Browsing by Subject "Astronomy"

Now showing 1 - 20 of 125
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    Accretion and outflow in black-hole x-ray binaries
    (2015) Dusoye, Avishek; Coriat, M; Woudt, Patrick Alan
    Black hole X-ray binaries (BHXBs) are stellar binary systems consisting of a black hole (BH) and a companion star. They are known to produce X-ray emission through the accretion of mass from the companion star onto the black hole via an accretion disc, as well as radio emission originating from their jets. My thesis splits into two projects. On one hand, I focus on the connection between the X-ray emitting accretion disc and the radio jets of BHXBs in general, by studying the quasi-simultaneous evolution of the radio fluxes and the X-ray fluxes from 17 BHXBs. This connection, also known as the radio/X-ray correlation has been studied and updated over the past years. New observations of new and known sources have shown that another population of X-ray binaries exists (referred to as outliers), lying below the standard radio/X-ray correlation. I investigate whether the mass of the black hole component of BHXBs can explain the existence of these outliers. In my second project, I focus on an exotic source, known as SS433. It has a supercritical accretion disc and displays precessing relativistic jets. I investigate whether these jets are made up of proton-electron plasma or electron-positron plasma. Circular polarization (CP) is a good diagnostics for understanding the particle composition of radio jets. Therefore we have observed the circular polarized flux densities of SS433 using the Australia Telescope Compact Array (ATCA) for a broad range of frequencies between 1:4 - 10 GHz. From those observations, a CP spectrum can be constructed and the spectral index can be estimated. There are 4 ways of producing CP emission and the spectral index helps us to constrain the CP production mechanism. In addition, the kinematics of propelling a proton-electron plasma in a jet is different from that of electronpositron plasma. I simulate various plausible models for the energy content of the jets and thereby aim to constrain the particle composition of the jets. Black hole X-ray binaries (BHXBs) are stellar binary systems consisting of a black hole (BH) and a companion star. They are known to produce X-ray emission through the accretion of mass from the companion star onto the black hole via an accretion disc, as well as radio emission originating from their jets. My thesis splits into two projects. On one hand, I focus on the connection between the X-ray emitting accretion disc and the radio jets of BHXBs in general, by studying the quasi-simultaneous evolution of the radio fluxes and the X-ray fluxes from 17 BHXBs. This connection, also known as the radio/X-ray correlation has been studied and updated over the past years. New observations of new and known sources have shown that another population of X-ray binaries exists (referred to as outliers), lying below the standard radio/X-ray correlation. I investigate whether the mass of the black hole component of BHXBs can explain the existence of these outliers. In my second project, I focus on an exotic source, known as SS433. It has a supercritical accretion disc and displays precessing relativistic jets. I investigate whether these jets are made up of proton-electron plasma or electron-positron plasma. Circular polarization (CP) is a good diagnostics for understanding the particle composition of radio jets. Therefore we have observed the circular polarized flux densities of SS433 using the Australia Telescope Compact Array (ATCA) for a broad range of frequencies between 1:4 - 10 GHz. From those observations, a CP spectrum can be constructed and the spectral index can be estimated. There are 4 ways of producing CP emission and the spectral index helps us to constrain the CP production mechanism. In addition, the kinematics of propelling a proton-electron plasma in a jet is different from that of electron-positron plasma. I simulate various plausible models for the energy content of the jets and thereby aim to constrain the particle composition of the jets.
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    Accretion disks in the cataclysmic variables
    (1976) Koen, Marthinus Christoffel
    The general equations describing the time-independent structure of gaseous circumstellar disks in vertical hydrostatic equilibrium are derived. The possibility of turbulent instablilty and the resulting viscosity laws are discussed.
