Transverse loop oscillations (TLOs) are standing oscillations of coronal loops that are interpreted as kink waves in Magneto-hydrodynamic theory. These waves are particularly important as they can be used to infer local plasma properties such as the magnetic field strength through the technique of coronal seismology. In this thesis TLOs are studied both observationally, primarily with the Atmospheric Imaging Assembly (AIA) instrument on the Solar Dynamics Observatory (SDO) satellite, and numerically using the Lare2D code. In the first observational study, eleven transversely oscillating loops in three active regions are analysed in detail. The time series analysis revealed periods between 1.7 and 10 minutes and damping times between 2.9 and 13 minutes and are compared with previously observed events. Coronal seismology of the kink mode is applied to determine the range of the internal Alfvén speed and the magnetic field strength for each loop. The magnetic field strength is found to have a range of 4 − 18 G. The second observational study presents the first observation of a transverse oscillation in a hot coronal loop following a linked coronal-flare mass-ejection event. The TLO is observed exclusively in the 131 Å and 94 Å bandpasses, indicating a loop plasma temperature in the range of 9 - 11 MK. Furthermore, the loop is not observed to cool into the other AIA channels, but just disappears from all bandpasses at the end of the oscillation. The time series analysis reveals a period of 302 ± 14 s (291 ± 9 s) and a damping time of 306 ± 43 s (487 ± 125 s) for the first (second) loop strand and a spatial phase shift along the loop of approximately 180° suggests that a higher order harmonic is observed. We show that the excitation mechanism of this unusual oscillation is directly connected with the reconnection processes that form the post flare loop. This differs from the blast wave excitation mechanism often proposed as the cause of cooler transverse loop oscillations. The third observation presents another unusual TLO event that is also shown to have a direct link to the local magnetic field topology and the flare reconnection processes. A transversely oscillating coronal loop seen in the 171 Å bandpass is seen to oscillate in antiphase with respect to adjacent larger loops seen in the 193 Å and 211 Å bandpasses. The local magnetic topology of the active region is modelled using potential field source surface (PFSS) extrapolation which reveals that the loops are anchored in different topological regions with footpoint locations identified either side of the EUV flare peak emission source. Lastly, numerical simulations using the Lare2D code are performed to further investigate the anti-phase TLO observations. Loops positioned in different topological regions above and below a magnetic X-point are impulsively excited and we observe inwardly directed anti-phase oscillations of the loops. In addition we show that both the loop oscillations and the oscillatory behaviour of the X-point are independent of the resistivity.

No description available.

The dynamical evolution of the matter content of the universe is modelled throughout this study as that of self and mutually gravitating Lagrangian fluids in the so called ΛCDM-Concordance cosmological framework which leads to the Hierarchical Clustering paradigm for the formation of cosmic structures. As a numerical tool for investigating galaxy formation scenarios in this context, we employed GADGET2 (see Springel 2005) and the more recent GADGET3 (see Springel et al. 2008): we describe the numerical solvers implemented in the code and test their behaviour in both gravitational and hydrodynamical setups of relevance for cosmological calculations (Tasker et al. 2008). Using the outputs of the MILLENNIUM simulation and the relative Semi Analytical galaxy catalogues produced by Croton et al. 2006, we developed an algorithm aimed at the identification of large spherical underdense regions in the simulated Large Scale Structure (LSS), at z = 0. Focusing on this peculiar environment, we found a confirmation in numerical simulations for the observations by Trujillo, Carretero & Patiri (2006). The Tidal Torque Theory can predict the spatial distribution of the orientation of both the angular momentum vector of Milky Way size galaxies located on the surface of large spherical voids, and of their host DM halos. We re–simulated the 5 GIMIC regions (Crain et al. 2009) following the gravitational evolution of the CDM component only. We then applied a Semi Analytical Model (SAM) of galaxy formation (De Lucia & Blaizot 2007) obtaining the galaxy catalogues and merger histories for the 5 different volumes simulated. It is not yet well understood if and how the LSS environment can influence the Star Formation (SF) histories of galaxies. Starting from the stellar mass content of semi–analytical galaxies at z = 0, we defined characteristic epochs for their build up and, as a preliminary study, investigated how these distribute as a function of different LSS environments.

