Coronal implosions - the convergence motion of plasmas and entrained magnetic field in the corona due to a reduction in magnetic pressure - can help to locate and track sites of magnetic energy release or redistribution during solar flares and eruptions. Although this conjecture was proposed almost two decades ago, observa- tions of such phenomena are still rare, and even our understanding of it is far from complete. In this thesis, following an introduction to the background and techniques used, we first generalise the implosion idea based on its spirit concerning about the relationship between magnetic energy release and field shrinkage, which allows us to unite and explain three different phenomena, that is, peripheral implosions, inflows and dipolarisations, using only one single principle. Previous observations of apparent contractions in the periphery of active regions are mainly in a face-on state, which cannot exclude the possibilty of field inclining instead of a real contraction as the cause. This then motivates us to study an excellent event observed near the solar disk center, and evidence from both observations and coronal magnetic field extrapolations is found to support the implosion idea. In a unification of three main concepts for active region magnetic evolution implied by the observation, namely the metastable eruption model, the implosion conjecture, and the standard “CSHKP” flare model, the contraction of the field is explained by the removal of the erupting filament originally underneath rather than local magnetic energy dissipation in a flare invoked by previous authors. However, the observation and extrapolation results in the work above are indirect and still not adequate, as the complex structure of the solar atmosphere, and the simplified assumption and preprocessing in the extrapolation may lead us to a wrong conclusion. Thus in the following four carefully seleted events with the continuously contracting loops in an almost edge-on geometry are for the first time investigated, demonstrating the reality of contraction of field lines in the global coronal dynamics unambiguously. Meanwhile, two categories of implosions, flare- and eruption-driven, are identified, which could be interpreted well in the framework of the implosion conjecture, disproving other authors’ proposal. We also revisit one of the original assumptions of the implosion conjecture which may fail when a heavily-mass-loaded filament is involved, and in this case implosions can be suppressed, possibly served as an alternative explanation for their observational rarity. In the end, we move on to one of the generalised implosion types, i.e., the inflow, and also study other reconnection flows associated with it. Intrinsic to the well- accepted reconnection picture of a solar eruptive event, particularly in the standard model for two-ribbon flares (“CSHKP” model), are an advective flow of magnetized plasma into the reconnection region, expansion of field above the reconnection region as a flux rope erupts, retraction of heated post-reconnection loops, and downflows of cooling plasma along those loops. However, the evidence of these flows is still circumstantial and rare. We report in this work on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic observations of a flare in which all four flows are present simultaneously. This also includes spectroscopic evidence for a plasma upflow at the edge of the active region claimed by previous authors, which we suggest decomposing into two components, one associated with open field at quasi- separatrix layers, the other with large-scale expanding closed arcade field. The reconnection inflows are symmetric, and consistent with fast reconnection, and the post-reconnection loops show a clear cooling and deceleration as they retract. Unlike previous events observed at the solar limb which are obscured by complex foregrounds and thus makes the relationship between the plasma flows, the flare ribbons, cusp field and arcades formed in the lower atmosphere difficult to interpret, the disk location and favorable perspective of this event studied here have removed these ambiguities giving a clear picture of the reconnection dynamics. We end with a brief chapter summarizing the thesis and suggesting some future work.

Star formation is a chaotic process, involving the evolution and interaction of a wide variety of structures. The interstellar medium exhibits substructure over a range of scales, and the clusters which form from the densest parts of this material may be imprinted with this clumpy distribution. In this thesis, we describe and evaluate statistical tools for quantifying structures that are important to the star formation process, in order to constrain the underlying physics and robustly compare observations, simulations and synthetic observations. We describe the basic theory and some common applications of fractal theory in astronomy. We show that some common measures of fractal structure are inconsistent and that comparing values derived from different data types (e.g. continuum data of molecular cloud maps and discrete data of star distributions) can lead to confusion. We introduce the Q+ algorithm which quantities the substructure in star clusters in terms of a fractal distribution. We describe the derivation and validation of this method and apply it to observed and simulated data sets. We examine the possibility of applying this same analysis to continuum data by converting the greyscale image into a statistically representative distribution of points. We introduce the J plots algorithm which uses the principal moments of inertia of a two-dimensional pixelated structure to quantify its shape. We show that this can be used to identify the shapes of structures extracted from astrophysical images using dendrograms. We apply this method (i) to data from the Hi-GAL survey to demonstrate the identification of ring-like shapes, and (ii) to simulations of _lament formation to quantify the differences in structure resulting from the nature of turbulence in the accreting material.