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    Accretion processes in magnetic cataclysmic variables - a detailed study of UZ Fornacis
    (2022) Khangale, Zwidofhelangani Ndamulelo; Woudt, Patrick Alan; Potter, Stephen Brian
    This thesis presents a multi-wavelength exploration of the emissions from magnetic cataclysmic variable (mCV) stars with specific focus on the eclipsing AM Her system, UZ Fornacis (hereafter UZ For). The main objective of this thesis is to investigate the underlying emission mechanisms as well as the accretion dynamics in this system with the intention of increasing our understanding of this class of objects as a whole. The presence of a strong magnetic field in UZ For makes it an interesting object in terms of how the magnetic field dominates the accretion dynamics and its influence on the emission processes. The eclipsing characteristics of this system, with a clearly defined ingress and egress of the main accretion spot, can be exploited to gain further insights through precise eclipse timings. The first part of this thesis presents 33 new mid-eclipse times spanning eight years of observations on UZ For. The new observations are used to test the two-planet model previously proposed to explain the periodic variations in the eclipse times of UZ For measured over the past ∼35 years. The results show that the proposed model does indeed follow the general trend of the new eclipse times. However, there are significant departures. In order to accommodate the new eclipse times, the two-planet model requires that one or both of the planets have highly eccentric orbits, i.e. e ≥ 0.4. Such multiple planet orbits are considered to be unstable. Whilst our new observations are consistent with two cyclic variations as previously predicted, significant residuals remain. This study explores the possibility that either additional cyclic terms, possibly associated with more planets, or other mechanisms, such as the Applegate mechanism are contributing to the eclipse time variations. The second part of this thesis presents phase-resolved spectroscopy of UZ For which allowed us to do a detailed Doppler tomography study of this target. The averaged blue spectrum is dominated by single- and/or double-peaked emission from HeII 4686 Å and the Balmer lines, as well as weak emission from HeI lines and the CIII/NIII blend at 4650Å. The averaged red spectrum shows strong emission from CaII lines at 8498 Å and 8542 Å and possibly weak emission from the NaI doublet at 8183 Å and 8194 Å. The strength of HeII 4686 Å line is comparable to that of Hβ line and this is typical for AM Her systems in a high state of accretion. The ratio of Hγ/Hβ ' 1 implying that these emission lines were formed in an optically thick region, that is, a region of high optical depth in the lines. Doppler tomography of the strongest features in the blue, HeII 4686Å and the Balmer lines (e.g. Hβ line), using both the standard and inside-out maps, revealed the presence of three emission regions: 1) the irradiated face of the secondary star, 2) the ballistic stream and the threading region, and 3) the magnetically confined accretion stream. The Doppler maps of emission lines in the red spectrum show the presence of emission from the irradiated face of the secondary star as well as emission from various parts of the ballistic and magnetically confined accretion stream. Also presented are the respective modulation Doppler maps of each line. These show that both the ballistic and magnetically confined accretion stream are modulated as well as the irradiated face of the secondary star. In addition to the above, the first simultaneous optical and MeerKAT radio observations of the eclipsing AM Her system UZ For are also presented. The optical data include SALT circular spectropolarimetry taken around the eclipse and SHOC and MeerLICHT photometry. The total intensity spectrum shows broad emission features with the continuum that rises in the blue. The percentage circularly polarized spectra show that UZ For is negatively polarized (up to ∼ −8%) in the blue and decreasing gradually towards the red. The circularly polarized spectrum shows the presence of three cyclotron emission features at ∼4500 Å, 6000 Å and 7700 Å, corresponding to harmonic numbers 4, 3 and 2, respectively. These features are dominant before the eclipse and disappears after the eclipse. The harmonics are consistent with the magnetic field strength of ∼57 MG seen at a viewing angle of 70◦ . To aid with the interpretation of the circular spectropolarimetry, simultaneous circular and linear photopolarimetric observations of UZ For obtained with the HIPPO instrument several weeks leading to the SALT observations were also analysed. The results show that UZ For is ∼ −5% circularly and 5% linearly polarized before the eclipse. A burst of linear polarization is seen just after the eclipse reaching ∼10%. After the eclipse, negative circular polarization reaching ∼ −5% is observed. Both linear and circular polarization are consistent with zero after phase 1.1. The MeerKAT radio observations show a faint source which has a peak flux density of 30.7 ± 5.4 µJy/beam at 1.28 GHz (L-band) at the position of UZ For. This marks the first detection of UZ For at L-band. This study demonstrates that multi-wavelength observations are essential to understanding the various emission processes that are at work in mCVs. The results presented in this thesis shows that UZ For is one of the most interesting AM Her systems known to date. The circular spectropolarimetry results are consistent with those from literature and shows that the SALT telescope will be able to observe other systems like this for detailed analysis. Furthermore, the eclipse times of this system is consistent with the presence of two planets and makes UZ For the second AM Her system after HU Aqr shown to harbour planets and in which additional observations still support earlier conclusions. The radio detection of this system with MeerKAT opens a window to studying other low flux density AM Her systems at L-band radio frequencies in the southern sky.
