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In this thesis I will present observational studies of transient systems that produce mildly to extremely relativistic outflows through a coupling to an accretion flow. I will focus on the analysis of data taken from three source classes: black hole X-ray binaries (BHXRBs; particularly the system MAXI J1820+070), gamma-ray bursts (GRBs; particularly the system GRB 171010A), and tidal disruption events (TDEs; particularly the systems ASASSN-14li and Swift J1644+57). I will present an extensive radio monitoring campaign on MAXI J1820+070 utilising five different interferometers, along with extensive X-ray observations, during the system's 2018 outburst. Together these data allowed me to probe the coupling between accretion and jet production throughout an entire outburst cycle, as well as during multiple hard accretion state only re-brightenings, tracked over a two year time frame. As MAXI J1820+070 transitioned from the hard to soft accretion state, contemporaneous time-series indicators of the launch of bipolar relativistic ejections were observed at both radio and X-ray frequencies (manifesting as a radio flare and an evolving quasi-period oscillation, respectively). I then confirmed the presence of these ejecta utilising multiple interferometers, and was able to track the evolution of both the approaching and receding ejecta for over ~150 d. Through utilising interferometers sensitive to very different angular scales, I was able to infer the internal energy of the ejection, and found it to be much larger than the value implied from the state transition radio flare. This is strong evidence for ongoing particle acceleration as ejections interact with the surrounding interstellar medium. In addition to the study of MAXI J1820+070, I will also present a broader population study of state transition radio flares from black hole X-ray binaries, and demonstrate that commonly employed models (which attribute flares to an optical depth evolution from an expanding region) are not appropriate for the majority of flares in the sample studied. I describe the ability of extended periods of particle acceleration to explain the flare profiles. GRB 171010A was a luminous and nearby long GRB detected at early times by the Arcminute Microkelvin Imager Large Array interferometer, as well as with the Swift X-ray telescope. Long GRBs produce highly relativistic outflows that are best studied through their interaction with the circumburst material. This interaction produced a broadband synchrotron afterglow. I present a study of the radio through X-ray afterglow of GRB 171010A in the context of the fireball model (which details the interaction of the jet and interstellar medium). By fitting the time evolving spectra, the values and evolution of the characteristic synchrotron frequencies can be inferred. GRB 171010A is one of the most energetic GRBs detected below z~0.5, allowing for our theoretical understanding of afterglows to be investigated. While I find general agreement with the canonical models (particularly the spectral indices either side of the minimum energy frequency) a number of deviations are seen. I discuss possible solutions to these deviations, which likely include the addition of a second spectral component resulting from a reverse shock. Finally, I will present late time radio and X-ray observations of the thermal TDE ASASSN-14li, and late time radio only observations of the relativistic TDE Swift J1644+57. Tidal disruption events occur when a star passes too close to a supermassive black hole and is torn apart by tidal forces. Approximately half of the stellar material is accreted and the rest is unbound. ASASSN-14li is a radio bright thermal TDE, and the origin of this radio emission is disputed. I will show that the late time radio properties are now consistent with background AGN activity, but that while the TDE was the dominant radio component, the radio emission was correlated with the X-ray emission. This provides evidence that thermal TDEs produce jets. It is not disputed that the relativistic TDE Swift J1644+57 produced a jet, and said jet's radio emission has now been monitored for ~10 yrs. I will present the most recent monitoring of Swift J1644+57 in the context of previously proposed jet models for the source. I will additionally discuss the up-to-date population of radio loud TDEs. My conclusions contain a comparison of the outflows produced by these sources, and how they are analysed in different frameworks.