Gravitational wave detectors are a new class of observatories aiming to detect gravitational waves from cosmic sources. All-reflective interferometer configurations have been proposed for future detectors, replacing transmissive optics with diffractive elements, thereby reducing thermal issues associated with power absorption. However, diffraction gratings introduce additional phase noise, creating more stringent conditions for alignment stability, and further investigations are required into all-reflective interferometers. A suitable mathematical framework using Gaussian modes is required for analysing the alignment stability using diffraction gratings. Such a framework was created, whereby small beam displacements are modelled using a modal technique. It was confirmed that the original modal-based model does not contain the phase changes associated with grating displacements. Experimental tests verified that the phase of a diffracted Gaussian beam is independent of the beam shape. Phase effects were further examined using a rigorous time-domain simulation tool. These findings show that the perceived phase difference is based on an intrinsic change of coordinate system within the modal-based model, and that the extra phase can be added manually to the modal expansion. This thesis provides a well-tested and detailed mathematical framework that can be used to develop simulation codes to model more complex layouts of all-reflective interferometers.

New Extreme Ultra-Violet (EUV) observations of the solar corona, obtained by the Coronal Diagnostic Spectrometer (CDS) instrument on board the NASA/ESA Solar and Heliospheric Observatory (SOHO) are presented. The CDS instrument for the first time has provided the opportunity of observing a large number of emission lines from a wide range of ions of different elements. The spectral and spatial resolution of the CDS instrument has allowed the simultaneous application of a wide range of spectroscopic diagnostic techniques to determine the temperature distribution, densities and elemental abundances in the solar plasma. A differential emission measure (DEM) diagnostic technique has been used to infer the temperature structure and the element abundances. The importance of including DEM effects in the element abundance analysis is demonstrated. A complete in-flight cross-calibration between all the CDS detectors (NIS and €115, 150-785 A), is presented here for the first time. The level of accuracy and completeness of the CHIANTI atomic database, used throughout this thesis, has allowed the identification of the many hundreds of spectral lines observed in the CDS spectra, resulting in several new line identifications. An assessment of the €115 solar spectra is presented, showing that, in spite of complexities, €115 spectra are useful for diagnostic analyses. Many discrepancies (in particular with the Li-like ions) between theory and CDS observations are highlighted. It is shown that some of these may be explained by inaccurate ionization equilibrium calculations. Coronal hole densities, temperatures, DEMs, and relative element abundances from both off-limb (plume and inter-plume regions) and on-disc observations are derived and compared with quiet sun values. These included (August 1996) a large equatorial hole, the Elephant's Trunk, probably the best example of a coronal hole observed by SOHO. Coronal hole densities in both coronal (N 1x10 8 cm 3 ) and transition region (Ne = lxlO'° cm 3 ) plasma were found to be about a factor of 2 lower than in the quiet sun, with much lower emission measures at temperatures above 106 K. Moreover, in the transition region, the cell-centres in both coronal holes and quiet sun regions show consistently higher densities (factor of 2) compared to those in the network. Relative element abundances show approximately photospheric values for the coronal holes, with an indication of element abundance variation .(neon in particular) with the supergranular structure. On-disc EUV observations of coronal hole plumes have been performed. This allowed a spectroscopic characterisation of plumes to be obtained for the first time, leading to the first identification of a low-latitude plume near sun-centre. Plumes are shown to be quasi-isothermal structures, with temperatures T 7 - 8x10 5 K. Abundance analyses (using transition region lines) reveal a small FIP effect in these plumes, together with a decreased Ne/O abundance ratio (compared to photospheric).