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    An analysis of beta cephei stars in NGC 3293 and the effects of stellar rotation
    (1994) Engelbrecht, Christian Albertus; Balona, Luis; Warner, Brian
    An intensive photometric study in the Johnson B band of ten β Cephei stars in the open cluster NGC 3293 is presented. High sampling rates of the stars' light curves allow the identification of many formerly unknown pulsation frequencies in these stars, by means of Fourier periodogram analysis. All of the stars are found to be multiperiodic, with up to five frequencies identified for individual stars. Physical parameters of the stars are determined from previous photometric results in the literature. These parameters are used to make a comparison of the observed frequencies with theoretical predictions, including the effects of rotation. The results imply a preference for first and second overtone quadrupole (l=2) pulsation in these stars, while a weak relationship between stellar mass and pulsation overtone is identified. One of the stars is identified as a member of an eclipsing binary system, only the second β Cephei star to be identified as such. Physical parameters of the system are determined. The effects of rapid rotation (specifically, the distortion of the stellar profile from a perfect sphere) on alternative methods of pulsation mode identification are investigated for a model β Cephei star. Ratios between light and velocity amplitudes show extreme differences between rotationally distorted and undistorted models, but these ratios show high sensitivity to other parameters as well. Thermally broadened spectral line profiles are calculated for the same model, for modes with l = 0 to 4 and equatorial rotation velocities up to 400 km/s. Neglect of rotational distortion is shown to lead to underestimation of true equatorial rotation velocities by as much as 253. This result confirms that observed B stars are probably rotating at break-up velocities. In summary, various aspects of the behaviour of the β Cephei stars in NGC 3293 in particular, and of β Cephei stars in general, including rapid rotators, are illuminated and augmented by this study.
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    The analysis of some bivariate astronomical time series
    (1993) Koen, Marthinus Christoffel; Zucchini, Walter
    In the first part of the thesis, a linear time domain transfer function is fitted to satellite observations of a variable galaxy, NGC5548. The transfer functions relate an input series (ultraviolet continuum flux) to an output series (emission line flux). The methodology for fitting transfer function is briefly described. The autocorrelation structure of the observations of NGC5548 in different electromagnetic spectral bands is investigated, and appropriate univariate autoregressive moving average models given. The results of extensive transfer function fitting using respectively the λ1337 and λ1350 continuum variations as input series, are presented. There is little evidence for a dead time in the response of the emission line variations which are presumed driven by the continuum. Part 2 of the thesis is devoted to the estimation of the lag between two irregularly spaced astronomical time series. Lag estimation methods which have been used in the astronomy literature are reviewed. Some problems are pointed out, particularly the influence of autocorrelation and non-stationarity of the series. If the two series can be modelled as random walks, both these problems can be dealt with efficiently. Maximum likelihood estimation of the random walk and measurement error variances, as well as the lag between the two series, is discussed. Large-sample properties of the estimators are derived. An efficient computational procedure for the likelihood which exploits the sparseness of the covariance matrix, is briefly described. Results are derived for two example data sets: the variations in the two gravitationally lensed images of a quasar, and brightness changes of the active galaxy NGC3783 in two different wavelengths. The thesis is concluded with a brief consideration of other analysis methods which appear interesting.
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    Asymptotic giant branch variables in NGC 6822
    (2010) Nsengiyumva, Francois; Whitelock, Patricia A; Menzies, John; Feast, Michael
    NGC 6822, a Local Group dwarf irregular galaxy, was observed in the nearinfrared JHKS bands using the Japanese-SAAO Infrared Survey Facility (IRSF) telescope over a period of about four years. An extensive search for large amplitude asymptotic giant branch variables over the observed 7.5' x 21.1' area of NGC 6822 found 162 probable variables. In particular, 30 Miras and 9 semiregulars have been identified and characterised.
