Coronal rain composed of cool plasma condensations falling from coronal heights along magnetic field lines is a phenomenon occurring in active region coronal loops. This work combines high-resolution observations and numerical simulations to understand the interplay between coronal rain and MHD oscillations. We analyse oscillations and kinematics of the coronal rain using high resolution observations. Two different regimes of transverse oscillations traced by the rain are detected: smallscale persistent oscillations driven by a continuously operating process and localized large-scale oscillations excited by a transient mechanism. The plasma condensations are found to move with accelerations largely below the free-fall rate. The observed evolution of the emission of the plasma at the loop top is found to exhibit clear signatures of a gradual cooling consistent with the limit cycle model and suggests the loop is going through a sequence of periodically repeating heating-condensation cycles. We further investigate the evolution and dynamics of coronal rain using 2.5D MHD simulations. We model the evolution of a cool plasma condensation in a gravitationally stratified coronal loop. The motion of plasma condensations is found to be strongly affected by the pressure of the coronal loop plasma. High coronal magnetic field or low condensation mass are found to lead to damped oscillatory motion of the condensations. The combined effect of plasma pressure gradients and magnetic tension force can therefore explain observed sub-ballistic motion and longitudinal oscillations of coronal rain. We finally address the possibility of excitation of loop oscillations by coronal rain. We carry out MHD simulations of a coronal loop containing a cool and dense condensation region near the loop apex. This is found to excite fundamental harmonic of a vertically polarised kink mode. As the condensations fall towards the loop footpoints, the fundamental mode period is found to decrease as a result of the change in distribution of mass along the loop. We also carry out simulations of a coronal loop with a siphon ow between the footpoints which is likely to arise in asymmetrically heated loops. The action of the centrifugal force associated with plasma moving along the curved axis of the loop is found to excite vertically polarised loop oscillations. We find that flows with realistic speeds are sufficient to excite oscillations with observable amplitudes. We therefore propose coronal rain as a possible excitation mechanism for transverse loop oscillations.

Perturbations of coronal structures by impulsive events such as solar flares generate waves which are interpreted with MHD theory. These waves allow plasma processes to be studied, and seismology of the local plasma parameters to be performed. The focus of this thesis is the detailed observational study of these waves. A statistically significant number of kink oscillations of coronal loops were analysed. The measured periods scale linearly with the estimated loop length, as expected from the standard interpretation of the waves as the global fundamental standing mode. A typical kink speed of Ck=(1300±50) kms−1 is obtained. A linear scaling of the damping time with period is observed, and non-exponential damping profiles were noted. The study was then extended to determine if there is any scaling between the quality factor of the oscillations and the oscillation amplitude. Selected events from the kink oscillation catalogue were analysed in detail, and it was found that the damping profiles of several oscillations were better fit by a Gaussian envelope than an exponential one. These damping profiles were then used to perform seismological inversions, including the transverse density structure of the loops. The obtained transverse density profile was compared to the observed intensity profile for one loop, using forward modelling and Bayesian inference, where good agreement was found. The intensity cross-sections of 233 coronal loops were analysed. Assuming an isothermal and cylindrical cross-section the transverse density structure of the coronal loop plasma was inferred. Several models for the transverse density profile were quantitatively compared. Very strong evidence was found for the existence of an inhomogeneous layer where the density varies smoothly between the rarefied background plasma and the dense centre of the loop. In a significant number of cases the width of this layer was high enough to conclude that the loop does not have a core at all and has a continuously varying transverse density profile. Finally, a flaring event was analysed which excites a series of propagating EUV intensity perturbations, and simultaneously produces a series of features in radio spectrometer data. This is the first observation which links quasi-periodic fast waves observed in the EUV band to quasi-periodic features in radio spectra. 2D numerical simulations of impulsively generated wave trains in coronal density enhancements are presented. This aims to establish how these waves are affected by initial perturbations which enter the non-linear regime, thereby establishing the feasibility of some of the mechanisms by which the observations presented could be explained.