A new era of gravitational wave astronomy has begun with the first direct detections of gravitational waves from the collision of binary black holes and a binary neutron star system. The scientific outcomes from these detections have been magnificent, however in order to increase the event rates for known sources, to be sensitive to new sources, to detect sources at greater distances, and to increase the signal to noise ratio for better extraction of source parameters, further research is required to increase the detectors sensitivity. The Advanced LIGO and Advanced Virgo detectors that enabled these first detections will ultimately be limited in their sensitivity by reaching the standard quantum limit (SQL). One novel technique to reduce the influence of quantum radiation pressure noise in a measurement of strain between two test masses is the speed meter topology. As a proof of concept experiment the Glasgow Sagnac Speed Meter experiment aims to show a reduction in quantum radiation pressure noise compared to an equivalent Michelson interferometer at audio-band frequencies. Two triangular cavities are the core of the experiment and consist of two 100g end test masses and one 1g input test masses per cavity, all suspended from multistage pendulums. In this combination the whole Sagnac Speed Meter experiment should be limited by quantum radiation pressure noise from about 100Hz to 1kHz and it is expected to achieve a reduction of quantum radiation pressure noise by a factor of 3-5 compared to an equivalent Michelson interferometer. This thesis presents the development, design, commissioning and testing of the three main types of suspensions in the Sagnac Speed Meter experiment. The longitudinal displacement noise requirement for both cavity suspension types is < 1.5 x 10-18m/√Hz over the measurement band between 100Hz and about 1kHz. In order to isolate the mirrors from seismic ground motion in the Sagnac Speed Meter experiment, they are suspended from multistage pendulums, resulting ideally in a 1/f^2n response for n pendulum stages above the pendulums rigid body modes. Reduction of thermal noise in the suspension elements (suspension thermal noise) is achieved by the introduction of high quality-factor materials in the lowest pendulum stage, making it fully monolithic. The 100g end test mass suspension is based on an existing design, originally developed for the AEI 10m prototype, as a triple suspension with two stages of vertical blade springs and a fully monolithic lowest pendulum stage. The 1g input test mass suspension, designed as a quadruple pendulum with a fully monolithic lowest pendulum stage, utilises the same vertical blade springs and top mass as the 100g end test mass suspension. The quadruple pendulum design enables passive damping of test mass motion at the penultimate stage. As passive damping introduces force noise due to thermal noise, a switchable passive damping system was developed and tested to mitigate limitation by this force noise. The auxiliary suspension, a double pendulum, serves to suspend the mirrors in the experiment that guide the beam towards the Sagnac Speed Meter, in between the cavities, and towards the balanced homodyne detector. As these are not part of the cavities, the longitudinal displacement noise requirement can be relaxed to < 8 x 10-15m/√Hz at 100Hz. The pendulum dynamics of the auxiliary and 100g end test mass suspension were measured in an optical lever set up and, in case of the auxiliary suspension, additionally with a vibrometer. With these measurements, the models were adjusted and could be used to estimate the longitudinal displacement noise due to coupling from seismic ground motion and thus verify the required performance of the suspensions. The research conducted in this thesis is an important step towards establishing the speed meter topology for consideration in future gravitational wave detectors. The developments in the scope of the monolithic assembly for the 100g end test masses will be applied to the AEI 10m prototype in order to enable sub-SQL measurements.