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    Atmospheric temperature structure in the RoAp stars
    (1996) Medupe, Rodney; Kurtz, D W
    The rapidly oscillating Ap (roAp) stars are a sub-group of the chemically peculiar stars of class 2 (CP2), which are characterised by peculiar spectra and anomalously strong lines of Sr, Cr, Eu and other rare earth elements. They have strong global dipole magnetic fields with effective strengths of up to a few thousand gausses. Stars showing these phenomena cover the spectral range B8p to F0 V-IV. About 20 years ago rapid non-radial pulsations were discovered in the coolest members of the CP2 group, namely the roAp stars. These pulsations are due to high over-tone, low degree p modes with periods between 5.6 and 15 minutes. Since then, studies of these rapid oscillations have revealed a lot of information about these stars. The eigenfrequency spectra of roAp stars can potentially reveal information such as their rotation periods, rotational inclinations, magnetic geometries, internal magnetic field strengths, radii, masses, luminosities and ages. Matthews et al. (1990, 1996) suggested a technique to empirically determine the T(Ƭ) relation for roAp stars. This technique involves comparing the pulsation amplitudes obtained from multi-colour photometry to the ones calculated from black-body pulsator models (assuming dipole mode pulsations). This comparison yielded limb-darkening coefficients which were used to determine T(Ƭ) in a way similar to what was done for the sun. Matthews et al. based their idea on the observed fact that pulsation amplitudes of roAp stars drop sharply with increasing wavelength. They thus explained this sharp decline of amplitude with wavelength in terms of the strongly wavelength dependent limb-darkening. The initial aim of this thesis was to investigate the technique proposed by Matthews et al., and to apply it to a number of roAp stars. However, when a linearised expression for the variation of the pulsation amplitude with wavelength, limb-darkening, inclination of the pulsation axis α, and ΔT/T₀ (where ΔT is the polar pulsational temperature semi-amplitude and T₀ is the atmospheric temperature) was derived, it was discovered that limb-darkening is too small an effect to account for the observed amplitudes. The result is based on the Wien approximation and uses the Planck function to represent intensity. Therefore, limb-darkening cannot be measured from the amplitude vs wavelength data. This analysis and the results thereof are reported in this thesis. Numerical models based on realistic treatment of the intensity spectra (obtained from model atmospheres) are used to confirm and refine the analytical results. The linearised expression mentioned above suggests that an important factor that explains the sharp decline of amplitude with wavelength is the variation of the ratio ΔT/T₀ with wavelength. Therefore, if the T(Ƭ) structure of a star is known a priori (from model atmospheres), the variation of ΔT with wavelength can be determined. This new technique, together with the variation of opacity with wavelength in the atmospheres of roAp stars, is applied to HD 134214:, HD 137949, HD 128898, HD 101065 and HR 3831 to determine ΔT cos α as a function of atmospheric depth. HR 3831 was observed at various rotation phases to investigate the effect of rotation on the derived ΔT cos α vs atmospheric depth relation. Preliminary results on this are included in this thesis. Bibliography: pages 107-113.
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    Bayesian model selection with applications to radio astronomy
    (2017) Mootoovaloo, Arrykrishna; Bassett, Bruce A
    This thesis consists of two main parts, both of which focus on Bayesian methods and the problem of model selection in particular. The first part investigates a new approach to computing the Bayes factor for model selection without needing to compute the Bayesian evidence, while the second part shows, through an analytical calculation of the Bayesian evidence, that Bayesian methods allow two point sources to be distinguished from a single point source at angular separations that are much smaller than the naive beam size at high signal to noise. In the first part, the idea is to create a supermodel by combining two models using a hyperparameter, which we call α. Setting α = 0 or 1 switches each of the models off. Hence, the ratio of the posterior of α at the two end points (0 or 1) gives the Bayes Factor. This effectively converts the problem of model selection into a Bayesian inference problem. One can then use a standard Markov Chain Monte Carlo method to map the posterior distribution of α and compute the Bayes factor. In the second part of this thesis, the Bayesian radio interferometry formalism of Lochner et al. (2015) is extended to take into account the gains of the antennae using the StEFCal algorithm, an important part of the calibration pipeline. Finally we study the case of a pair of sources and show that they can be resolved using an analytical computation of the Bayesian evidence. This demonstrates that Bayesian methods allow super-resolution: the pair of sources can be distinguished from a single source at arbitrarily small scales compared to the naive beam size, as long as the measurements have sufficient signal to noise.