Lyman break galaxies (LBGs), characterised by the eponymous spectral break at the electron transition energy of neutral hydrogen, are young, intensely star-forming, compact galaxies in the distant Universe. Due to the presence of hot, young stars within them, these galaxies are ideal candidates for the types of sources which contributed ionizing flux during the Epoch of Reionization by z ∼ 6. Not only can their study help us to learn more about the process of reionization, but LBGs are also primary sources for our understanding of the formation and evolution of galaxies. However, due to their large luminosity distance, and hence their apparent faintness and small projected sizes, they are difficult to study directly. A local analogue population, selected to reproduce the observed properties within their distant cousins, can be used to greatly improve the interpretation of distant LBGs. Such local Lyman break analogue (LBA) populations have been established for z ∼ 3 LBGs; however, there is significant evolution in the physical properties within galaxies between z ∼ 3 and z ∼ 5 − 7 (representing a difference of over one billion years in the age of the Universe), hence making it inappropriate to use the same analogue sample for both epochs. The establishment and study of an analogue sample of z ∼ 5 − 7 is described in this thesis. Building on the work of Stanway & Davies [2014], I selected an LBA candidate sample of 180 local (0.05 < z < 0.25) galaxies whose ultraviolet luminosity and colours matched those observed in the distant z ∼ 5 − 7 LBG population. I fit the spectral energy distribution (SED) of the candidates, deriving their stellar masses, dominant population ages, dust reddening, metallicities, star formation rates, and star formation rate densities. Comparing these properties with those found in the distant LBG sample, and depending on the age and mass cuts applied, approximately ∼ 40 − 70% of candidates are good local analogues. This confirms that galaxies exist in the local Universe whose physical properties are akin to those found in the distant cosmos. I have reduced and interpreted radio observations of a subset of the LBA candidates, confirming them to be young, star formation driven systems that do not host AGN. In fact, their stellar populations appear to be so young that in most sources no significant supernovae-driven radio continuum has been established. Their low ages also indicate that commonly used star formation rate (SFR) indicators, typically calibrated for stellar populations at ages > 100 Myr, are likely to underestimate their true SFRs. In order to determine the projected density of genuine LBA, I analysed low-resolution AAOmega spectroscopy for a sample of ∼ 230 photometrically selected LBA candidates. Combining this with the results of the SED fitting, the spatial density of genuine local Lyman break analogue galaxies lies between 24 and 40 per square degree. These findings have important implications for the distant LBG population, suggesting that galaxies with comparable star formation rate densities exhibit similar physical properties, and indicating that both LBAs and the distant LBG sources are undergoing bursty episodes of star formation (as opposed to a continuous star formation rate). From the high ionization parameters found in the local LBA sources, predictions can be made about potentially observable emission lines in the z ∼ 5 − 7 LBG population. Future surveys, such as the JWST and the LSST, are likely to shed further light on both this local analogue populations and their distant cousins.

Cataclysmic variables (CVs) are short–period (' 80 min to ' day) binaries in which a white dwarf accretes from a low–mass companion. Since both stellar components are structurally simple and there is a sufficiently large number of relatively bright CVs known, it is possible to carry out detailed observational population studies. Therefore CVs are one of the best–suited classes to test, constrain, and further develop our understanding of the evolution of all interacting compact binaries, such as black hole binaries, X-ray transients, milli–second pulsars and, more importantly, Type Ia Supernova (SN Ia), our yardsticks for measuring distances on cosmological distance scales. In this thesis, I present the study of a large sample of CV white dwarfs. Their effective temperatures are determined from the analysis of their ultraviolet Hubble Space Telescope (HST ) spectra and provide a test for the current models of CV evolution. Our results highlight the presence of a number of discrepancies between current population models and observations, particularly the lack of period bounce systems, i.e. highly evolved CVs with brown dwarf companions, which are predicted to make up for ' 40 . 70 % of the present day Galactic CV population. The combination of the HST data with optical phase–resolved X–shooter observations is a powerful tool to identify period bouncers. We study the spectral energy distribution of QZ Lib, in which we identify the coolest white dwarf at the longest orbital period below the period gap and we spectroscopically confirm the presence of a brown dwarf donor. These characteristics make QZ Lib the strongest period bouncer candidate identified so far. Although successful, this method requires the use of expensive space–based and large ground–based facilities, indicating the need for a more efficient observing strategy. We present here CHiCaS, the Compact binary HIgh CAdence Survey which represents the first systematic attempt to identify, via the detection of their eclipses, the elusive population of period bouncers. By the end of next year, this program will deliver one minute cadence lightcurves for ' 2:5 million objects as faint as g ' 21:5, along with full colour information. CHiCaS will also provide a complete and unbiased view into the short term variability of thousands of binaries, eclipsing systems, pulsating stars and CVs in the period gap. In our HST sample, we identify another remarkable CV: SDSS J123813.73–033932.9. Its lightcurve shows sudden increases in brightness, up to ' 0:45 mag, occurring quasi–periodically every 8–12 hours (the “brightenings”) and a nearly sinusoidal variation at half the orbital period (the “double–humps”). The HST observations show that these phenomena arise from the heating and cooling of a fraction of the white dwarf, which we conclude being caused by a modulation in the accretion rate owing to spiral density waves and thermal instabilities in the accretion disc.

