Browsing by Author "Van Der Heyden, Kurt"

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    Open Access
    Detecting the magnetic cosmic web through deep radio polarization imaging
    (2017) Burnham-King, Lauren S; Van Der Heyden, Kurt; Taylor, Russ
    The polarisation of radio emission is one of the most powerful probes of magnetic fields in the cosmos. Faraday rotation of polarized radiation provides one of the methods to observe magnetic fields. Measuring the rotation of the polarisation angle of radiation from an extragalactic source over a broad radio bandwidth allows us to infer the properties of the magnetic fields that the radiation passed through on the path to the observer. In the last few decades, the presence of structure in the matter distribution of the universe has been observed. It remains an open question whether there are magnetic fields associated with this large-scale structure. Large-scale universe simulations allow us to investigate the effect of extragalactic magnetic fields on the spatial distribution of Rotation Measure (RM) of radio sources that will be detected in deep radio images with MeerKAT. We constructed lightcones out to z = 1 from large-scale universe simulations as a base for our model and assemble a routine to trace large scale structures, attach magnetic fields to the structure and construct RM observations. The aim is to explore whether deep MeerKAT continuum observations will be able to detect magnetic fields associated with large-scale structure (the so-called magnetic cosmic web).
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    Open Access
    Gas flows and feedback in star-forming galaxies
    (2018) Tafere, Melaku S.; Vaisanen, Petri; Van Der Heyden, Kurt
    In this MSc thesis, I investigated the kinematics of Hα emission from the hot ionized and NaD absorption from cool neutral gas in a sample of 40 nearby Luminous Infrared Galaxies (LIRGs: LIR ≥1011L) from Supernovae and starbursts in the infrared, SUNBIRD survey imaged with the Southern African Large Telescope (SALT). This project can be seen in two major areas. The first is analyzing the emission and absorption spectra, in this case from the hot ionized gas and cool gas respectively. Two routines were used for the profile fitting, to get the centroid wavelength of both the emission and absorption lines; Gaussian and Gaussian-Hermite functions were used. The fitting result provides an indication of the origin of an outflowing gas; nuclear or disk driven?. Secondly, I plotted and compared the velocities of hot ionized and cool neutral gas. Three different methods were used to estimate the systemic velocity Vsys for the accurate estimation of outflowing velocity. For instance, an outflowing NaD upto 380 kms−1 and inflowing gas up to -100 kms−1 relative to Hα is seen at the optical nucleus of the NGC6240[PA=45] and NGC1204[PA=253] galaxies respectively. I tested if there was a relation between star formation rate (SFR) and an outflowing gas and also whether the inflowing gas triggering new nuclear SF. I find the gas of two of our targets escaping the potential well of the host galaxy. There was also an expectation that the out flowing velocity would correlate with SFR, unfortunately I do not see any correlation, though it needs further investigation of all offset velocities of the gas with respect to stellar absorption lines and it also requires detail analysis of the of projection or orientation effect on the galaxy.
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    Open Access
    Probabilistic methods for radio interferometry data analysis
    (2017) Natarajan, Iniyan; Van Der Heyden, Kurt; Smirnov, Oleg M; Zwart, Jonathan T L
    Probability theory provides a uniquely valid set of rules for plausible reasoning. This enables us to apply this mathematical formalism of probability, also known as Bayesian, with greater flexibility to problems of scientific inference. In this thesis, we are concerned with applying this method to the analysis of visibility data from radio interferometers. Any radio interferometry observation can be described using the Radio Interferometry Measurement Equation (RIME). Throughout the thesis, we use the RIME to model the visibilities in performing the probabilistic analysis. We first develop the theory for employing the RIME in performing Bayesian analysis of interferometric data. We then apply this to the problem of super-resolution with radio interferometers by performing model selection successfully between different source structures, all smaller in scale than the size of the point spread function (PSF) of the interferometer, on Westerbork Synthesis Radio Telescope (WSRT) simulations at a frequency of 1.4 GHz. We also quantify the change in the scale of the sources that can be resolved by WSRT at this frequency, with changing signal-to-noise (SNR) of the data, using simulations. Following this, we apply this method to a 5 GHz European VLBI Network (EVN) observation of the flaring blazar CGRaBS J0809+5341, to ascertain the presence of a jet emanating from its core, taking into account the imperfections in the station gain calibration performed on the data, especially on the longest baselines, prior to our analysis. We find that the extended source model is preferred over the point source model with an odds ratio of 109 : 1. Using the flux-density and shape parameter estimates of this model, we also derive the brightness temperature of the blazar (10¹¹-10¹² K), which confirms the presence of a relativistically boosted jet with an intrinsic brightness temperature lower than the apparent brightness temperature, consistent with the literature. We also develop a Bayesian criterion for super-resolution in the presence of baseline-dependent noise and calibration errors and find that these errors play an important role in determining how close one can get to the theoretical super-resolution limit. We then proceed to include fringe-fitting, the process of solving for the time and frequency dependent phase variations introduced by the interstellar medium and the Earth's atmosphere, in our probabilistic approach. Fringe-fitting is one of the first corrections made to Very Long Baseline Interferometry (VLBI) observations, and, by extending our method to include simultaneous fringefitting and source structure estimation, we will be able to perform end-to-end VLBI analysis using our method. To this end, we estimate source amplitude and fringe-fitting phase terms (phase offsets and delays) on 43 GHz Very Long Baseline Array and 230 GHz Event Horizon Telescope (EHT) simulations of point sources. We then perform model selection on a 5 μas extended Gaussian source (one-fourth the size of the PSF) on a synthetic 230 GHz EHT observation. Finally we incorporate turbulent time-varying phase offsets and delays in our model selection and show that the delays can be estimated to within 10-16 per cent error (often better than contemporary software packages) while simultaneously estimating the extended source structure.