This thesis is conceptually divided into two parts. The first and main part concerns the generation of gravitational radiation that is emitted from merging black-hole binary systems using Numerical Relativity methods. The second part presents the methodology of the search for gravitational-wave bursts that are emitted in core-collapse Supernovae. My approach to Numerical Relativity is to utilise the late-time gravitational radiation of merging binary black holes to extract key astrophysical parameters from such systems via standard parameter estimation techniques. I begin with an introductory chapter that outlines the standard theories of stationary and perturbed black holes. In addition, up-to-date techniques in performing binary black hole simulations, the current and near-future status of the global network of gravitational-wave detectors, as well as the most promising gravitational-wave sources. I conclude the chapter with elements on parameter estimation techniques, such as Bayesian analysis and the Fisher information matrix. In Chapter 2, I discuss detection issues and parameter estimation results from the late-time radiation of colliding non-spinning black holes in quasi-circular orbits, and propose a practical test of General Relativity, as well as of the nature of the merged compact object. Chapter 3 involves similar parameter extraction calculations, but involves a more realistic approach, whereby the effect of the various angular parameters on parameter estimation is considered, placing an emphasis on the actual science benefit from measuring the gravitational radiation from perturbed intermediate and supermassive black holes. In Chapter 4 we present the results of an extensive Numerical Relativity study of merging spinning black hole binaries and argue that the mass ratio and individual spins of a non-precessing progenitor binary can be measured solely by observing the late-time radiation. Chapter 5 presents the methodology in carrying out searches for gravitationalwave bursts (GWB) from core-collapse Supernovae with a dedicated for GWB searches pipeline (X-Pipeline) and presents the sensitivity performance of XPipeline in detecting GWBs associated with certain Supernova candidates during the two most recent LIGO-Virgo-GEO Science Runs.

No description available.

The detection of gravitational waves from the coalescence of two compact objects has been brought to within touching distance by the construction and operation of a global network of laser-interferometer detectors. However, the amplitude of the radiation from these events is so low that direct detection will require the combined innovations of advanced interferometry and detector characterisation, along with powerful methods of extracting weak, but modelled, signals from the background detector noise. This work focuses on enhancing the probability of such detection through improved identi�cation of noise artefacts in the instrumental data, and improved signal processing and extraction. We begin with a recap of the theory of gravitational waves as derived from Einstein's theory of gravity, and the mechanisms that allow propagation of this radiation away from a source. We also catalogue a number of promising astrophysical progenitors, with a focus on compact binary coalescences. We detail the interactions between gravitational waves and an observer, and describe the layout of the large-scale laser interferometers that have been built to enable direct detection. A description of the operation of these detectors during the last science run is given, focusing on their stability and sensitivity, isolating a number of key instrumental noise mechanisms and how they a�ected astrophysical searches over the data. Additionally, we illustrate a new method to improve the identi�cation of seismic noise bursts, allowing their removal from search data, improving search sensitivity. The LIGO and Virgo gravitational-wave detectors operated as a network during the last joint science run. A summary is given of the analysis pipeline used to search for gravitational waves signals from compact binary coalescences using a coincidence-based method, including details of the results of that analysis. Details are also given of the pipeline used to search for gravitational waves associated with short, hard gamma-ray bursts, in which a new coherent method was tuned to search over the reduced parameter space constrained by the electromagnetic counterpart. Finally, we present a new pipeline adapting the coherent method to the blind, all-sky, all-time search, allowing for a more sensitive analysis, as demonstrated by direct comparison.

This thesis describes the use of the rarest high-mass and high-redshift galaxy clusters to constrain cosmology, with a particular focus on the methodology of Extreme Value Statistics (EVS). Motivated by the prospect that even a single sufficiently high mass and high redshift cluster can provide strong evidence against a given cosmology, we first use exact EVS to construct the probability density function (PDF) for the mass of the most-massive cold dark matter (CDM) halo within a fixed redshift volume. We find that the approximation of uncorrelated haloes is valid for high mass haloes 10¹⁵ and large volumes 100⁻¹Mpc, which are also required before the shape of the PDF converges to an asymptotic Generalised Extreme Value (GEV) form. Furthermore, we show the GEV shape parameter γ to be a weak discriminant of primordial non-Gaussianity on galaxy cluster scales. We then extend this analysis to real observations, predicting the PDF for the most-massive galaxy cluster within an observational survey, showing no cluster so far observed is significantly larger than the most-massive expected at its redshift in a concordance cosmology. We also show how the predictions for most-massive cluster with redshift are changed in cosmologies with primordial non-Gaussianity or coupled scalar field dark energy. Finally, we consider why this result appears at odds with some previous analyses, reaffirming that they make use of a biased statistic and showing how an equivalent unbiased one may be constructed. This is then used to rank a comprehensive sample of galaxy clusters according to their rareness, with the cluster ACT-CLJ0102-4915 found to be the most extreme object so far observed. However, the observation of this (and all other clusters so far seen) is shown to be a not unusual event in a concordance universe.