The field of exoplanet atmospheres has advanced rapidly in the 16 years since the detection of sodium in the atmosphere of HD209458b. Giant planets on short period orbits (`hot Jupiters') have been key to this progression, due to their relatively characterisable atmospheres. Optical transmission spectroscopy revealed the first detection of an exoplanet atmosphere and has continued to play a vital role in atmospheric characterisation. In this thesis I present optical transmission spectroscopy of three hot Jupiters and place these in context within the LRG-BEASTS transmission spectroscopy survey. My ground-based transmission spectrum of WASP-52b revealed the presence of clouds in the planet's atmosphere and evidence for stellar faculae on the host. This study demonstrated that HST precision could be achieved from the ground. I also present ground-based transmission spectroscopy of HAT-P-18b. My precise transmission spectrum was well fitted with a Rayleigh scattering slope at the equilibrium temperature of the planet. The absence of broad sodium and potassium absorption led me to conclude that a high altitude haze was present. Finally I present a transmission spectrum of WASP-80b. I found that a haze was likely present in this planet's atmosphere and my transmission spectrum was inconsistent with a previously claimed detection of pressure-broadened potassium. My results are in agreement with the emerging correlation that cooler planets are less likely to have clear atmospheres. However, this correlation is still tentative. The LRG-BEASTS survey will test this relation and help improve our understanding of the underlying physics driving the formation of clouds and hazes in exoplanet atmospheres.

Galaxy groups and clusters (GGCs) hold a privileged position within the cosmological hierarchy. As the most recent structures to have formed, their abundances, spatial distribution and individual properties bear the indelible imprint of the background cosmology, initial conditions and their formation history, making them valuable probes of both cosmology and astrophysics. It has, however, become increasingly clear over the past decade that making use of these probes for precision cluster cosmology requires detailed, realistic predictions for the observed properties of GGCs. Producing these necessitates the use of large cosmological hydrodynamical simulations with realistic ‘sub-grid’ prescriptions for baryonic physics. One mechanism in need of addressing is the effect of the cosmic background of massive neutrinos. As these remain relativistic to relatively late times, they will free-stream out of overdensities, altering the formation of large-scale structure (LSS). If this effect can be accurately modelled, it presents an independent method of constraining the value of the neutrino mass by means of LSS observations. This thesis makes use of the cosmo-OWLS and BAHAMAS cosmological hydrodynamical simulation suites to explore the separate and combined effects of baryon physics (particularly feedback from active galactic nuclei, AGN) and the free-streaming of massive neutrinos on large-scale structure. I focus on five diagnostics: i) the halo mass function; ii) halo mass density profiles; iii) the halo mass concentration relation; iv) the clustering of haloes; and v) the clustering of matter; and I explore the extent to which the effects of baryon physics and neutrino free-streaming can be treated independently. In comparing to the GAMA observations, I find that these data provide insufficient statistical power to constrain the value of the summed neutrino mass. This is primarily due to the intrinsic scatter in the stellar mass - halo mass relation, and the strong dependence of the sensitivity on precise mass binning. As a consequence, more precise estimations of the halo mass will be required in future work seeking to utilise this metric. Finally, I find that the clustering of simulated BAHAMAS groups is remarkably consistent with that of observed GAMA groups. This lends additional weight to the argument that BAHAMAS accurately reproduces the properties of the GGC population, and supports its use as a cosmological tool.

This thesis describes the design, construction and characterisation of a spectrograph operating in the optical range 4000-8000 Å and its application in the observation and classification of transient astronomical sources.

This thesis aims to understand more about the developmental histories of galaxies and their internal components by studying the wavelength dependence of their spatial structure. I use a large sample of low-redshift galaxies with optical–near-IR imaging from the GAMA survey, which have been fitted with Sersic and Sersic + exponential functions in nine wavebands simultaneously, using software developed by the MegaMorph project. The first section of this thesis examines how the sizes and radial profiles of galaxies vary with wavelength. To quantify the wavelength dependence of effective radius I use the ratio, R, of measurements in two restframe bands. The dependence of Sersic index on wavelength, N, is computed correspondingly. I show that accounting for different redshift and luminosity selections partly reconciles variations between several recent studies. Dividing galaxies by visual morphology confirms the behaviour inferred using morphological proxies, although our quantitative measurements allow me to study larger and fainter samples. I then demonstrate that varying dust opacity and disc inclination can account for features of the joint distribution of R and N for late-type galaxies. However, dust does not appear to explain the highest values of R and N. The bulge-disc nature of galaxies must also contribute to the wavelength-dependence of their structure. The second section of this thesis studies radial colour gradients across the galaxy population. I use the multi-wavelength information provided by MegaMorph analysis of galaxy light profiles to calculate intrinsic colour gradients, and divide into six subsamples split by overall Sersic index (n) and galaxy colour. I find a bimodality in the colour gradients of high- and low-n galaxies in all wavebands which varies with overall galaxy luminosity. Global trends in colour gradients therefore result from combining the contrasting behaviour of a number of different galaxy populations. The ubiquity of strong negative colour gradients supports the picture of inside-out growth through gas accretion for blue, low-n galaxies, and through dry minor mergers for red, high-n galaxies. An exception is the blue high-n population which has properties indicative of dissipative major mergers. In the third section of this thesis I apply bulge-disc decompositions to my sample of galaxies, in order to discover the structural origin of the wavelength dependences found in the previous two chapters. I find that most galaxies with a substantial disc, even those with no discernible bulge, display a high value of N. The increase in Sersic index to longer wavelengths is therefore intrinsic to discs, apparently resulting from radial variations in stellar population and/or dust reddening. Similarly, low values of R (< 1) are found to be ubiquitous, implying an element of universality in galaxy colour gradients. I also study how bulge and disc colour distributions vary with galaxy type. I find that, rather than all bulges being red and all discs being blue in absolute terms, both components become redder for galaxies with redder total colours. I even observe that bulges in bluer galaxies are typically bluer than discs in red galaxies, and that bulges and discs are closer in colour for fainter galaxies. Trends in total colour are therefore not solely due to the colour or flux dominance of the bulge or disc.