Supermassive black hole (SMBH) masses correlate tightly with various properties of their host galaxies, implying close co-evolution. However, the mass measurements underpinning this conclusion have large uncertainties and are limited by selection biases. With the advent of modern millimetre/sub-millimetre interferometers, molecular gas dynamics are now suitable tracers of the SMBH-dominated regions of galaxy potentials. In this thesis, I present new mass measurements for the SMBHs in the nuclei of the local elliptical galaxies NGC 524 and NGC 7052. These measurements exploit exceptionally high angular resolution observations of carbon monoxide (CO) J=2−1 emission using the Atacama Large Millimeter/sub-millimeter Array (ALMA). I refine earlier dynamical modelling methods, allowing for non-axisymmetric gas distributions and accounting for previously underestimated formal uncertainties when fitting models with very large numbers of constraints. The measurement in NGC 524 is consistent with an earlier stellar dynamical measurement, while that in NGC 7052 shows that an earlier gas dynamical measurement underestimated the SMBH mass (possibly by neglecting pressure support). The dynamical models of these galaxies also reveal a small inflow/warp in NGC 524, and that the central hole in NGC 7052 is consistent with the region within which tidal forces disrupt molecular clouds. Using spatially-integrated CO spectra from both single-dish telescopes and interferometers I further show a correlation exists between large-scale CO emission line widths and SMBH masses. This new correlation is no less tight than most, allowing it to be used to estimate SMBH masses in the absence of the (often harder to obtain) data required to exploit tighter correlations. Finally, I discuss the dynamical features revealed in new ALMA observations of 11 other SMBH mass measurement candidate galaxies, and estimate the SMBH masses in the three most promising. This thesis demonstrates the potential of molecular gas dynamics to advance our understanding of SMBH-host coevolution, and indicates the next steps in understanding the molecular contribution to AGN fuelling and feedback.

Despite their name, there is clear theoretical evidence that black holes emit radiation. The nature and spectrum of this radiation depends on many features of the black hole and the emitted particles. With the profound implications that black hole evaporation has on our understanding of the marriage of quantum mechanics and general relativity, it is essential to study the details of this radiation in new contexts. In this thesis we examine certain important modifications to the spectrum of radiation from black holes that can occur. Firstly, we study the case that the particles emitted by the black hole are spatially extended, rather than pointlike, and find that the rate of emission is suppressed by a factor which depends sensitively on the size of the particle. We then study the emission of electrically charged particles from black holes from a perspective that elucidates the physical picture of emission; namely, in terms of tunnelling of particles both through the horizon and the external electric field. We finally consider the cosmological consequences of modifying the mass-temperature relation of black holes by supposing that they can radiate into higher dimensions. In particular, we argue that current constraints on the density of black holes in the universe are evaded in certain extra-dimensional theories, in which black holes are substantially colder than their four-dimensional counterparts.

We calculate the dynamical tides raised by a close planetary companion on non-rotating stars of 1 M⊙ and 1:4 M⊙. Using the Henyey method, we solve the fully non-adiabatic equations throughout the star, both for the case of frozen convection and for the case where an approximation for the perturbation to the convective flux is used. The horizontal Lagrangian displacement is found to be 10 to 100 times larger than the equilibrium tide value in a thin region near the surface of the star. This is because non-adiabatic effects dominate in a region that extends from below the outer edge of the convection zone up to the stellar surface, and the equilibrium tide approximation is inconsistent with non-adiabaticity. We derive analytical estimates which give a good approximation to the numerical values of the magnitude of the ratio of the horizontal and radial displacements at the surface. We calculate the conversion of these oscillations into observable spectroscopic and photometric signals. Observables are calculated for some real planetary systems to give specific predictions. Time-dependent line broadening and the radial velocity signal during transit are both investigated as methods to provide further insight into the nature of the stellar oscillations. The photometric signal is predicted to be roughly proportional to the inverse square of the orbital period, P-2, as in the equilibrium tide approximation. However, the radial velocity signal is predicted to be proportional to P-1, and is therefore much larger at long orbital periods than the signal corresponding to the equilibrium tide approximation, which is proportional to P-3. The prospects for detecting these oscillations and the implications for the detection and characterisation of planets are discussed.

Giant stars are believed to lose significant fractions of their total mass over their lifetimes, but the mechanisms responsible for this are ill-understood. One possible mechanism is dynamical mass loss - a hydrodynamical process in which matter is ejected from the stellar surface in ballistic outflows. In this thesis, dynamical mass loss is studied in three stellar regimes: common-envelope objects, asymptotic giant branch stars, and red supergiants. Using hydrodynamical simulations performed with the stellar evolution code MESA, we examine the dynamical behaviour and stability of stars in each of these regimes. We examine the dynamical properties of common-envelope objects during the slow spiral-in phase using a parameterised 1-dimensional model of orbital dissipatory heating. We find that the envelope becomes unstable to high-amplitude dynamical pulsations that can lead to repeated mass-ejection events capable of removing the entire envelope and terminating the common-envelope phase. We estimate this process's α efficiency value and suggest how these results might be employed in parameterised common-envelope models. We employ coupled evolutionary and hydrodynamical simulations of AGB stars to study their dynamical properties as they traverse the TP-AGB and examine their dependence on basic stellar properties and on the thermal pulse cycle. We find that these models experience large amounts of dynamical mass loss, and we construct a parameterised model to estimate its strength. We find that this model is successful at locating the termination of the AGB. We apply a similar approach to a study of RSGs, and find that dynamical mass loss also emerges in this regime. We estimate the conditions under which this occurs and discuss how this mechanism may resolve theoretical problems relating to the Humphreys-Davidson limit and the progenitors of SNe IIn. We conclude that dynamical mass loss is likely to form a vital part of the mass-loss histories of cool giant stars.