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    Bent tail radio sources as tracers of galaxy clusters at high redshift and SMBH mass estimates
    (2021) Mguda, Zolile Martin; Van Der, Heyden Kurt; Vaisanen, Petri
    Bent tail radio sources (BTRSs) are radio galaxies which have jets that show a characteristic C‐shape that is believed to be due to ram pressure caused by the motion of the galaxy through the ambient medium. They are generally found in galaxy clusters in the local Universe. They have already been used in observations as tracers of galaxy clusters at redshifts of up to z _ 1. They have, however, been shown to be numerous in galaxy groups as well. The ability to find high redshift galaxy clusters is important in cosmology because they are important cosmological probes. According to the _ CDM model, galaxy clusters form around redshift of z _ 2 and finding clusters of halo mass greater than 1014 M_ at redshift greater than z = 2:5 would disprove the current concordance model. Finding galaxy clusters at those redshifts is more feasible with the new generation of radio telescopes and the upcoming square kilometer array (SKA). In this work we look at some SMBH mass measurements, which are crucial in the determination of the correlations between the SMBH mass and some galaxy characteristics including jet length and luminosity. The high redshift SMBH mass measurement methods are calibrated using local Universe correlations. This makes SMBH mass measurement an important aspect in the study of high redshift radio galaxies and hence BTRSs. We use cosmological simulations from the MareNostrum Universe simulation to look at the efficacy of using BTRSs as tracers of clusters assuming the ram pressure is the cause of the jet bending. This is the first step in predicting the possible number of BTRSs that we may observe with the SKA. We find that SMBH masses can be measured up to redshift of z = 4:5 using the virial mass estimator method. The BTRSs are equally likely to be found in galaxy clusters and galaxy groups in the local Universe. This means that around 50% of the BTRSs that we are likely to find at high redshift will be in galaxy clusters. However, finding a pair of BTRSs in close proximity is a sign of a galaxy cluster environment. These results are still dependent on the resolution of degeneracies in our understanding of the duty cycles of AGN radio jets, projection effects of the radio jets, the environmental dependence of radio‐loudness in galaxies and other open questions.
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    The Cape rapidly oscillating Ap star survey
    (1993) Martinez, Peter; Kurtz, D W
    This thesis describes a survey, the Cape Survey, which was started with the intention of discovering more roAp stars suitable for asteroseismological studies and also to identify the limits of the roAp phenomenon in temperature and luminosity. This is the most extensive survey of the roAp phenomenon to date. Prior to the start of the Cape Survey, only 14 roAp stars, discovered over a period of 12 years, were known. The Cape Survey has yielded another 10 new roAp stars in the past three years. The candidates for the Cape Survey were mostly drawn from the Ap SrCrEu stars in the Michigan Spectral Catalogue.
