The detection of remnant planetary systems at white dwarfs allows the end stages of planetary evolution to be explored observationally. This thesis presents observations of white dwarfs and describes the contributions they make to planetary science. Firstly, white dwarf science probes the end states of the majority of known planetary systems, including the Solar system. In Chapter 3 I present the discovery of strongly variable emission lines from gas in a debris disc around the white dwarf SDSS J1617+1620. Time-series spectroscopy obtained during the period 2006–2014 has shown the appearance and then complete disappearance of strong double-peaked Ca ii emission lines. These observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact onto the debris disc of a single small rocky planetesimal, or interactions between material in a highly eccentric debris tail. I also use observations of white dwarfs to contribute to exoplanet science more generally. Metal pollution from planetary debris is visible in spectra of white dwarfs, providing the only technique to directly measure the bulk chemical composition of rocky extrasolar material. Chapter 4 presents a detailed study of the metal-polluted white dwarf SDSS J0845+2257, using high-resolution HST/COS and VLT spectroscopy to detect hydrogen and eleven metals originating in an orbiting debris disc. The chemistry of the debris is broadly similar to the Earth, but enhanced abundances of core material (Fe, Ni) suggest that the planetesimal from which the debris formed may have lost a portion of its mantle. Conversely, in Chapter 5 I focus on the detection of just carbon and oxygen, but at 16 different white dwarfs to search for hypothetical “carbon planets”. I find no evidence for carbon-rich planetesimals, with C/O< 0:8 by number in all 16 systems. I place an upper limit on the occurrence of carbon-rich systems at < 17 percent. The range of C/O of the planetesimals is consistent with that found in the Solar System. White dwarfs can also be fascinating objects in their own right. In Chapter 6 I present HST observations of the mysterious white dwarf GD 394, a hot, extremely metal polluted white dwarf. Extreme ultraviolet observations in the mid 90s revealed a 1.15 day periodicity with a 25 per cent amplitude, hypothesised to be due to a surface accretion spot. I obtained phase-resolved HST/STIS high-resolution FUV spectra of GD 394 that sample the entire period, along with a large body of supplementary data. I use these data to test the hypothesis of an accretion spot, search for variability in accretion rates over decades-long timescales, and probe the immediate circumstellar environment of GD 394.

Despite the coherent cosmological framework provided by the λCDM model that astronomers have to work within, there are still a lot of unanswered questions regarding galaxy formation and evolution. Measuring the clustering of galaxies can provide information about the different environments that different types of galaxies reside in. Also, measuring the clustering of similar samples of galaxies at different redshifts can provide insights into how galaxies have evolved over time. Previous clustering analyses, particularly at high redshift, have often been restricted to galaxy samples which are small, selected on observable properties and/or contain an unknown mixture of different spectral types. Small samples lead to limited statistics and the inability to break the sample into interesting subsamples based on properties, e.g. by luminosity or star formation rate. Selecting samples based on observable properties leads to varying intrinsic properties with redshift and hence makes interpreting the evolution of clustering difficult. Mixing spectral types makes it impossible to separate the contribution to the clustering signal from early and late-type galaxies which tend to cluster very differently. This thesis overcomes some of the limitations of earlier clustering analyses by using the Spitzer Wide area InfraRed Extragalactic (SWIRE) photometric redshift catalogue of Rowan-Robinson et al.(2008) to measure the clustering of galaxies. The SWIRE catalogues covered multiple fields and large volumes providing large samples of galaxies over 0.1 < z < 1.5. The template fitting procedure also provides spectral classifications as well as intrinsic properties such as stellar mass and star formation rate estimates. The clustering of elliptical and spiral galaxies detected in SWIRE is measured as a function of stellar mass over 0.1 ≤ z ≤ 1.5. The clustering of spiral galaxies selected on star formation rate is also investigated over the same redshift range. Such measurements can help constrain theories of galaxy evolution. Another Spitzer dataset, the dark field, is used in an attempt to place one of the first observational constraints on the detection rate of population III supernovae. The dark field is an extragalactic data set with repeat imaging on a monthly basis over a baseline of approximately 2 years. The unprecedented depth and multi-epochal nature of this data makes it ideal for a first foray into trying to detect supernovae from the first stars.

In this thesis we present the XCS DR3 cluster candidate list. This represents the first major update of the XMM Cluster Survey since 2005. The candidate list comprises of 1365 entries with more than 300 detected counts distributed over 229 deg2. We note that a larger area (523 deg2) is available for the study of X-ray point sources and that the new XCS point source sample has more than 130,000 entries. After redshift follow-up and X-ray spectral analysis, these 1365 clusters will comprise the largest homogeneous sample of medium to high redshift X-ray clusters ever compiled. The future science applications of the XCS DR3 clusters include the study of the evolution of X-ray scaling relations and a measurement of cosmological parameters. In support of these science applications, we also present in this thesis detailed selection functions for the XCS. These selection functions allow us to quantify the number of clusters we didn't detect in our survey regions. We have taken two approaches to the determination of the selection function: the use of simple (circular & isothermal) β models and the use of ‘observations' of synthetic clusters from the CLEF N-body simulation. The β model work has allowed us to explore how the selection function depends on key cluster parameters such as luminosity, temperature, redshift, core size and profile shape. We have further explored how the selection function depends on the underlying cosmological model and applied our results to XCS cosmology forecasting (Sahlen et al. 2009). The CLEF work has allowed us to explore more complex cluster properties, such as core temperature, core shape, substructure and ellipticity. In summary, the combination of the cluster catalogues and selection functions presented herein will facilitate field leading science applications for many years to come.