In 2015, a gravitational wave (GW) signal from a binary black hole merger passed through the arms of the US-based Advanced LIGO (aLIGO) interferometers, resulting in the first direct detection of gravitational waves. This long-awaited observation made worldwide news one hundred years after Einstein first predicted the existence of GWs in 1916. Since the first detection, four more binary black hole inspiral events have been detected, as well as the ground-breaking GW observation of a binary neutron star inspiral. To detect these signals, ground-based GW detectors like aLIGO and the French-Italian detector, Advanced Virgo, need to be sensitive to changes in separation of close to 10^-19m between freely suspended test masses spaced up to 4km apart. This has always been a challenge to achieve, thus 50 years of technological developments were needed to make these first detections possible. Following the first observations of coalescing black holes and neutron stars, it is essential to pursue technological advancements that improve the sensitivities of ground-based detectors. Doing so will increase the signal-to-noise ratio of future detectors, which will allow for the better extraction of astrophysical source parameters. Observing more types of astrophysical sources, and at greater distances from the Earth will further the field of GW astronomy. One such area of advancement is to pair the operation of detectors at cryogenic temperatures with improvements in mirror and suspension design, with the aim of improving sensitivities by lessening the effects of thermal noise. Fused silica, currently used for the mirror substrates and suspension fibre elements in all detectors that operate at room temperature, cannot be used in detectors that operate at cryogenic temperatures due to its unfavourable thermo-mechanical properties. Thus a change of mirror substrate and suspension material is necessary for the construction of cryogenic detectors. There are two promising candidates for cryogenic mirrors and suspension elements, sapphire and silicon. Currently one cryogenic detector, the Japan-based KAGRA observatory, is under construction using sapphire as a material for its mirrors and some suspension elements. Other future detectors currently in the design phase, such as the Einstein Telescope (ET) in Europe and Voyager, in the USA may use silicon or sapphire material in their mirror suspensions. In all ground-based detectors the test masses are supported in multi-stage pendulum suspensions, where the last stages are quasi-monolithic. In the quasi-monolithic stage, the test masses are suspended from penultimate masses via fibres, welded to an interface piece, or "ear". Currently these ears are connected to the test masses using a method called hydroxide catalysis bonding, which creates a strong, low noise joint. This bonding technique has been used successfully in room temperature detectors for 17 years. This thesis details research into hydroxide catalysis bonding, with a focus on its use to create cryogenic crystalline suspensions for future ground-based detectors. The use of indium as an alternative bonding technology for joints in low temperature crystalline suspensions is also investigated. The aim of this study is to research possible ways to implement indium bonding into suspension design along with hydroxide catalysis bonds to create a more versatile and easily repairable system. This work was completed with the aim of investigating novel ways of implementing bond techniques into GW detectors, and studying their material properties. The breaking stress and stability of different bond technologies were investigated, as well as their thermal noise levels and impact on overall detector sensitivity. The majority of substrate materials used in this thesis were sapphire and silicon, as these are the two materials of choice for use in future cryogenic detectors. Measurements of the Young's modulus of hydroxide catalysis bonds between fused silica were also completed and used to model the thermal noise contribution of bonds in a prototype test mass for the possible room temperature upgrade to aLIGO, A+. In Chapter 1 an overview of the field of gravitational wave research is given. An explanation of GW sources and a history of the different types of ground-based GW detectors are summarised here, with a focus on Michelson-type interferometric detectors, used to make the first direct GW detections. The noise sources that affect the sensitivity of interferometric detectors are also reviewed. In Chapter 2 there is a summary of several different bonding techniques that could be considered for making joints between the test masses and suspension elements of GW detectors. The mechanisms of bond formation as well as the advantages and disadvantages to each approach are covered, especially in the context of the requirements for use in a GW detector. Finally hydroxide catalysis and indium bonding are introduced as possible techniques to join the suspension and mirror elements in GW detectors. In Chapter 3 the breaking stresses of hydroxide catalysis bonds between c-plane sapphire substrates as a function of time is studied. The aim of this experiment is twofold. The breaking stress of bonds that have been allowed to cure for shorter lengths of time is investigated to gain insight into the chemical processes of the bonds as they develop. Additionally, it is crucial to know