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    Characterization of the atmospheric turbulence at the Sutherland site and conceptual design study and optimization of an Adaptive Optics system for the Southern African Large Telescope
    (2017) Catala, Laure; Crawford, Steve M; Whitelock, Patricia A; Buckley, David A H
    To support the potential development of an AO system for SALT, a site monitoring campaign of the Sutherland site was initiated in 2010. This campaign also led to the participation in the development of a new instrument to characterize the atmospheric turbulence. The results from five years of site testing were used in AO simulations in order to demonstrate the potential capabilities of an AO system on SALT. The site testing study produced up-to-date seeing values and provided a measurement of the atmospheric turbulence profiles. I found a median seeing value of 1.51". The main contributor to the turbulence is clearly the ground layer, below 1 km, responsible for 83% of the turbulence. The next most significant contributor is the wind shear layer around 3 km. Seasonal trends show that slightly worse seeing conditions occur during the winter months due to predominant East, South-easterly winds that are associated with degraded seeing conditions. In addition to the main site testing campaign, I helped develop the "Profileur de Bord Lunaire" (PBL, Profiler of Moon limb in English), a new instrument that uses the Moon limb to measure the atmospheric turbulence profile. The work on the data processing and inversion method led to the extraction of high altitude-resolution profiles of the turbulence strength. I present here those results along with a comparison with profiles obtained with the Multi-Aperture Scintillation Sensor (MASS). Using the results from the site testing campaign along with the SALT optical design, I simulated the general dimensioning of a system for SALT that would use a single natural guide star (NGS). The trade-off between performances and sky coverage resulted in a 34x34 system using NGS in the range 10 to 14 magnitude in R-band. The 34x34 dimensions refer to the number of subapertures of the Shack-Hartmann wavefront sensor. I conclude with the significant improvement in spectroscopic performance for SALT that could be achieved by implementing an AO system. The gains in encircled/enslited energy are most significant in the near infrared where gains of 183% could be achieved at 1600 nm for the planned Near-infrared upgrade to the Robert Stobie Spectrograph (RSS). The gains in enslited and encircled energy at 700 nm for the visible arm of RSS and the High-Resolution Spectrograph (HRS) are limited to a maximum of 22% and 34%, respectively, due to the large apertures adapted to seeing-limited observations. Further gains could be achieved by designing the next generation of SALT instrumentation to take full advantage of an AO system.
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    Characterizing the digital planetarium as a teaching and learning space
    (2023) Sivitilli, Alexander; Allie, Muhammad; Marchetti Lucia
    The modern planetarium exists today as a digital immersive facility with a multitude of capabilities and applications while also being tied to its analog roots among instructional media in astronomy education. I characterized the use of the digital planetarium as a teaching and learning space by following two lines of inquiry: a) how students engage with a digital planetarium and b) what shapes the teaching and learning space in the planetarium. For these two questions, two sources of data respectively formed the basis of the investigation: student responses and observational data. The data were analyzed using the Grounded Theory Method. Student responses were gathered from a cohort of university students during two separate planetarium visits which constituted part of their introductory astronomy course. An instrument was designed to probe both how the students responded to the overall planetarium experience and how they engaged with the educational content. In the second student visit, the show content was specifically designed by me, and the instrument had been modified based on the results of the first visit. Individual student responses were analyzed for key ideas which were grouped into several categories that emerged from the data. These categories allowed me to develop a localized mid-level model of student engagement. This led to the notion of a “spectrum of attentiveness” that strongly influenced how students engaged. The data also suggested an optimal level for relevant engagement that was influenced by the nature of coincident distractions. Detailed observations were documented throughout the investigation that included multiple additional visits to the planetarium. These visits familiarized me with the planetarium and its use as an educational space. Categories were constructed from the observational data, allowing the development of a second localized mid-level model that described the key contextual factors that influenced the planetarium teaching and learning space. In addition to addressing the initial lines of inquiry, the localized models were then supplemented with broader cognitive models, in particular Working Memory (WM) and Cognitive Load Theory (CLT). By combining both models with the explanatory power from WM and CLT, a Model for Curriculum Design in the Planetarium (MCDiP) was developed to systematically shape the digital planetarium as an effective teaching and learning space.