Brown dwarfs (BDs) are substellar mass objects with masses ≤80MJup, the hydrogenburning limit. Deuterium fusion occurs in objects with ≥13MJup, lasting only a very short period (4-50 Myrs). This means that brown dwarfs are constantly cooling over their lifetime, passing through temperature ranges equivalent to those of exoplanets and planets in the Solar System. This aging and cooling creates a spectral sequence (hottest to coolest: M, L, T, Y) as various molecules and condensates begin to form. The atmospheres created during this sequence are extremely similar to gas giant planets. As brown dwarfs are free-floating objects, it is possible to attain much higher resolution spectra than with exoplanets because we do not have to subtract out the light from the host star, and thus they provide the perfect testbed for atmospheric modelling of objects beyond our Solar System. The aim of this thesis is to characterise the coolest brown dwarf atmospheres, spectrally classified as Y dwarfs, and the associated change in their atmospheric structure and composition with decreasing temperature. In order to achieve this ultimate goal, a number of steps are taken. The first is the updating of the molecular and atomic data to include the latest line lists and pressure broadening parameters, which enables us to more accurately model BDs with our radiative transfer calculations. Along the way, we discuss the most effective method to calculate gaseous absorption, and illustrate the effects of using sub-optimal techniques. With the radiative transfer calculations optimised, we proceed to implement a new inverse method to our NEMESIS retrieval code: Nested Sampling. This algorithm employs state-of-the-art Bayesian statistics to fit spectra and derive probability distributions for each parameter of interest. We verify our new Bayesian method and radiative transfer code against two other codes from separate groups, and investigate the model dependency of parameters using realistic atmospheric models of exoplanets that may be probed by the James Webb Space Telescope. We discuss the model-dependency of each parameter in the context of changing signal-to-noise, finding that some retrieved parameters are more model-dependent than others. Finally, having updated and verified our code through the methods previously discussed, we apply it to a range of ultracool T and Y dwarf atmospheres. The retrieved probability distributions of each parameter representing the atmospheric structure and composition of the brown dwarfs are compared to one another to elucidate trends with decreasing temperature. For the coldest object that we analysed, we discovered a cloud that is consistent with a Na2S cloud, which is expected to form at these temperatures. This is the first Bayesian detection of such a feature on a Y dwarf. We also discuss the plausibility that this cloud may instead be a water cloud. The analysis of these objects and the work presented here helps elucidate the atmospheric structure and composition of the Y dwarf population. We hope that this work will assist not only in future analyses of Y dwarf atmospheres, but also more generally in any atmosphere within or beyond our Solar System.

Gyrochronology, which describes the empirical relationship among stellar mass, rotation period, and age, is a potentially powerful tool for determining the ages of stars, upon which the understanding of stellar evolutionary models hangs. Well-calibrated with open clusters up to the age of ~2.5 Gyr, predictions from gyrochronology are in conflict with asteroseismic observations of older stars, and the underlying physics are poorly understood. The solar-age, open cluster M67, recently observed by the NASA K2 mission, offers the opportunity to test both gyrochronology and more physically-motivated angular momentum evolution models. Here, we seek to gain insight into the angular momentum evolution of solar-age stars through the analysis of K2 data. Using our own pipeline, we extract and process K2 light curves that encompass the inner and outer portions of M67. We describe various rotation period detection methods, focusing on the Lomb-Scargle periodogram, autocorrelation function (ACF), and Gaussian processes (GPs), before presenting our own preliminary results. Observed scatter between colour and period, combined with disagreement from the first published sets of K2 M67 periods, prompted a comprehensive round of sinusoidal injection tests to probe our absolute detection limits with K2 data using two different light curve preparation pipelines. Finding low sensitivity for 25 d, 0.1% amplitude signals and lingering systematics on the order of 25 d and greater, we use a combination of the Lomb-Scargle, ACF, and GP methods to determine our 'final' M67 rotation periods from K2 data. We find agreement between tidal synchronization theory and observed rotation periods for our M67 binaries. While angular momentum evolution models may perform slightly better than gyrochronology, we cannot make any conclusive statements regarding the validity of either due to the inherent scatter and small sample size. Future work at refining detection methods is needed, along with complementary data from upcoming missions such as LSST and PLATO.

