Includes bibliographical references (p. 111-113)

Includes abstract. Includes bibliographical references (leaves 95-100).

Planetary Nebulae (PNe) are defined as the ionized shells of circumstellar gas ejected through an intense stellar wind at the end of the star's life. PNe come in different shapes, from spherical to highly complex, non-spherical shapes. Mass loss in AGB stars is presumed as the shaping mechanism but how it results in different PNe morphologies is still unclear. Binary central stars that have undergone common envelope evolution are thought to be a possible solution to this longstanding problem. Using photometry from the OGLEIV survey, we present the newly identified close binary central stars of Planetary Nebulae (CSPNe), six in total. Of the six PNe with close binary CSPNe, one looks spherical which presents a very interesting argument in terms of our understanding of PNe evolution. The orbital distribution is derived and compared against current orbital distribution for binary CSPNe, with most binaries from the distribution exhibiting orbital periods less than a day. A binary fraction estimate of 6% is presented and possible cases are discussed that might have influenced our estimation to be different from the expected 10-15%.

A detailed study of the cosmological evolution in a particular vector-tensor theory of gravity with a potential and a Galileon-motivated interaction terms is presented. The evolution of vector field self interactions that are relatively related to Galileon fields throughout the expansion history of the universe is considered and a classification of the parameters M (mass term) and H (Hubble parameter) according to the behaviour of the field in each cosmological epoch is carried out. In particular, we obtain conditions for the parameters so that the field grows exponentially or oscillates with decreasing amplitude. We also obtain an autonomous system for the inflationary case. The general features of the phasemaps are given and the critical point is appropriately characterised. It is not possible to obtain an autonomous system for radiation and matter dominated epochs hence, we consider other analytical methods. We obtain eigenvalues and hence, phasemaps. The general features of the phasemaps are given and the point to which the trajectories on the phasemaps converge is appropriately characterised. Therefore, we show that it is possible to obtain a wide variety of behaviours or interesting phenomenologies for the cosmological evolution of vector field self-interactions that are relatively related to Galileon fields by choosing suitable values for the parameters M and H of given conditions.

Planetary Nebulae (PNe) are the product of Asymptotic Giant Branch (AGB) evolution. Evolved from Solar-like intermediate mass stars (0.8 – 8M), they have a hot, radiating core that ionizes the gas of the expelled envelope, producing a glowing nebula. The core eventually evolves into a white dwarf (WD), following the WD cooling track. Complex, aspherical morphologies are observed in PNe and binary central stars (CSs) have been the favoured explanation for deviations from spherical symmetry. Finding and characterizing the population of binary CSs is thus important to understand the physics behind their morphologies. The objects of this study are Hen3-1333, Hen2-113 and Hen2-47, all with WolfRayet (WR) CSs that commonly exhibit fast, dense stellar winds. All exhibit multipolarity in their young nebulae, Hen3-1333 has a disk and dual-dust chemistry, while the other two have central stars offset from the geometric centre of their nebulae. The objects were chosen because most of these features, especially multipolar morphologies, are not well represented amongst PNe with known binary CSs. Here we develop a quantitative time-series analysis to determine whether these objects have binary CSs and develop constraints to permissible orbital parameters. The High Resolution Spectrograph (HRS) of the Southern African Large Telescope (SALT) was used to collect échelle spectroscopic data over 3 years and The Exoplanet Survey Satellite (TESS) was used to obtain photometric data for the objects. The medium resolution (MR) mode (R ≈ 40000) was chosen and 58, 60, and 35 spectra were collected for Hen3-1333, Hen2-113 and Hen2-47, respectively, with an average S/N of 40 at 4495 Å. The TESS data had continuous sampling (30 min cadence) recorded for an orbit length of 27.4 days. Using cross-correlation and Gaussian line fitting, radial velocity (RV) time-series were compared to lightcurves determined from the TESS data. Lomb-Scargle periodograms were used to search for periodic variability in the RV and photometry time-series data. The results were discussed based on short (0 – 10 days), intermediate (10 – 103 days) and long (103 – 104 days) orbital period ranges. Compatible scenarios for each range were estimated by combining observational constraints with different parameters expected for assumed companion star types. The quantitative variability analysis excludes short orbital period binary systems, suggesting that if their multiple features are due to binary interactions, the most likely case is the long orbital period range. If the variability observed is due to a companion, rather than pulsations from the CS, the companion masses, 0.10 – 1.36 M for Hen3-1333, 0.043 – 1.27 M for Hen2-113 and 0.077 – 1.36 M for Hen2-47, correspond to main sequence stars and dwarfs.