the breaking stress over longer periods of curing time to be assured that they will not fail in the long term. In fact, this study found that hydroxide catalysis bonded sapphire shows an initial drop in breaking stress, which then levelled off at 15-16MPa. These results agree with similar trends found in shorter curing time tests on sapphire and fused silica completed in the past. In Chapter 4 the effect of crystal orientation on the tensile strength of hydroxide catalysis bonded sapphire is investigated. Specifically, the breaking stress of bonds between a-a and m-m planes of sapphire jointed with hydroxide catalysis bonds is studied, using samples of the same geometry and jointed using the same bonding procedures as those presented in Chapter 3. These samples were allowed to cure at room temperature for 4 weeks, then the samples were strength tested. The breaking stresses were recorded and compared with the breaking stress results of c-c plane sapphire, also cured for 4 weeks at room temperature, reported in the previous chapter. In Chapter 5 a non-destructive technique of measuring the Young's modulus of hydroxide catalysis bonds between silica and between sapphire is developed. This approach uses acoustic pulses from an ultrasonic transducer transmitted through the bonded samples, and the portion of the acoustic wave that is reflected back from the embedded bond layer is recorded and studied. The bond Young's modulus was extracted from the data by analysis of the amplitudes of the acoustic pulses reflected from the bonds. A Young's modulus value of 15.3+/-5.2GPa for \hcbed sapphire and 21.5+/-6.6GPa for bonded fused silica was found with this approach. A Bayesian analysis model of the reflected acoustic signal and the underlying noise background was developed to analyse the low SNR signals of bonds between fused silica. A value of 18.5+/-2GPa, with a 90% confidence range was found with this approach, agreeing well with the results from the pulse amplitude analysis. In Chapter 6 the new Young's modulus value found in Chapter 5 is used to assess the mechanical loss and thermal noise budgets of hydroxide catalysis bonds in different mirror suspension geometries. Two room temperature test masses were modelled; a bonded aLIGO mass and a bonded prototype test mass, of a design suitable for use in A+. Three different cryogenic masses were also modelled; first a sapphire KAGRA mass, followed by a prototype sapphire ET mass, and a prototype silicon ET mass. / The thermal noise budgets of the bonds in all of these cases were found to be below the anticipated technical noise requirement for bonds, which is based on each detector's current design sensitivity curves. This indicates that hydroxide catalysis bonds are suitable for use in current detectors and for the design of future ones. In Chapter 7 different approaches to creating indium bonding procedures for use in cryogenic ground-based detectors are studied. Hybrid suspension designs that utilize both indium and hydroxide catalysis bonding are being considered in cryogenic detector designs such as KAGRA or ET. It is proposed that the \hydroxide catalysis bonds would be used to fix the test masses to the suspension elements. This takes advantage of their high breaking stress under shear and peeling, as has been successfully demonstrated in the past for room temperature detectors such as Virgo, aLIGO, or the Germany-based detector GEO600. Indium's low tensile strength means it cannot be used as a joint under tensile or shear load. However it is being considered for use in compressive joints, such as between the fibres and ears or between the fibres and blade springs. This would be done for contingency reasons, since indium can be de-bonded and re-bonded relatively easily, whereas hydroxide catalysis bonds cannot. In the event of a fibre break or a test mass upgrade, the whole bonded test mass assembly could be removed by de-bonding the indium bond interface. It could then be replaced by re-bonding it, making it a good option for future cryogenic mirror suspensions. Two indium bonding approaches are investigated, diffusion bonding and induction bonding. In both cases the substrates used were polished silicon, and the indium layers between them were made with different combinations of thin thermally deposited films and foils. The tensile strength and a post-break visual inspection of the indium bonds were used as a standard by which to judge bond quality and repeatability.

Gravitational waves are considered as ripples in the curvature of space-time and were predicted by Einstein in his general theory of relativity. Gravitational waves interact very weakly with matter which makes them very difficult to detect. However, research groups around the world are engaged in building a network of ultra sensitive ground and space based interferometers for the first detection of these signals. Their detection will open a new window in the field of astronomy and astrophysics. The nature of gravitational waves is such that when incident on a particle, they stretch and squeeze the particle orthogonally thus producing a tidal strain. The strain amplitude expected for gravitational waves which may be detected on earth are of the order of hrms ~10-22 to 10-23 (over a frequency range from few Hz to a few kHz). A network of instruments based on the Michelson interferometer design currently exists around the world. These