First observed as early as redshift z = 7 and now thought to be found at the centre of every massive galaxy in the local Universe, the evolution history of supermassive black holes (SMBHs) spans over 13 billion years. In this thesis, the coevolution between SMBHs and their host galaxies is studied using a set of hydrodynamical simulations to isolate different components of the interaction between black holes and cosmic gas. The simulations range from black hole accretion in an idealised context to the impact of feedback in the cosmological simulations of the HORIZON suite. The origin of SMBHs during the first billion years of the Universe is a highly non-linear problem, where small-scale behaviour influences large- scale behaviour and vice versa. Gas fuelling a black hole flows from the cosmic web, through its host galaxy and into the black hole's gravitational potential, before eventually reaching its event horizon. Even discounting the complex physical processes at play, resolving the 19 orders of magni- tude in spatial scale involved is beyond the capabilities of current simula- tions. Some of the length scales therefore have to be covered by sub-grid algorithms which need to be able to handle a wide range of environments. Idealised accretion simulations presented in this thesis show that the Bondi-Hoyle-Lyttleton (BHL) accretion algorithm is sufficiently versatile. It automatically determines the accretion rate onto the black hole by the mass flux into its accretion region when the black hole's gravitational po- tential becomes resolved. The accretion rate onto the black hole therefore naturally converges to the correct solution once the size of the accretion region approaches the physical size of the black hole. A drag force algo- rithm that compensates for unresolved dynamical friction, on the other hand, produces a force on the black hole that can unphysically accelerate it relative to the bulk flow of the gas. It needs to be switched off when gas properties are measured within the black hole's gravitational potential. A study of black hole accretion within an isolated cooling halo confirms that the accretion algorithm is able to handle the flow configurations en- countered within an evolving galaxy. To ensure gas is always accreted within the black hole's gravitational potential, a refinement algorithm called "zoom-within-zoom" is introduced in this thesis. It allows the black hole environment to be resolved by orders of magnitude above that of its host galaxy. A low mass seed black hole with a strong drag force early on takes advantage of this extra information during the black hole's early evolution. In the longer term, resolving gas clouds in the black hole vicin- ity to sub-pc scales has a lasting impact on both the mass evolution and duty cycle of massive black holes. Sub-pc size clumps also play a deciding role in the first 200 Myr of evo- lution of a SMBH progenitor in a full cosmological context: 90% of its mass is gained through interactions with dense clumps, which fuel super- Eddington accretion bursts. Once the gas within the host galaxy settles into a rotationally supported disc, star formation and black hole accre- tion slow down. As both primarily occur within the central 30 pc of the compact host galaxy, star formation in proto-galaxies has a major impact on black hole accretion even in the absence of feedback. At low redshift, on the other hand, feedback becomes the crucial link between a SMBH and its host galaxy. A comparison of two simulations from the HORIZON suite, run with and without active galactic nuclei (AGN) feedback respectively, shows that AGN feedback is able to prevent as much as 90% of the stellar mass from forming in the most massive galaxies. Quenching proceeds via a combination of AGN driven outflows and reduced inflows and evolves with redshift as the MSMBH - M* relation flattens from z = 5 to z = 0. In conclusion, neither the evolution of galaxies nor that of black holes can be understood without the context of the other. At high redshift, the competition between star formation and black hole accretion inside the compact host galaxy intrinsically links the origin of SMBHs to the early evolution of galaxies. At low redshift, AGN feedback modulates the gas supply of the host galaxy, which has a lasting impact on star formation. The coevolution of black holes and galaxies therefore spans their entire history.

