Philosophiae Doctor - PhD / We present a new sample of distant powerful radio galaxies, in order to study their host-galaxy properties and provide targets for future observations of Hi absorption with new radio telescopes. We cross-match the Sydney University Molonglo Sky Survey radio catalogue at 843 MHz with the VISTA Hemisphere Survey near-infrared catalogue using the Likelihood Ratio technique, producing contour plots as a way to inspect by eye a subset of bright sources to validate the automated technique. We then use the Dark Energy Survey optical and near-infrared wavelength data to obtain photometric redshifts of the radio sources. We find a total of 249 radio sources with photometric redshifts over a 148 square degree region. By fitting the optical and near-infrared photometry with spectral synthesis models, we determine the stellar masses and star-formation rates of the radio sources. We find typical stellar masses of 1011−1012 M for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. We then report results from the first search for associated Hi 21 cm line absorption with the new MeerKAT radio telescope (shared-risk early science programme). We used a 16-antenna sub-array of MeerKAT to carry out a survey for Hi absorption in the host galaxies of nine powerful (L1.4 GHz > 1026 W Hz−1 ) radio galaxies at cosmological distances (z = 0.29 to 0.54). We found no evidence of absorption with 5σ optical depth detection limits. We only obtain a tentative absorption towards a radio source 3C 262 at z = 0.44 with significant ongoing star formation at a rate of 10.5 M yr−1 . The source consists of two radio lobes separated by 28.5 kpc with no evidence of a compact core. If the absorption arises from neutral gas from an extended disc, the line is redshifted by 79(21) km s−1 with respect to the nucleus and has an average Hi column density across the source of NHI ∼ 7 × 1019−20 cm−2 , which is consistent with the rate of star formation. But after further tests, we find that the Hi detection towards 3C 262 is likely to be an artefact. We conclude that the new correlator with 32 k channel resolution will be needed before searching for its associated absorption in MIGHTEE data.

No description available.

No description available.

Philosophiae Doctor - PhD / The standard model of cosmology (the ɅCDM model) has been very successful and is compatible with all observational data up to now. However, it remains an important task to develop and apply null tests of this model. These tests are based on observables that probe cosmic distances and cosmic evolution history. Supernovae observations use the so-called `standard candle' property of SNIa to probe cosmic distances D(z). The evolution of the expansion rate H(z) is probed by the baryon acoustic oscillation (BAO) feature in the galaxy distribution, which serves as an effective `standard ruler'. The observables D(z) and H(z) are used in various consistency tests of ɅCDM that have been developed. We review the consistency tests, also looking for possible new tests. Then the tests are applied, first using existing data, and then using mock data from future planned experiments. In particular we use data from the recently commissioned Dark Energy Survey (DES) for SNIa. Gaussian Processes, and possibly other non-parametric methods, used to reconstruct the derivatives of D (z) and H (z) that are needed to apply the null tests of the standard cosmological model. This allows us to estimate the current and future power of observations to probe the ɅCDM model, which is the foundation of modern cosmology. In addition, we present an improved model of the HI galaxy number counts and bias from semi-analytic simulations, and we use it to calculate the expected yield of HI galaxies from surveys with a variety of phase 1 and 2 SKA configurations. We illustrate the relative performance of the different surveys by forecasting errors on the radial and transverse scales of the BAO feature. We use the Fisher matrix method to estimate the error bars on the cosmological parameters from future SKA HI galaxy surveys. We find that the SKA phase 1 galaxy surveys will not contend with surveys such as the Baryon Oscillation Spectroscopic Survey (BOSS) whereas the full "billion galaxy survey" with SKA phase 2 will deliver the largest dark energy Figure of Merit of any current or future large-scale structure survey. / South African Square Kilometre Array Project (SKA) and German Academic Exchange Service (DAAD)

No description available.