In this thesis, I present molecular line & continuum data of a sample of small, southern, isolated cores. I present a multi-wavelength view of the cores, by utilising optical images, 2MASS extinction maps, CO integrated intensity maps and 1.2mm continuum images of each of the cores. Spitzer data are used to identify young stellar objects local to each core, which may influence the evolution of the core. Column densities and masses are calculated for each core. The column densities calculated from the CO and 1.2mm continuum tracers are shown to be in excellent agreement with each other, and with the peak extinction, as seen on the 2MASS extinction maps. A comparison of column density derived from 1.2mm continuum and C18O observations suggest that a fraction of the gas has frozen out onto the dust grains in the densest parts of the core. The masses derived from 13CO, C18O and 1.2mm continuum observations are compared with the virial mass calculated from the observed linewidths. The cores are found to be within 3� of virial equilibrium in all cases, which suggests that all cores may be gravitationally bound. I find that the observed linewidths of the isolated cores are consistent with models of star formation by turbulent dissipation. The C18O linewidth is observed to be narrower than the 13CO linewidth, which is narrower than the 12CO linewidth in all cases. This suggests that as the density of the tracer increases, the linewidth decreases. Therefore, turbulent support against collapse is removed from the inside out, resulting in stars forming in the densest parts of the cores. I also present a proposed evolutionary diagram, based on the observed 12CO and ratio of 12CO/C18O linewidths. I hypothesise that a young core will have large 12CO and C18O linewidths. In an older core, the turbulence will have had time to dissipate in the core centre, and so the C18O linewidth will be narrower. For the oldest cores, the dissipation of turbulence will have occurred in the outer parts of the core and so the 12CO/C18O ratio will be small, indicating a more evolved core.

The way in which stars form from the interstellar medium is poorly understood. In this thesis we investiage the process star formation in molecular clouds via core fragmentation using Smoothed Particle Hydrodynamics (SPH). The initial conditions of the simulations are informed as closely as possible by observations of Ophiuchus. We run large ensebles of individual core simulations and compare the collective results with observations of stars and brown dwarfs. We use observations of Ophiuchus by Motte et al. (1998) and Andre et al. (2007) to calibrate a lognormal distribution from which we draw correlated masses, sizes and velocity dispersions. We assume that the cores are intrinsically triaxial. The distribution of core shapes is then inferred by fitting a single parameter family of ellipsoidal shapes to the observed core aspect ratios. Each core is given the density profile of a critical Bonnor-Ebert sphere and a turbulent velocity field which is modified to include ordered rotation and radial excursions. We evolve one hundred of the model prestellar cores using the Seren SPH code (Hubber et al., 2011). The simulations are repeated with continuous accretion heating, no accretion heating and episodic accretion heating (Stamatellos et al., 2012). We find that simulations with episodic accretion heating can reproduce the general features of the Chabrier (2005) initial mass function. This includs the ratio of stars to brown dwarfs and the turn-over at 0:2M�. We demonstrate that the mass of a star is not related to the mass of the prestellar core in which it formed. Low mass cores with Mcore � 0:1M� tend to collapse into single objects whereas higher mass cores with Mcore & 1M� can fragment into tens of objects. We finally show that the multiplicy statistics of the protostars formed in these simulations are well matched by observations. Multiplicity frequencies are higher than those of field stars and we note the presence of long-lived quadruple, quintuple and sextuple systems.

The structure and dynamics of the solar wind interaction with two small scale obstacles (of the order of a pickup ion gyroradius) is examined. These are a comet, comparable to Grigg-Skjellerup, and a weakly ionospheric planet. We also perform a pilot study of an intrinsically magnetized planet in such flow, in preparation for a future three-dimensional simulation. Here, we use two-dimensional hybrid simulations (particle ions, fluid electrons) and consider different solar wind Alfven Mach number flow (MA) and interplanetary magnetic field orientation relative to this plane. This allows control of the available wave types. The cometary simulations display magnetosonic "turbulence" as MA is increased, when the field is perpendicular to the simulation plane. If we allow parallel propagating modes by setting the field parallel to the plane, we find the "turbulence" significantly changes in scale and extent, suggesting resonant growth of Alfven ion cyclotron waves in the presence of magnetosonic "turbulence" occurs. Free energy is available from picked up cometary ions. The process depends on the cometary ion density, which strongly varies, and we conclude this explains the broadband nature of the disturbances. In the perpendicular field orientation, the planetary source produces a novel two tail structure which continuously strips the planetary ionosphere. We find these tails have very distinct characteristics, resulting in the wake being filled relatively quickly downstream, by complex structure. At higher MAl magnetosonic "turbulence" again appears. Switching the field parallel to the plane causes massive field line draping and pile-up, and causes instability. A long lasting wake appears, and we conclude that a three-dimensional simulation is required. The magnetized ionospheric planet pilot study proved difficult to scale accurately in two dimensions. The planetary field failed to penetrate the solar wind, however it appears the simulation would be stable and achieve equilibrium in three dimensions.