Technological advances coupled with a decline in digital storage costs have resulted in a profusion of data being created, collected and consumed. These data give rise to new challenges and opportunities in many disciplines ranging from science and engineering to biology and finance. An example of a future project in radio astronomy that promises both Big Data and Big Discoveries is the Square Kilometre Array (SKA) radio telescope project. Astrophysicists are confident that the Big Data amassed by the SKA will not only answer fundamental questions regarding the Universe but also contain big discoveries not yet postulated. The transformational potential of the SKA and its ensuing data and algorithmic challenges, in particular for the discovery and study of pulsars, drive the research of this thesis. Discovering all pulsars beaming towards Earth is one of the key science goals of the SKA. However, in addition to low signal strengths, searching for pulsars is extremely difficult due to the intrinsic weakness of their signals, propagation effects and the presence of anthropogenic interferences. Numerous techniques have been developed to overcome some of these difficulties and to assist in the quest to find more pulsars. However, despite the success of these techniques, the number of pulsars discovered in recent surveys (Swiggum et al. 2014, Lazarus et al. 2015) has fallen well short of the number predicted by pulsar population synthesis models (Lorimer 2011). This shortfall in pulsar detections can be attributed to radio frequency interference (RFI), red noise and scintillation (Lazarus et al. 2015). For this thesis, and in order to investigate and quantify these claims, I first developed a new technique to simulate pulsar search data that contain different types of RFI and varying noise baselines (i.e. red noise). This surrogate modelling technique was then used in a framework that I developed to inexpensively explore the sensitivity of pulsar search pipelines for different noise and RFI settings. The results from this framework highlight the necessity to develop algorithms that are able to identify and remove non-stationary variations from the data before RFI excision and searching is performed in order to limit false positive detections. To address the shortcomings identified with the framework which assessed the performance of existing pulsar search pipelines, I developed a new real-time algorithm for excising RFI while simultaneously normalising the variability in time and frequency inherent to pulsar observations. Processing synthetic data with the algorithm resulted in an expansion of the noise/pulsar spin period parameter space for which we are able to successfully detect pulsars. Furthermore, the algorithm is shown to reduce the number of false positive detections. In conclusion, the insights gained from the work presented in this thesis and the improvements achieved will contribute to the development of a new realtime pulsar search pipeline adept at dealing with the challenges posed by the SKA.

The problem of angular momentum transport in the radiative zone of the Sun and other stars is currently one of the most discussed in stellar astrophysics. A wide range of local and global instabilities acting in differentially rotating stars have been studied and incorporated in stellar evolutionary codes. In this thesis, I revisit two of the hydrodynamic processes that are thought to operate, namely the Eddington-Sweet (ES) circulation and a global motion induced by the Goldreich-Schubert-Fricke (GSF) instability, and investigate stellar models in which these mechanisms are suppressed. In particular: The data from helioseismology show that the upper radiative zone of the Sun is in near-uniform rotation, but they have a significant uncertainty. I discuss the possibility of patterns of differential rotation compatible with the constraint of perfect radiative equilibrium, which implies the absence of ES circulation. I show that this constraint leads to a set of ordinary differential equations for the stellar rotation profile, which are solved numerically. I find that these models can be quite close to uniform rotation, and compatible with the data from helioseismology. I investigate the dynamical stability of these models. It is shown that while a naive approach would suggest that the GSF instability would distrupt the rotation of these stars, this does not happen in reality. I present a study of the local stability of stratified, weakly magnetized, differentially rotating uids to non-axisymmetric perturbations with finite heat conductivity, kinematic viscosity, and resistivity. The models are found to be locally stable under rather general circumstances. Even if the molecular viscosity is very small compared to the heat conductivity, it suffices to suppress the GSF instability. I examine the onset of the GSF instability in the core of red giant stars, where, according to recent observations, strong differential rotation may be present. I derive constraints on the amount of shear necessary for the GSF instability to operate at different depths, and show that very strong shear is required. In general, the effectiveness of the GSF instability in transporting angular momentum is currently overstated.