Galaxy environment plays a significant role in galaxy evolution. While most work has focused on the effect of cluster and group environment, large scale structures, such as filaments and voids, may provide additional contributions. Recently, various authors have found correlations between galaxy morphology, stellar mass and colour, and the distance to cosmic web filaments in both simulations and data. However, the effect of filaments on the gas supply of galaxies is still under investigation due to conflicting results. I use data from the Resolved Spectroscopy of a Local Volume (RESOLVE) survey, which is a low-redshift galaxy census complete down to log M?/M= 8.9, to study the relationship between galaxy properties and the cosmic web. I use the Discrete Persistence Structures Extractor (DisPerSE), a topology-based software package, to map filaments in the RESOLVEA field. This work shows that galaxies in RESOLVE-A have, higher stellar masses close to filaments. When accounting for the additional effect of groups, I find no variation in the colour of galaxies with respect to their distance to filament. Low-mass (log M?/M< 9.7) galaxies increase in gas fraction with increasing distance to filament, which may indicate that low-mass galaxies lose gas as they enter filaments.

Open clusters are essential laboratories for understanding stellar evolution, as they allow constraints to be placed on stellar ages and luminosities. As distance indicators they are also important tracers of star formation in the Milky Way. One such cluster is Trumpler 27 that we identified for detailed study.The aim of the study is to estimate the distance, radial velocity, age, membership and reddening of the cluster using both spectroscopic and photometric techniques. We used new spectroscopic data collected from SAAO's 1.9m telescope together with existing photometric data from catalogues in the study of Trumpler 27. The spectra collected were classified using spectral atlases to determine the reddening in the field. Stars of the cluster were identified using selection techniques that made use of both infrared and optical Q parameters, spatial distribution and photometric techniques. The result from this work suggest that Trumpler 27 is made up of ~ 55 stars which are at different stages of evolution. The stars include main sequence stars, blue supergiants, two cool supergiants and maybe two WR stars.

The Southern African Large Telescope Fabry-Pérot interferometer has been used in its medium resolution mode to observe three of 30 galaxies of the MHONGOOSE galaxy sample for which very deep HI observation (typically 200 hours/galaxy) will be obtained with MeerKAT. So optical high spatial resolution of 2 arcsec data of NGC 7361, NGC 7424 and NGC 7793 have been obtained. The major object of this thesis was to test SALT Fabry-Pérot medium resolution data in order to pursue the survey of all the MHONGOOSE sample, and to be able to compare the accuracy of the kinematic results. Through this work, some FORTRAN based routines have been improved and they allow us to compute kinematic maps with good accuracy. Indeed, the velocities measured from the profiles of the Hα emission in the data cube are accurate with the range of 1 km s⁻¹ to 10 km s⁻¹. So, we computed the kinematic maps and the rotation curves of the three galaxies using DiskFit and ROTCUR. For NGC 7361 and NGC 7424 we compared the rotation curves derived by both methods. For NGC 7793 we were also able to compare our results with previous studies.

To date, our understanding of the formation and evolution of galaxies attributes a very important role to the neutral hydrogen (HI) gas since it constitutes the reservoir of fuel out of which galaxies form stars. In their evolution, galaxies interact with each other and with their environment, and very often these interactions leave fingerprints in the HI distribution. The extended HI envelopes of galaxies are sensitive tracers of those tidal interactions. In the present study, we map the HI distribution of galaxies in a ~1.5° X 2.5° region of the Virgo cluster using the Karoo Array Telescope (KAT-7) and the Westerbork Synthesis Radio Telescope (WSRT). With a total observing time of ~78 hours with the KAT-7 and 48 hours with the WSRT, we search for low HI column density features in the region. Despite the different observing time and beam size of the two telescopes, we reach similar column density sensitivities of NHI ~1 X 10¹⁸ atoms cm⁻² over 16.5kms⁻¹. With a new approach, we combine the two observations to map both the large and small scale structures. We detect, out to an unprecedented extent, an HI tail of ~60 kpc being stripped off NGC 4424, a peculiar spiral galaxy. The properties of the galaxy, together with the shape of the tail, suggests that NGC 4424 is a post-merger galaxy undergoing a ram pressure stripping as it falls into the centre of the Virgo Cluster along a filamentary structure. We also give the HI parameters of the galaxies detected.

This thesis aims at using the Infrared Survey Facility (IRSF) JHKₛ bands, Two Micron All Sky Survey (2MASS) JHKₛ bands and Wide Field Infrared Survey Explorer (WISE) W1-W4 bands isophotal magnitudes to understand the cosmic flow associated with Zone of Avoidance (ZoA) galaxies.