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    Classification of multiwavelength transients with machine learning
    (2019) Sooknunan, Kimeel; Lochner, Michelle; Bassett, Bruce
    With the advent of powerful telescopes such as the Square Kilometre Array (SKA), its precursor MeerKAT and the Large Synoptic Survey Telescope (LSST), we are entering a golden era of multiwavelength transient astronomy. The large MeerKAT science project ThunderKAT may dramatically increase the detected number of radio transients. Currently radio transient datasets are still very small, allowing spectroscopic classification of all objects of interest. As the event rate increases, follow-up resources must be prioritised by making use of early classification of the radio data. Machine learning algorithms have proven themselves invaluable in the context of optical astronomy, however it has yet to be applied to radio transients. In the burgeoning era of multimessenger astronomy, incorporating data from different telescopes such as MeerLICHT, Fermi, LSST and the gravitational wave observatory LIGO could significantly improve classification of events. Here we present MALT (Machine Learning for Transients): a general machine learning pipeline for multiwavelength transient classification. In order to make use of most machine learning algorithms, "features" must be extracted from complex and often high dimensional datasets. In our approach, we first interpolate the data onto a uniform grid using Gaussian processes, we then perform a wavelet decomposition and finally reduce the dimensionality using principal component analysis. We then classify the light curves with the popular machine learning algorithm random forests. For the first time, we apply machine learning to the classification of radio transients. Unfortunately publicly available radio transient data is scarce and our dataset consists of just 87 light curves, with several classes only consisting of a single example. However machine learning is often applied to such small datasets by making use of data augmentation. We develop a novel data augmentation technique based on Gaussian processes, able to generate new data statistically consistent with the original. As the dataset is currently small, three studies were done on the effect of the training set. The classifier was trained on a non-representative training set, achieving an overall accuracy of 77.8% over all 11 classes with the known 87 lightcurves with just eight hours of observations. The expected increase in performance, as more training data are acquired, is shown by training the classifier on a simulated representative training set, achieving an average accuracy of 95.8% across all 11 classes. Finally, the effectiveness of including multiwavelength data for general transient classification is demonstrated. First the classifier is trained on wavelet features and a contextual feature, achieving an average accuracy of 72.9%. The classifier was then trained on wavelet features and a contextual feature, together with a single optical flux feature. This addition improves the overall accuracy to 94.7%. This work provides a general approach for multiwavelength transient classification and shows that machine learning can be highly effective at classifying the influx of radio transients anticipated with MeerKAT and other radio telescopes.
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    Compact and Extended Radio Sources Classification using Deep Convolutional Neural Networks
    (2019) Alhassan, Wathela; Taylor, A R; Vaccari, Mattia
    Upcoming surveys with new radio observatories such as the Square Kilometer Array will generate a wealth of imaging data containing large numbers of radio sources. Different classes of radio sources can be used as tracers of the cosmic environment, including the dark matter density field, to address key cosmological questions. Classifying these sources based on morphology is thus an important step toward achieving the science goals of next generation radio surveys. Extended Radio Sources have been traditionally classified as Fanaroff-Riley (FR) I and II, although some exhibit more complex 'bent' morphologies arising from environmental factors or intrinsic properties. In this work we present the FIRST Classifier, an on-line system for automated classification of Compact and Extended radio sources. We developed the FIRST Classifier based on a trained Deep Convolutional Neural Network Model to automate the morphological classification of compact and extended radio sources observed in the FIRST radio survey. Our model was trained independently for 20 times and achieved an average accuracy, precision, recall and F1 of 0.98. The current version of the FIRST classifier is able to identify the morphological class for a single source or for a list of sources as Compact or Extended (FRI, FRII and BENT).
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    A comparison of the HI and CO velocity dispersions of nearby galaxies
    (2012) Mogotsi, Keoikantse Moses; De Blok, W J G
    Velocity dispersions are used to determine the stability of galactic disks against gravitational collapse to form stars, in some star formation recipes, in studies of gas dynamics, and to determine how much turbulence there is in the interstellar medium. Atomic hydrogen (HI) dispersions have been used in the studies of star formation and large-scale turbulence, despite stars forming in molecular clouds and the inner regions of galaxies being dominated by molecular gas. Carbon monoxide (CO) has been used as a tracer for molecular gas. In this work HI and CO dispersions were determined for a sample of nearby galaxies and they were compared to determine what the relationship between HI and molecular gas dispersions is.
