The Local Group's peculiar motion with respect to the cosmic microwave background has been measured with high precision and found to be ~627±22 km s-1, but the source of this motion is not clear. Studies to constrain the fractional contribution of the local mass overdensity, particularly in the Great Attractor region and the more distant Shapley Supercluster, remain contradictory. This work is an attempt to disentangle these contributions by measuring a redshift-independent distance to the Norma cluster, which lies at, or close to, the core of the Great Attractor, in order to determine its peculiar velocity. A reliable measurement of the distance and peculiar velocity of Norma will help clarify the velocity flows in the Great Attractor region. The challenge is with observing in the Zone of Avoidance since star crowding and Galactic extinction effects are severe at such low Galactic latitudes. High quality near-infrared images have been used in the photometric analysis for the Norma cluster sample so as to minimise the effect of Galactic extinction. In addition, the imaging cameras used have a small pixel scale resulting in well resolved images for reliable foreground star-subtraction hence accurate photometry. In order to determine the redshift-independent distance, I applied two independent, complemen- tary methods: the Fundamental Plane and the Tully-Fisher relations. The redshift-independent distance and the peculiar velocity of the Norma cluster were measured using 1) the Ks-band Fundamental Plane analysis for 31 galaxies using the 3.6m New Technology Telescope (NTT) at the European Southern Observatory (ESO) 2) the J- and Ks-band Fundamental Plane analysis for 31 galaxies using the Japanese 1.4 m InfraRed Survey Facility (IRSF) at the South African Astronomical Observatory (SAAO) 3) the combined NTT and IRSF Ks-band Fundamental Plane analysis for 41 galaxies 4) the Ks-band Tully-Fisher analysis for 12 galaxies using near-infrared images from the IRSF and HI profiles obtained from the 64-m Parkes radio telescope.

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.

Includes bibliographical references (p. 167-175)

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.

Bibliography : p. 199-207. / Using a phenomenological approach, I have investigated the nonlinear properties of the pulsations of two white dwarfs, namely DA G29-38 and DB GD358. The data at my disposal comprised numerous single- and multi-sites time series photometric campaigns, including light curves from four Whole Earth Telescope runs conducted respectively in 1988 and 1992 on G29-38, and in 1990 and 1994 on GD358. Thanks to their length and quality, several of these individual data sets have the beating between the excited eigenmodes resolved. Amplitude changes are nevertheless visible between these various amplitude spectra, suggesting the presence of intrinsic nonlinear processes. However, I showed that only when the spectral changes are drastic have time-dependent nonlinear phenomena to be invoked; no matter how long the data sets, mild seasonal amplitude variations can often be accounted for by beating between the eigen-modes and high order cross-frequencies when harmonic distortion is strong enough. From third order of perturbation, cross-frequencies naturally appear in the direct vicinity of the normal modes, which not only alter the simple eigenmultiplet structures thus rendering the mode identification more difficult, but also generate long and complex beating processes. In GD358, for instance, 153 such third order combination frequencies are expected to appear in the frequency range of each eigentriplet; a light curve spanning at least 9 months is thus necessary to resolve the period structure of this star. Drastic spectral changes were nevertheless recorded in both G29s38 and GD358 which could not be accounted for by such high order beating processes. I could securely conclude that the pulsations of both these stars experience intrinsic amplitude variations on time-scales ranging from days to years, suggesting that different nonlinear processes dominate the evolution of the pulsations at different time.

Bibliography: p. 321-330. / 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.

Includes abstract. Includes bibliographical references.

Bibliography: p. 243-248. / 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.

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.

Bibliography: leaves 75-82. / The purpose of this thesis is a discussion of the generation and influence of magnetic fields in gaseous disks. It builds mainly on the work of Vainstein and Ruzmaiken (1972) on the generation of galactic magnetic fields. Emphasis here is on application to accretion disks in the cataclysmic variables (CVs). These include novae, dwarf novae and nova-like variables. Disks are maintained by mass transferred from a Roche-lobe filling secondary star to a white dwarf. More information can be found in Warner (1976) and Robinson (1976). As is pointed out in the final chapter, the basic theory should have wider applicability.