What drives the transition of galaxies from the disc dominated, star forming blue cloud to the elliptical dominated, quiescent red sequence? What role does the morphology, central supermassive black hole and galaxy environment play in this transition? I have attempted to answer these questions by using Bayesian statistics to infer a simple star formation history (SFH) describing the time, tq, and exponential rate, τ, that quenching occurs in a galaxy. I use both the optical and NUV photometry of a galaxy in order to infer the posterior distribution of its SFH across the two dimensional [tq, τ] parameter space. I then utilise the Galaxy Zoo 2 morphological classifications to obtain a morphology weighted, combined population distribution across each quenching parameter for a sample of galaxies. I apply this method across the blue cloud, green valley and red sequence of a sample of 126,316 galaxies and find a clear difference between the quenching timescales preferred by smooth and disc weighted populations, with three major routes through the green valley dominated by smooth (rapid rates, attributed to major mergers), intermediately classified (intermediate rates, attributed to galaxy interactions) and disc morphologies (slow rates, attributed to secular evolution). I hypothesise that morphological changes occur in systems which have undergone quenching with an exponential rate, τ < 1.5 Gyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. I repeat this SFH analysis for a sample of 1,244 Type 2 AGN host galaxies and find statistical evidence for recent, rapid quenching, suggesting that this may be caused by AGN feedback. However I find that rapid quenching rates cannot account for all the quenching across the AGN host population; slow quenching rates, attributed to secular evolution, are also significant in the evolution of AGN host galaxies. I investigate this possible secular co-evolution of galaxies and black holes further by measuring the black hole masses of a sample of 101 bulgeless AGN host galaxies and compare them to typical black hole-galaxy scaling relations. I find that the measured black holes of the bulgeless galaxies are ~1-2 dex more massive than they should be, given their lack of bulges. This suggests that black hole-galaxy scaling relations may arise due to mutual correlations to the overall gravitational potential of the dark matter halo of the galaxy. I also considered the effect of the group environment on the time and rate that quenching occurs, with respect to the group-centric radius, for 4,629 satellite galaxies. I find that although mergers, mass quenching and morphological quenching are all occurring in groups, environmentally driven quenching mechanisms are also prevalent. However, I find that these environmentally driven quenching processes are not correlated with the velocity of a satellite within a group, ruling out ram pressure stripping as a possible mechanism. I discuss how all of these quenching mechanisms are likely to affect a galaxy across its lifetime, acting in concert to reduce the SFR, which in turn produces the wide distribution of quenching timescales seen across the colour-magnitude diagram. I discuss ideas for future work using the method employed in this work, including applying it to forthcoming data from large integral field unit surveys.

Stellar ages will play a big role in the next generation of astronomy. Useful to exoplaneteers and galactic archaeologists alike, this relatively under-exploited stellar property is currently limited by the precision of dating techniques. The work presented in this thesis contributes incrementally to a greater understanding of rotation period decay in Main Sequence (MS) stars as a proxy for stellar age. Inferring stellar ages from rotation periods, 'gyrochronology', is the only dating method with the potential to provide ages for stars on the hundreds-of-thousands scale. Unfortunately however, it suffers from being poorly calibrated as the sample of cool, MS stars with precise ages is extremely sparse. Using light curves of spotted, rotating, MS, FGK stars with asteroseismic ages from the Kepler spacecraft, I attempted to recalibrate the relation between rotation period, colour and age. I demonstrate that the simple, 'straight line' gyrochronology relations used in the past are unable to explain the new asteroseismic sample. Questions are raised about the power of gyrochronology---does it accurately predict ages for old stars? To fully answer this question, it will be necessary to exploit new data from the K2 (the repurposed Kepler mission). K2 has observed (and is still observing) several open clusters and asteroseismic field stars which may provide new insight into stellar rotational evolution. Unfortunately, systematic features in K2 light curves produced by Kepler's reduced pointing precision inhibit the detection of astrophysical signals in the data. These systematic features can be removed by modelling and subtracting them from the time series, 'detrending', but this process can remove some signals and can even inject noise. For this reason I developed a method for detecting periodic signals in K2 light curves without detrending: the Systematics-Insensitive Periodogram (SIP). This method is particularly useful for red giant asteroseismology. Precise ages can be inferred for oscillating red giants using the SIP and will be useful for galactic archaeology and open cluster age inference. In the next chapter of this thesis I return to the problem of stellar rotation period inference. Current methods for rotation period inference can produce inaccurate, imprecise periods with poorly approximated uncertainties and often without uncertainties altogether. I present a new method for inferring precise, accurate, probabilistic rotation periods with accurate uncertainties using Gaussian processes. Although expensive to compute, this method is ideal for applying to individual targets. I hope to continue to develop this method and apply it to a large ensemble light curves from Kepler and other photometric surveys in the future. Star spots and acoustic (p-mode) oscillations are not the only mechanisms that produce variability in dwarfs and giants. A combination of asteroseismic pulsations and granulation on the stellar surface produces variability on short timescales. It has been shown that the amplitude of this short-term variability, called 'flicker' is strongly correlated with both surface gravity and stellar density (Bastien et al., 2013, Bastien et al., 2016, Kipping et al., 2014). However, there is substantial additional scatter in these relations that is not accounted for by the observational uncertainties. I provide a new calibration of these relations which models this level of additional, astrophysical variance using hierarchical probabilistic inference. In the final chapter of this thesis I explore rotation period recovery with the Large Synoptic Survey Telescope (LSST). With its ten year baseline, LSST light curves will be sensitive to long rotation periods which are characteristic of old and low-mass stars. If the rotation periods of such stars can be measured from LSST light curves, it may be possible to improve the gyrochronology relations. We find that LSST is most sensitive to rotation periods between 10 and 20 days. Its sensitivity falls at short periods due to the sparsity of its sampling and at longer periods due to the lower amplitudes of variability and smaller apparent magnitudes of slow rotators.