detectors are undergoing a major upgrade and once online by 2015-16 the improved sensitivity and increased sky coverage may lead to the first detection of the gravitational waves signals. The Institute for Gravitational Research in the University of Glasgow in collaboration with the Albert Einstein Institute in Hannover, Golm and the University of Cardiff has been actively involved in the research for the development of instruments and data analysis techniques to detect gravitational waves. This includes construction of a long ground based interferometer in Germany called GEO 600 (upgraded to GEO-HF) having an arm length 600 m and strong involvement in the larger detectors of the LIGO (Laser interferometer gravitational wave observatory) project in USA having arm lengths of 4 km (Operated by MIT, Boston and CALTECH, Pasadena). An upgrade to LIGO called Advanced LIGO (aLIGO) is currently under construction with significant input from the University of Glasgow. Thermal noise is one of the most significant noise sources affecting the sensitivity of the detector at a range of frequencies. Thermal noise arises due to the random fluctuations of atoms and molecules in the materials of the test mass mirrors and suspension elements, and is related to mechanical loss in these materials. The work presented in chapter 3 of this thesis is devoted to the analysis of aspects of mechanical loss and thermal noise in the final stages of the GEO suspension. GEO-600 is currently undergoing an upgrade to GEO-HF targeting sensitivity improvements in the kiloHertz region. However, the planned upgrade requires access to the vacuum tanks enclosing the fused silica suspension system. There is a risk of damaging the suspension, which has led to a repair scenario being developed in Glasgow, to reduce the downtime of the detector. An optimised design of the fused silica fibre has been proposed. A study of mechanical loss has been undertaken through Finite Element Analysis (FEA) modeling techniques. The mechanical loss of the optimised fibre is estimated to be lower than the original GEO fibre by a factor of ~4. In terms of thermal noise performance the optimised fibre gives an improvement of ~1.8. The repair scenario of the monolithic suspension has led to the development of tools and welding procedures. Three prototype suspensions involving metal masses were successfully built, before fabricating the monolithic fused silica suspension in Glasgow. The work in chapter 4 focuses on the theory of photoelasticty and birefringence techniques. The production and use of various forms of polarised light has been discussed. A setup of plane and a circular polariscope using two polarisers and two-quarter wave plates has been shown. The retardation of light due to the birefringence in the sample can be measured using the Tardy method of compensation and a Babinet-Soleil compensator. Finally a discussion on the stress-optic law has shown that the relative stress in a sample can be measured once the retardance is known. The silica fibres in the aLIGO detector would be laser welded using a 100 W CO2 laser. The laser welding would lead to high temperature and development of thermal gradients. This could result in residual thermal stress in fused silica, which could lead to an additional mechanical loss. A study of mechanical and thermal stress induced in fused silica has been discussed in chapter 5 of this thesis. To understand the working of photoelastic techniques learned in chapter 4, a study of mechanical stress was undertaken by applying a load on the sample to induce temporary birefringence. The estimated values of stress showed a good agreement when compared with the theoretical predictions and FEA modelling. Thermal stress was induced in fused silica by applying a 25 W CO2 laser beam for 10 seconds and the relative stress was measured using photoelastic birefringence techniques. Thermal modelling of the stressed sample was performed using the techniques developed in FEA. The experimental values show a good agreement with the estimated 1st principal stress (FEA model) and equivalent stress. A study of thermal stress in fused silica welds has also been presented in chapter 5. Two fused silica samples were welded using CO2 laser welding and the relative stress at different points were measured. The stress in the weld region was measured to be relatively lower than other areas. At a distance of 3 mm away from the weld line the maximum stress was measured which was greater than the stress in the weld region by a factor of ~5. The work discussed in chapter 6 focuses on the study of the suspension thermal noise in aLIGO detector for applying incremental upgrades. To further enhance the sensitivity of the aLIGO detector, incremental upgrades could be applied to the suspension system to improve the thermal noise. The incremental upgrades focused on two aspects: improving the dissipation dilution factor, and obtaining a lower mechanical loss than the aLIGO baseline. Based on the results from FEA, two designs were compared, each having a suspension of length 100 cm but different stock diameter - 3mm and 5 mm. A comparison with the aLIGO baseline showed that these two models obtained a lower mechanical loss by a factor of 3.4 to 6.8. In terms of suspension thermal noise there was an improvement by factor of 2.5 to 3.7, which could lead to rise in the sensitivity of the detector by a factor of 2.5.