Inflation is currently the most promising paradigm of the Early Universe. The simple paradigm involves a single canonical scalar field minimally coupled to gravity slowly rolling down a potential. In this thesis, we discuss an extension to the simple paradigm, multifield inflation, in which inflation is driven by more than one scalar field. Unlike in the single field paradigm, isocurvature perturbations could be non-vanishing and source curvature perturbation on superhorizon scales. Analytic model predictions during the slow-roll regime in some classes of multifield inflation models have been worked out in the literature. However, curvature perturbation may continue to evolve after slow-roll as isocurvature perturbations are not necessarily exhausted when inflation ends. In this thesis, by using the δ N formalism, we investigate the effects of perturbative reheating on the curvature perturbation and related observables in multifield models. By considering various two-field models, we demonstrate that the subsequent (p)reheating evolution is significant and must be taken into account even for perturbative reheating. How the model predictions evolve during reheating is a model dependent question, implying that models of multifield inflation cannot be compared to observations directly without specifying how reheating takes place. We also discuss a different class of two-field models, conformal inflation, which is locally scale invariant. Universal behaviour emerges as a critical phenomenon near the enhanced SO(1,1) or shift symmetry point, leading to model independent predictions. Going beyond the original model proposed by Kallosh and Linde, we show that this universal behaviour extends to more generalised models involving higher order derivatives for slow-roll potential driven inflation.

Jointing materials is an inevitable step in the fabrication of many high performance optical devices. Today there is an increasing demand, both from industry and academic research, for reliable techniques for jointing of optical components which can be performed simply and inexpensively, where the bonds possess high strength and precision with low mechanical loss. Several methods of bonding have been defined and employed over time. Currently, the most common techniques of bonding are optical contacting, diffusion, epoxy and glass frit. Each of these techniques has positive aspects which make them appropriate for a range of applications, but not ideal in cases where a thin optically transparent and mechanically strong joint between optical compounds is required. Hydroxide-catalysis bonding is an interesting candidate in such cases: it has already been demonstrated to have excellent performance with respect to mechanical stability, precision and strength in numerous applications, such as in space systems, optics and gravitational wave detectors. At present, there are not many papers that talk exhaustively about the optical properties of hydroxide-catalysis bonding. Most of the time, the published results are closely dependent on requirements imposed by the experiment that is being conducted. Since there are a lot of interesting potential applications and increasing industrial interest in usage of hydroxide-catalysis bonding, it is important to develop techniques to characterise the optical properties of these bonds. In this thesis, a non-destructive technique for measuring the optical properties of hydroxide-catalysis bonding is reported. More specifically, the bond refractive index and thickness are determined from reflectivity measurements. By applying this method, it will be possible to understand how the optical properties and chemistry of a hydroxide-catalysis bond vary when different bonding solutions and substrate materials are used and, consequently, to tailor better the bonds to various utilisations of interest. Sodium silicate solution at different concentrations with water and potassium hydroxide solution are used to bond fused silica and sapphire substrates. Curing at room temperature and at 100 °C for eight hours is chosen to study the influence of temperature on the properties of a hydroxide-catalysis bond. It was found that the bond optical reflectivities are less than 1% for fused silica samples and less than 10% for sapphire samples and they decrease over time. Bond refractive indices start from a value close to the refractive index of water and approach the refractive index of fused silica as the cure proceeds. Bond thicknesses cured at room temperature decrease over time plateauing at a more or less constant value, different for each case studied (about a few hundred nanometres), whereas bond thicknesses cured at high temperature seem to increase over time (less than about 400 nanometres).