Neutron stars are amongst the most exotic objects known in the universe; more than a solar mass of material is squeezed into an object the size of a city, leading to a density comparable to that of an atomic nucleus. They have a surface magnetic field which is typically around a trillion times stronger than the magnetic field here on Earth, and we have observed them to spin up to around 700 times per second. The existence of neutron stars was first proposed by Baade and Zwicky in 1934 but later graduated from theory to fact in 1967 as the first pulses were detected by Jocelyn Bell-Burnell, a then graduate student at the University of Cambridge. There are now well over 2000 neutron stars whose radio emission beams point at, and have been detected on Earth. We call these objects pulsars. Because of their remarkable properties, pulsars are very useful to physicists, who can employ them as precision timing tools due to the unwavering nature of their emission and of their rotation. Having an array of ultra-accurate clocks scattered throughout our galaxy is very useful for performing astrophysical experiments. In particular, precise pulsar timing measurements and the models that explain them, will permit the direct detection of gravitational radiation; a stochastic background initially, and potentially the individual signals from supermassive black hole binaries. Our models of pulsar behaviour are so precise that we are now able to notice even slight departures from them; we are starting to see that unmodelled variability in pulsars occurs over a broad range of timescales, both in emission and in rotation. Any unmodelled variability is, of course, detrimental to the pulsar's utility as a precision timing tool, and presents a problem when looking for the faint effects of a passing gravitational wave. We are hoping that pulsar timing arrays will detect gravitational radiation in the coming decade, but this depends, in part, on our ability to understand and mitigate the effects of the unmodelled intrinsic instabilities that we are observing. One important clue as to the nature of the variability in pulsar emission and rotation, is the emerging relationship between the two; we sometimes observe correlation on timescales of months and years. We have been observing pulsars for almost fifty years and our expanding datasets now document decades of pulsar behaviour. This gives us the ability to investigate pulsar variability on a range of timescales and to gain an insight into the physical processes that govern these enigmatic objects. In this thesis I describe new techniques to detect and analyse the emission and rotational variability of radio pulsars. We have employed these techniques on a 24 year pulsar dataset to unearth a striking new example of a dramatic and simultaneous shift in a pulsar's emission and rotation. We hypothesise that this event was caused by an asteroid interaction, although other explanations are also possible. Our variability techniques have also been used to analyse data from 168 young, energetic pulsars. In this thesis we present results from the nine most interesting. Of these, we have found some level of correlated variability in seven, one of which displays it very strongly. We have also assessed the emission stability of the NANOGrav millisecond pulsars and have found differing degrees of variability, due to both instrumental and astrophysical causes. Finally, we propose a method of probing the relationship between emission and rotation on short-timescales and, using a simulation, we have shown the conditions under which this is possible. Throughout the work, we address the variability in pulsar emission, rotation and links between the two, with the aim of improving pulsar timing, attaining a consolidated understanding of the diverse variable phenomena observed and elucidating the evolutionary path taken by pulsars.

