The questions were administered to a cohort of introductory astronomy students at the University of Cape Town in 2019. A grounded analysis of the student responses was carried out to identify key ideas. The categories that emerged from the analysis showed clear evidence of students using different, unconnected types of explanations rather than simple extrapolations of one idea. A conceptual transition was identified relative to the body position of the respondents: body calibration and self-propelled body motion (or journeying). What was striking was that time was rarely mentioned explicitly. The way in which students expressed themselves was assumed to be an expression of the way in which they were thinking about different distance domains and suggestive of the cognitive perspective offered by "Embodied Cognition". Of particular interest was that nonstatic explanations were centered around the notion of a journey, and one of the key "thinking templates" in Embodied Cognition; the SOURCE-PATH-GOAL "Image Schema". Part II of the thesis summarizes key elements of Embodied Cognition that are pertinent to the present work and describes a pilot activity for teaching astronomical distances based on this account. Part II Theories of cognition can roughly be divided into two camps: those that assume that thinking is a "mentalese activity" involving symbolic manipulation. Most importantly, these symbolic elements are "amodal" in that they are not derived from the sensory modalities. On the other hand, Embodied Cognition assumes that these symbols arise from the sensory modalities, hence all thinking arises from bodily experience and its interactions with the environment in infancy. While there are several strands that feed into Embodied Cognition, of direct interest to the present work is that of Cognitive Linguistics and the notion of Conceptual Metaphor. In this view metaphors are not regarded as (mere) linguistic devices but as conceptual expressions that reflect cognitive schematic structures that relate to the bodily infant experience. These cognitive schematic structures or "Image Schemas" arise from repeated bodily actions repeatedly activating particular neural networks and form the basic building blocks of all abstract thought. A fair amount of such Image Schemas (or "thinking templates") have been identified of which the SOURCE-PATH-GOAL resonates most clearly with the data described earlier. This Image Schema comes about in infancy when a child learns that a toy on the far side of a room cannot be reached by grasping only but that moving the body from one place to another (crawling) is required. This is the basis of "Life is Journey or the Ph.D. Journey", for example. Another aspect of Embodied Cognition holds that understanding involves a mental simulation using the cognitive resources that are activated at the time. In order to see if activating the SPG / Journey "thinking template" prior to engaging with the teaching material would help in comprehending astronomical distances a two-part teaching activity (A and B) was developed around the notion of a journey. Part A was presented to the students as 'Journey to the observable edge of the UNIVERSE along UNIVERSity avenue" and required students to walk the length of the campus in a structured manner that is described in detail in the thesis. Part B, engagement with the teaching material, was carried out immediately afterwards in the Main Hall of the University. Thus, the thinking behind the two-part activity, piloted in 2020 just prior to Covid related lockdown, was that "journey" cognitive resources would be activated by the experience and would therefore be used in engaging with the teaching material regarding astronomical distances. Student evaluations were gathered in order to probe how students had engaged with the activity, including if any of the resources associated with journeying were expressed. A post-test ranking task showed that while results were mixed relative to previous studies overall there was a marked improvement for the present cohort. In summary the work shows clearly that there were two different modes of thinking about distances (i) based on counting and (2) based on the notion of the journey/journey-ing. Results were interpreted as the activation of schema described by embodied cognition. The difficulty that students experienced with astronomical distances was attributed to the lack of activating the Source-Path-Goal schema. In order to see whether there was a way to activate the Source-Path-Goal schema, an activity involving students walking was designed. The outcomes from the activity, indicated promising results with regard to student engagement with astronomical distance.
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    Seeing our world through science
    (2013) Sithaldeen, Riashna; Jarrett, Thomas
    We live in a time of exciting discoveries and developments in the field of science, allowing us to 'see' more than ever before, including 'seeing' into the past, the present and the future. For anyone who wants to learn more about seeing our world through science.
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    Stellar Halos: modelling formation in the L-Galaxies 2020 semi-analytic model
    (2020) Murphy, Geoff; Yates, Robert; Cunnama, Daniel; Mohamed, Shazrene
    A study was carried out to determine how well the L-Galaxies 2020 semi-analytic model simulates the stellar halos of galaxies and the intracluster stellar (ICS) components of galaxy clusters. Two galaxy disruption models were tested, namely instantaneous disruption and gradual disruption. Furthermore, two stellar halo profiles were applied to the simulation results: a power-law profile with slope γ = −3.5 and a Navarro-Frenk-White (NFW) profile. In the latter case, the stellar halo stars follow the distribution of the galaxy's dark matter. It was found that a combination of an NFW profile and gradual disruption provided the best results across the widest range of literature data, namely measurements of stellar halo mass, total stellar mass, stellar mass fractions, and stellar halo iron abundances. Gradual disruption of satellite galaxies also resulted in the central galaxies having more massive stellar halos in comparison to instantaneous disruption. Additional stellar halo formation mechanisms, such as in-situ star formation, were not needed, as the stellar halo masses seen in observations can be obtained in L-Galaxies by considering only tidal disruption of infalling satellite galaxies. The number of high mass accretions into the halos of Milky Way-mass galaxies in the gradual disruption model agreed well with simulation literature. It was found that while central galaxies can induce many disruptions of satellite galaxies (over a thousand in some cases), the majority of the Milky Way-sized stellar halos in L-Galaxies are formed by the disruption of one to fourteen satellite galaxies, in good agreement with simulation literature. A population of galaxies with unexpectedly low stellar halo iron abundances was found. These were determined to be a result of disruptions of high mass, low metallicity satellite galaxies. Furthermore, rather than iron or oxygen, carbon was found to be the dominant element produced by stellar halo stars for the majority of redshifts in most high mass central and satellite galaxies, due mainly to asymptotic giant branch stars. The relative contribution of stellar halo stars was found to be minor, however, with circumgalactic medium enrichment from halo stars in comparison to outflows from galactic stars being on average . 1%. For clusters with virial masses exceeding 1.6 × 1014M, the brightest central galaxy and ICS (BCG+ICS) stars contained 42.44% of the total cluster stellar iron content, while the fraction MICS MBCG+MICS was found to be 82.50%, both results being in good agreement with observation.
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    THE SDSS-II SUPERNOVA SURVEY: PARAMETERIZING THE TYPE Ia SUPERNOVA RATE AS A FUNCTION OF HOST GALAXY PROPERTIES
    (2012) Smith, Mathew; Nichol, Robert C; Dilday, Benjamin; Marriner, John; Kessler, Richard; Bassett, Bruce; Cinabro, David; Frieman, Joshua; Garnavich, Peter; Jha, Saurabh W; Lampeitl, Hubert; Sako, Masao; Schneider, Donald P; Sollerman, Jesper
    Using data from the Sloan Digital Sky Supernova Survey-II (SDSS-II SN Survey), we measure the rate of Type Ia supernovae (SNe Ia) as a function of galaxy properties at intermediate redshift. A samp ...