We are now firmly in the era of multi-messenger astronomy. The detection of the first binary black hole (BBH) merger in GW150914 [1] opened up the era of gravitational wave astronomy, with a further 9 such mergers being detected during the first two observing runs (O1 and O2) of the LIGO Scientific and Virgo Collaborations (LVC). The first – and currently only – multi-messenger source was detected during O2 and was caused by the merger of two neutron stars in a binary system (BNS) [2]. The electromagnetic (EM) counterparts to GW170817 [3] were observed across the EM spectrum by numerous observing facilities, with implications across a vast range of scientific disciplines. Optical/nearinfrared observations demonstrated that the emission was due to a kilonova powered by the radioactive decay of r-process material produced during the merger. For the first time short gamma-ray bursts were convincingly linked to BNS mergers, as observed in GRB170817A [3]. The third LVC observing run (O3) began 2019 April 1 and concluded 2020 March 27. The signal from GW190425 [4] was likely caused by the coalescence of two neutron stars, with the system having a larger total mass than any currently known BNS system. Furthermore, the detection of GW190412 revealed the first BBH merger with a clearly unequal mass ratio of q = m2/m1 = 0.28 along with significant higher-multipole gravitational radiation [5].

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.

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.

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.

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.

I have conducted a systematic survey of eleven nearby novalike cataclysmic variables in the radio band, using the MeerKAT radio interferometer. Radio emission is detected from four out of the eleven systems: IM Eri, RW Sex, V3885 Sgr and V603 Aql. While RW Sex, V3885 Sgr and V603 Aql had been previously detected, this is the first reported radio detection of IM Eri. These observations have doubled the sample of non-magnetic cataclysmic varaibles with sensitive radio data. I observe that at these radio detection limits, a specific optical luminosity &amp; 2.2 × 1018 erg s−1 Hz−1 (corresponding to MV . 6.0) is required to produce a radio detection. I also find that the X-ray and radio luminosities of the detected novalikes are on an extension of the LX ∝ L ∼0.7 R power law originally proposed for non-pulsating neutron star low-mass X-ray binaries. No other correlations are found between the radio emission and emission in other wavebands or any other system parameters for the existing sample of radio-detected novalikes. In-band (0.9–1.7 GHz) radio spectral indices are measured, and are found to be consistent with reports from earlier work. I construct broad spectral energy distributions for this sample from published multi-wavelength data, and use them to place constraints on the mass transfer rates of these eleven systems. Finally, I also present the results of time-resolved optical spectroscopy of two little-studied systems in the sample: V5662 Sgr and LSIV -08 3. I obtain orbital periods for these two systems that are consistent with previous measurements in the literature.

We aim to characterise the X-ray binary population as a function of the local stellar population (in terms of age, metallicity, and stellar density) in the Magellanic Bridge, the interconnecting region between the Small and Large Magellanic Cloud. Gardiner and Noguchi (1996) suggest that closest approach between Small and Large Magellanic Cloud, as evidenced by dramatic phase shift in star formation, occurred approximately 200 Myr ago. During the approach, gas had been tidally stripped (most likely from the Small Magellanic Cloud) into the interconnecting Bridge. According to models of star formation history (Harris, 2007), alongside optical surveys of the Bridge (Skowron et al., 2014), there is strong evidence to suggest that the young, low metallicity stellar population formed in situ, rather than being tidally stripped from either Magellanic Cloud. Three fields located near the Western Bridge observed by Harris (2007) were also observed with the XMMNewton. Cross-matching between optical and X-ray sources was performed, and any interesting matches were followed up with spectroscopic analysis, using the 1.9 m telescope located in Sutherland. A Be/X-ray Binary (BeXRB) candidate is discussed, and if confirmed, will be the furthest known BeXRB from the SMC.

Bibliography: pages 136-138. / The design and construction of an astronomical polarimeter is described and an evaluation made of its performance. Extensive observations of cataclysmic variables with emphasis on the AM Her and DQ Her classes are then presented. After consideration of the basic principles involved in the development of an efficient and accurate polarimeter, a design using two super-achromatic retarders (a 1/4 wave and a 1/2 wave) rotating above a fixed analyser was adopted. This permitted simultaneous linear and circular polarisation measurements, or, by rearranging the order of the retarders in the beam, linear polarisation measurements alone, or circular polarisation measurements alone, with enhanced efficiency. The polarimeter was found to have extremely low instrumental polarisations and, because of the superachromatic retarders used, the efficiency correction factors were very close to 1 at all wavelengths. The polarisations are calculated at the telescope and the light curve at a higher time resolution may also be recorded if this is required. Extensive sets of observations using the polarimeter were obtained for six of the ten AM Her variables (or "polars"). EF Eri and El405-451 were observed most. Evidence was found in El405-451 for movement of the apparent location of the accretion region on the primary star and the inclination and magnetic dipole off set from the rotation axis was determined. This allowed a comparison to be made between the competing models for the cyclotron emission, showing that those which take into account the temperature structure of the accretion region provide the best results. Observations and an analysis of the polarisation data from H0139-68, E2003+225, VV Puppis and PG1550+191 are also presented in some detail. A final chapter presents results from observations made to detect a modulation in the polarisation at the rotation period of the primary in the DQ Her variables. Upper limits are set for 4 members of the class and the implications of the results are discussed.

No description available.

Includes abstract / Includes bibliographical references. / Long-term variability in all types of X-ray binaries is a well established characteristic, but due to observation limitations and lack of long-term monitoring capability these variations have not been studied before in a systematic way. In this thesis, we exploit the ~ 16 yr optical light curves from the MACHO and OGLE databases, and combine these with archival XMM-Newton X-ray observations to study the long-term properties of Be/X-ray binaries and Supersoft X-ray Sources in the Magellanic Clouds.