Central massive objects like supermassive black holes and stellar nuclear clusters are common in all type of galaxies. I use N-body simulations to study the formation and evolution of nuclear clusters and to investigate the influence of the dynamical evolution of disc galaxies on the structural and kinematical properties of the host galaxy. I show that the second moment of velocities determine a lower limit on the dissipative formation process, which is about 50% in the case of the nuclear cluster in the late-type spiral galaxy NGC 4244. The vertical anisotropy of nuclear clusters can be used to determine an upper limit on the formation process due to merger or accretion of star clusters, which is about 10% for the nuclear cluster in NGC 4244. This is the first time that we have strong evidence of a hybrid formation scenario for nuclear clusters. In a set of 25 galaxy simulations I study bar formation in disc galaxies. I show that bar formation lead to the increase in mass in the central region of galaxies. This mass increase raises the velocity dispersion of stars in the disc and bulge component, which explains the offset of barred galaxies in the relation between the mass of the supermassive black hole, Ml, and the velocity dispersion of stars in the bulge, se , the Ml - se relation (Gueltekin et al. 2009). While Graham et al. (2011) argued that the orbital structure of stars within the bar could be responsible for the observed offset of barred galaxies from the Ml - se relation of unbarred galaxies, I show that the effect of stellar orbits in bars on se is less than 15% compared to the increase in mass which raises se by 40%. The offset I find in the simulation is comparable to the offset using the recent sample of Ml measurements of elliptical, unbarred and barred disc galaxies from Gueltekin et al. (2009).

The chemistry of galaxies provides a powerful probe of the underlying physics driving their evolution, complementing the traditional tools of morphology, kinematics,and colours. This dissertation examines several aspects of the galactic chemical evolution of late-type galaxies - both disc-like and dwarf - using a suite of cosmological hydrodynamical simulations, which incorporate the nucleosynthetic pollution of the interstellar medium, supplemented with classical analytical models of Local Group dwarfs. Throughout the work, these models are confronted with extant observations of both local and high-redshift systems, in order to identify both the strengths and weaknesses of the current generation of galaxy models. The work here has been presented across four primary science chapters which follow on from the Introduction and Motivation, prior to closing with the Conclusions and Future Directions. The first science result (Chapter 2) derives from an examination of the cold (neutral)gas content of the first-ever simulated bulgeless dwarf disc galaxies (Governato et al. 2010), and builds upon the work first presented in Pilkington et al. (2011). The focus of the work is on comparing the observables inferred from the simulated interstellar media, with those seen in nature (including The HI Nearby Galaxies Survey and the Magellanic Clouds), including their velocity dispersion profiles, disc flaring, and the distribution of power within the ISM’s structure, on different scales. Going beyond the work in Pilkington et al. (2011), two additional simulations from the Governato et al. (2010) suite are included, and the original work has been extended to include an analysis of the chemical properties of the dwarf galaxies. The second science result (Chapter 3) examines the role of feedback, metal diffusion, and initial mass function selection, on the resulting chemistry of a new grid of M33-like disc simulations. The emphasis of the analysis is upon the resulting age-metallicity relations and metallicity distribution functions (in particular, the extreme metal-poor tail). Aspects of the work have been presented by Pilkington et al. (2012b), enhanced here by a further examination of the satellites associated with their respective host galaxies. The satellites are seen to be free of gas, with star formation histories which make them not unlike Local Group dwarf spheroidals. The third science result (Chapter 4) is based upon an analysis of the temporal evolution of metallicity gradients in Milky Way-like systems, and derives from the work presented in Pilkington et al. (2012d). A large suite of simulations, sampling a range of numerical codes (particle- and grid-based, in addition to classical Galactic Chemical Evolution (GCE) models), each with different treatments of star formation, energy feedback, and assembly histories, was employed. The analysis focussed on both the radial and vertical abundance gradients, emphasising the role of feedback in shaping the gradients, and demonstrates the critical role that new observations of in situ gradients at high-redshift can play in constraining the uncertain nature of feedback within simulations. This work has been complemented by a brief examination of the azimuthal abundance variations in the massive discs. The fourth science result (Chapter 5) expands upon our earlier exploration of the chemical properties of simulated dwarf galaxies, but now employs a classical semi-numerical GCE approach. By coupling colour-magnitude diagram-constrained star formation histories with our GEtool GCE code, we attempt to constrain the relative rates of gas infall and outflow, for the Carina, Fornax, and Sculptor Local Group dwarfs, in order to match their empirical chemical abundance patterns and metallicity distribution functions. This builds upon the preliminary work, as presented by Pilkington & Gibson (2012a).