We introduce a statistical exploration of the parameter space of the Munich semi-analytic model built upon the Millennium dark matter simulation. This is achieved by applying a Monte Carlo Markov Chain (MCMC) method to constrain the 6 free parameters that define the stellar mass function at redshift zero. The model is tested against three different observational data sets, including the galaxy K-band luminosity function, B −V colours, and the black hole-bulge mass relation, to obtain mean values, confidence limits and likelihood contours for the best fit model. We discuss how the model parameters affect each galaxy property and find that there are strong correlations between them. We analyze to what extent these are simply reflections of the observational constraints, or whether they can lead to improved understanding of the physics of galaxy formation. When all the observations are combined, the need to suppress dwarf galaxies requires the strength of the supernova feedback to be significantly higher in our best-fit solution than in previous work. We interpret this fact as an indication of the need to improve the treatment of low mass objects. As a possible solution, we introduce the process of satellite disruption, caused by tidal forces exerted by central galaxies on their merging companions. We apply similar MCMC sampling techniques to the new model, which allows us to discuss the impact of disruption on the basic physics of the model. The new best fit model has a likelihood four times better than before, reproducing reasonably all the observational constraints, as well as the metallicity of galaxies and predicting intra-cluster light. We interpret this as an indication of the need to include the new recipe. We point out the remaining limitations of the semi-analytic model and discuss possible improvements that might increase its predictive power in the future.

These are privileged times to be a cosmologist. Recent years have witnessed unprecedented progress in observational and computational techniques and we now are able to quantify cosmological properties with unprecedented accuracy. My work builds upon this observational accuracy by establishing a connection with viable theoretical models. I focus on two specifics eras of the universe's evolution, namely inflation and today's cosmic acceleration. In the context of single field inflationary models I illustrate the relation between the spectra of curvature and gravitational wave perturbations. I conclude that their mutual interdependence extends beyond the usual amplitude consistency relation and can be traced all the way to infinite order of accuracy. This yields an infinite hierarchy of consistency relations between these spectra and their derivatives. On a observational perspective, using WMAP's data, I explore the dependence of CMB constraints on inflation with the cosmological scale at which these are chosen to be presented. I develop a technique that allows for an appropriate choice of this scale and show that this way constraints may be improved by as much as 5 times. In the context of the particle physics motivated quintessence models I have looked at the ability of early universe probes - namely Big Bang Nucleosynthesis - for distinguishing between different dark energy proposals when combined with standard distance modulus or the Hubble rate techniques. I conclude that more yet more accurate measurements are required if observations are to successfully confirm or rule out these models as potential candidates against a cosmological constant. I also analyze possible effects that may mimic or underlie cosmic acceleration effects. I focus on a potential lack of knowledge of the precise values of particular cosmological parameters such as the curvature and matter content of the universe. I find that even a small uncertainty in any of this two quantities leads to significant bias on the reconstruction of dark energy properties, when typical probes like the distance luminosity and the Hubble rate are considered. I conclude that in order to disentangle between these effects a combination of distance and expansion history measurements is required.