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    Correcting the rotation curve of spiral galaxies for the non-circular motions induced by a bar
    (2017) Randriamampandry, Toky Herimandimby; Carignan, Claude
    The mass distribution of disk galaxies is usually determined through the use of rotation curves. This determination relies on two key assumptions; that the gas moves on circular orbits and that this motion traces the underlying gravitational potential. In the case of barred spiral galaxies the first assumption is false as the bar induces non-circular streaming motions in the gas. Therefore, the rotation curves of barred galaxies need to be corrected for the non-circular motions before being used for mass model analysis. In this dissertation, we use numerical simulations to quantify and correct for the non-circular flows induced by a bar. The aim is to investigate and quantify the effect of the bar properties on the amplitude of the non-circular motions. This is done by comparing the observational data such as rotation curves and bar properties with the kinematics and bar properties obtained from mocked galaxies. In chapter III, we examine the performance of ROTCUR and DISKFIT for deriving rotation curves from velocity maps of barred spiral galaxies using mock observations. Our results confirm that ROTCUR under-/overestimates measured rotation curves if the bar is aligned with one of the symmetry axes. The DISKFIT algorithm, which is specifically designed for barred galaxies only works for galaxies of intermediate bar orientations. In chapter IV, we quantify the magnitude of the non-circular flows and constraint the range of bar orientation angles where DiskFit fails by using Tree-SPH simulations from the GalMer database by Chilingarian et al . (2010 ). We found that the rotation curve obtained from ROTCUR was 40% smaller/larger than the expected velocities calculated from the gravitational potential when the bar is aligned with the major/minor axis. For the DISKFIT analysis, we found that DiskFit produces unrealistic values for all the models when the bar is within ten degrees of the symmetry axes. New hydrodynamic simulations of three disc galaxies (NGC 1300 , NGC 1530 and NGC 3621 ) are presented in chapter IV. Our objective is to create more realistic simulated galaxies that replicate the bar properties and velocity elds of the galaxies of our sample. The initial conditions for our simulations are determined through a Bayesian analysis of the azimuthally averaged rotation curve, the stellar surface brightness, and the gas surface density. The parameters posterior distribution functions (PDFs) combine with the disc stability parameters PDFs are used as criterion to select the model parameters. The velocities of the gas particles are transformed into velocity maps and compared with the observed galaxies. We are able to reproduce the bar properties and kinematics of the three galaxies in our sample. These findings imply that a tailored simulation is an effective way of investigating non-circular flows in disc galaxies especially when the bar orientation is close to the minor or major kinematical axis of the galaxy.
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    Deep learning for supernovae detection
    (2017) Amar, Gilad; Bassett, Bruce
    In future astronomical sky surveys it will be humanly impossible to classify the tens of thousands of candidate transients detected per night. This thesis explores the potential of using state-of-the-art machine learning algorithms to handle this burden more accurately and quickly than trained astronomers. To this end Deep Learning methods are applied to classify transients using real-world data from the Sloan Digital Sky Survey. Using cutting-edge training techniques several Convolutional Neural networks are trained and hyper-parameters tuned to outperform previous approaches and find that human labelling errors are the primary obstacle to further improvement. The tuning and optimisation of the deep models took in excess of 700 hours on a 4-Titan X GPU cluster.
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    A deep near infrared survey along the Norma Wall
    (2010) Riad, Ihab F; Kraan-Korteweg, Renée C;Woudt, Patrick Alan
    The multi-wavelength surveys have improved our understanding of the extent and shape of the Norma Wall. However, the most central part of the Norma Wall remains poorly understood because of the high levels of extinction and star-crowding at these low Galactic latitudes. In this thesis, I present an imaging survey in the three near-infrared bands (J, H, Ks) along the most obscured part of the Norma Wall, the so called Norma Wall Survey (NWS). It has the goal to uncover the galaxy distribution in this area. The near-infrared waveband was selected because it is less affected by dust extinction than the optical. In addition, near-infrared surveys are sensitive to early-type galaxies which are better tracers of the mass in galaxies.
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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 deep optical ZoA galaxy catalogue in Vela first indications of previously hidden large-scale structures
    (2012) Von Maltitz, Kosma; Kraan-Korteweg, Renée C; Woudt, Patrick Alan
    This thesis presents a deep optical galaxy catalogue of the Vela region in the Zone of Avoidance (I b < 10°.245 < I < 280. This region was searched for galaxy candidates by optical inspection of IIIaJ (3950 A to 5400 A) film copies of the ESO/SRC sky survey as part of an effort to reduce the ZoA.