We address the issue of kinematic heating in disc galaxies by analysing a suite of cosmological Milky Way-type disc simulations run with different particle-and grid-based hydrodynamical codes and different resolution, and compare them with observations of the Milky Way. By studying the kinematics of disc stars in these simulations, we seek to determine whether or not the existence of a fragile thin disc is possible within a cosmological framework, where multiple mergers and interactions are the essence of galaxy formation. We study the velocity dispersion-age relation for disc stars at $z=0$ and find that four of the simulations, the stellar disc appears to undergo continual/secular heating. Two other simulations suggest a “saturation” in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth, and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations, which are born relatively hot. The models which are in better agreement with observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger. By running a set of isolated Milky Way-type simulations with different resolution and different density thresholds for star formation we conclude that, on top of the effects of mergers, there exists a ``floor'' in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. A persistent issue in simulations of disc galaxies is the formation of large spheroidal components, and disc galaxies with larger bulge to disc ratios than is observed. This problem is alleviated by supernova feedback. We found that by increasing the feedback in the simulations, we decrease the amount of stars that are accreted onto the main galaxy. The star formation is quenched more efficiently in low mass satellites when stronger feedback is implemented as well as in the main halo. These effects result in a disc galaxy, which has formed less stars overall, but more importantly, contains less accreted stars. As the strong stellar feedback quenches the star formation in the small building blocks, the metallicity of the accreted stars is lower than in the case where less feedback was used. In the context of hierarchical formation, mass assembly is expected to be scale free. Yet the properties of galaxies depend strongly on their mass. We examine how baryonic physics has different effects at different mass scales by analysing three cosmological simulations using the same initial conditions that are scaled to three different masses. Despite their identical dark matter merger history, we show that the simulated galaxies have significantly different stellar accretion histories. As we go down in mass, the lowest mass progenitors are unable to form stars, resulting in a low mass galaxy with less accreted stars. The overall chemical properties are also distinct at the different mass scales, as one might expect from the mass-metallicity relation of observed galaxies. We examine gradients of chemical abundances with radius and with height above the disc, and look for properties that are retained at different mass scales and properties which change, often dramatically. We analyse the kinematic and chemical properties of their accreted and in-situ populations. Again, trends can be found that persist at all mass scales, providing signatures of hierarchical structure formation. We find that accreted populations in the high mass simulation did not resemble any of the populations in the lower mass galaxies, showing that the chemical properties of proto-galaxies, which merge at high redshift to form massive galaxies, differ from the properties of low mass galaxies that survive at z=0. We probe further the signatures of hierarchical structure formation at smaller scales, in dwarf galaxies. We analysed the morphologies, kinematics and chemical properties of two simulated dwarf galaxies with different merger histories. We again analyse the accreted and in-situ populations. Observations of dwarf galaxies have found that they are comprised of multiple components. Our simulated dwarfs indicate that such populations may indeed be a manifestation of the hierarchical formation process in action in these lower mass galaxies. In one simulated dwarf, the in-situ stellar component forms a thin disc and a thick disc. We show that the thick disc in this simulation forms from in-situ stars that are born kinematically hot in the disc from early gas-rich mergers. The thin disc is formed quiescently from the later infall of gas. The accreted stars in the simulation were found to form an extended stellar halo. Chemical signatures of the three populations are also explored. The second dwarf we analysed has different galactic components, a result found to be due to the different merger history of this galaxy. The last major merger in this simulation occurs early on in the formation process between two proto-galaxies of similar mass. The result is a dwarf galaxy comprised of a disc formed of in-situ stars and a flattened rotating stellar halo formed of accreted stars. The angular momentum of the accreted and old in-insitu stars is obtained from the last major merger. We discuss the resemblance of this flattened rotating stellar halo to fast rotating flattened elliptical galaxies, and propose that such structures may explain some of the observed extra-galactic thick discs. These studies show that galactic properties emerge through the complex inter-play between hierarchical structure formation, star formation, and feedback from supernovae. Different modelling of these processes will alter the simulated galaxy's properties, and detailed comparisons with observations can then be made to determine the dominant processes responsible for different galactic properties. We remain optimistic that further improvement in modelling will allow deeper insights into the processes of galaxy formation and evolution.