The work presented here looks at several aspects of searching for continuous gravitational waves from pulsars, often referred to simply as continuous waves or CWs. This begins with an examination of noise in the current generation of laser interferometer gravitational wave detectors in the region below ~100 Hz. This frequency region is of particular interest with regards to CW detection as two prime sources for a first CW detection, the Crab and Vela pulsars, are expected to emit CWs in this frequency range. The Crab pulsar's frequency lies very close to a strong noise line due to the 60 Hz mains electricity in the LIGO detectors. The types of noise generally present in this region are discussed. Also presented are investigations into the noise features present in the LIGO S6 data and the Virgo VSR2 data using a program called Fscan. A particular noise feature present during VSR2 was discovered with the use of Fscan, which I report on and show how it degrades the sensitivity of searches for CWs from the Vela pulsar using this data. I next present search results for CWs from the Vela pulsar using VSR2 and VSR4 data. Whilst these searches did not find any evidence for gravitational waves being present in the data, they were able to place upper limits on the strength of gravitational wave emission from Vela lower than the upper limit set by the pulsars spin-down, making it only the second pulsar for which this milestone has been achieved. The lowest upper limit derived from these searches confines the spin-down energy lost from Vela due to gravitational waves as just 9% of Vela's total spin-down energy. The data from VSR2 and VSR4 are also examined, analysis of hardware injections in these datasets verify the calibration of the data and the search method. Similar results are also presented for a search for CWs from the Crab pulsar, where data from VSR2, VSR3, VSR4, S5 and S6 are combined to produce an upper limit on the gravitational wave (GW) amplitude lower than has been previously possible, representing 0.5% of the energy lost by the pulsar as seen through its spin-down. The same search method is also applied to analyse data for another 110 known pulsars, with five of these being gamma-ray pulsars that have been timed by the Fermi satellite. GWs from the pulsars timed by Fermi are expected at frequencies below 40 Hz, the LIGO detectors are not calibrated below these frequencies but the Virgo detector is. Hence the data used to search for GWs from these pulsars is the Virgo VSR4 data. The other 105 pulsars were analysed using out of date ephemerides obtained for the LIGO S5 run and the data analysed was from the LIGO S6 run, hence the results obtained for these pulsars are presented as an indication of what results can be expected with updated ephemerides only. For these 110 pulsars the spin-down limits were not able to be beaten, although there are a few pulsars for which this may be able to be achieved with an analysis combining all the possible datasets, in particular J1913+1011. The final part of this thesis reports extensions to the search method used for the analyses previously described. The first way in which this search method is extended is the use of a nested sampling algorithm to perform the parameter estimation stage of the analysis which was previously preformed using a MCMC. The nested sampling code also allows for model selection through the computation of the Bayesian evidence, I present results from characterisation tests of this nested sampling search code that demonstrate the equivalence of its results to those from the MCMC and grid based codes. The other extension to the search method looks at a new CW emission mechanism from a neutron star with a pinned superfluid core that is misaligned from the star's principle axes. This emission model predicts CWs at both the stars spin frequency f and twice its spin frequency 2f, providing an extra data channel with which to perform a search when compared to the triaxial rotator model which only emits at 2f. I present the development of a search for the emission from this new model, tests of the algorithm developed using simulated data, and results from a search using actual data from the VSR4 run for CWs from the Crab pulsar. The testing of the search algorithm shows that the posterior for the model is sufficiently complex to inhibit useful parameter estimation, but that the computation of the Bayesian evidence allows one to distinguish between this model and the triaxial rotator given a low SNR signal in the f data channel.

This thesis concerns the analysis of continuous gravitational waves from neutron stars with non-axisymmetric rotational motion using data from ground-based interferometric gravitational wave detectors, the development of a computationally efficient algorithm for analysis of this data and the use of this algorithm in follow up searches, which were previously too computationally expensive to consider.

The process of converting gas into stars underpins much of astrophysics, yet many fundamental questions surrounding this process remain unanswered. For example -- how sensitive is star formation to the local environmental conditions? How do massive and dense stellar clusters form, and how does this crowded environment influence the stars that form within it? How do the most massive stars form and is there an upper limit to the stellar initial mass function (IMF)? Answering questions such as these is crucial if we are to construct an end-to-end model of how stars form across the full range of conditions found throughout the Universe. The research described in this thesis presents a study that utilises a multi-scale approach to identifying and characterising the early precursors to young massive clusters and high-mass proto-stars, with a specific focus on the extreme environment in the inner few hundred parsecs of the Milky Way -- the Central Molecular Zone (CMZ). The primary sources of interest that are studied in detail belong to the Galactic centre dust ridge -- a group of six high-mass (M ~ 10^(4-5) Msun), dense (R ~ 1-3 pc, n > 10^(4) cm^(-3)), and quiescent molecular clouds. These properties make these clouds ideal candidates for representing the earliest stages of high-mass star and cluster formation. The research presented makes use of single-dish and interferometric far-infrared and (sub-)millimetre observations to study their global and small-scale properties. A comparison of the known young massive clusters (YMCs) and their likely progenitors (the dust ridge clouds) in the CMZ shows that the stellar content of YMCs is much more dense and centrally concentrated than the gas in the clouds. If these clouds are truly precursors to massive clusters, the resultant stellar population would have to undergo significant dynamical evolution to reach central densities that are typical of YMCs. This suggests that YMCs in the CMZ are unlikely to form monolithically. Extending this study to include YMCs in the Galactic disc again shows that the known population of YMC precursor clouds throughout the Galaxy are not sufficiently dense or central concentrated that they could form a cluster that then expands due to gas expulsion. The data also reveal an evolutionary trend, in which clouds contract and accrete gas towards their central regions along with concurrent star formation. This is argued to favour a conveyor-belt mode of YMC formation and is again not consistent with a monolithic formation event. High angular resolution observations of the dust ridge clouds with the Submillimeter Array are presented. They reveal an embedded population of compact and massive cores, ranging from ~ 50 - 2150 Msun within radii of ~ 0.1 - 0.25 pc. These are likely formation sites of high-mass stars and clusters, and are strong candidates for representing the initial conditions of extremely massive stars. Two of these cores are found to be young, high-mass proto-stars, while the remaining 13 are quiescent. Comparing these cores with high-mass proto-stars in the Galactic disc, along with models in which star formation is regulated by turbulence, shows that these cores are consistent with the idea that the critical density threshold for star formation is greater in the turbulent environment at the Galactic centre.

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.