Studying the interplay of accretion and star formation is crucial to our understanding of galaxy evolution. The new generation of radio surveys are able to play a key role in this area, since both processes produce radio emission. As we probe low radio-luminosities, the two main populations are star-forming galaxies and radio-quiet quasars (RQQs). How they contribute to the total radio emission is under debate, with previous authors arguing that star formation dominates. In this thesis I investigate the relative levels of radio emission due to accretion and star formation in RQQs. Firstly, I select a sample of 74 quasars from the VISTA Deep Extragalactic Observations (VIDEO) survey, whose depth allows me to study very low accretion rates and/or lower-mass black holes. By comparing radio emission from the quasars with that from a control sample of galaxies, and calculating two independent estimates of star-formation rate, I show that this emission is predominantly related to the accretion process. A second sample of 72 RQQs is obtained from the Spitzer-Herschel Active Galaxy Survey, spanning a factor of ~100 in optical luminosity over a narrow redshift range at z~1. This enables evolutionary effects to be decoupled when comparisons are made with the VIDEO sample. I reduce radio data from the Karl G. Jansky Very Large Array (JVLA) for these objects, and find further support that accretion makes a significant contribution to the radio emission in RQQs. In addition, the levels of accretion and star formation appear to be weakly correlated with each other, and with optical luminosity. These findings offer an insight into how the mechanisms behind galaxy evolution may interact differently in RQQs compared to their radio-loud counterparts. They also have important implications for modelling radio populations below 1mJy, which is necessary for the development of the Square Kilometre Array.

Stellar evolution models underpin much of astrophysics, yet they are severely under-constrained on the pre-main sequence. The masses, radii, temperatures and luminosities of detached double-lined eclipsing binaries (EBs) can be determined in a model-independent manner from the light and radial velocity curves of the system. When these reach a precision of a few percent or less, they provide one of the most powerful observational tests of stellar evolution theory. Furthermore, young binary star systems display significant photometric and spectroscopic variability over a range of timescales and wavelengths. EBs offer powerful test-beds for characterising such variability, as the stellar parameters and orbital geometry are known to high precision. This thesis presents the characterisation of EBs discovered by the CoRoT and Spitzer space missions in the ~3 Myr old NGC 2264 star forming region. A sample of EBs in a young open cluster such as this, constitutes a particularly strong test of stellar evolution models because the EBs share the same age and composition, yet span a wide range of masses. We present the discovery of CoRoT 223992193, a detached, double-lined eclipsing binary, which comprises two pre-main sequence M-dwarfs and shows evidence of a circumbinary disk. We develop innovative approaches, based on Gaussian process regression, to determine the fundamental stellar parameters. At the time of discovery this was only the ninth pre-main sequence eclipsing binary (PMS EB) with component masses below 1.5 M? and provides a useful test of stellar evolution models. CoRoT 223992193 is the first low-mass PMS EB to show evidence of a circumbinary disk and displays significant photometric and spectroscopic variability. The two stars are found to possess slightly different rotation periods: the primary is consistent with synchronisation while the secondary rotates slightly supersynchronously. The photometric and spectroscopic variations are consistent with the picture of two active stars possibly undergoing non-steady, low-level accretion; the system's very high inclination provides a new view of such variability. The techniques developed in characterising CoRoT223992193 are applied to the NGC2264 EB sample. These work well for near equal-mass systems and two main sequence EBs are solved. However, primarily due to the one-dimensional nature of the radial velocity determination, the majority of the sample remains unsolved; more powerful methods are presented and are suggested for further work. Four low-mass EBs show evidence for youth, with three of these displaying low-to-extreme brightness (and hence mass) ratios. No such PMS systems have been reported in the literature. Once complete the full NGC 2264 sample will form one of the most stringent tests of PMS stellar evolution models to date. Finally, we present a visual multiple containing three EBs spanning B-M spectral types. Two of these systems show evidence for youth and cluster membership; the third is more ambiguous. Further observations are required to confirm the nature of this multiple, but if all three EBs are constituent members, it would be the first triply-eclipsing multiple system discovered, and will provide a powerful test of star formation and stellar evolution models in dense cluster environments.

New observations of multiple molecular gas tracers in nearby early- and late-types galaxies are presented and are used to study the physical conditions of the gas within different morphological structures. The CO Tully-Fisher relation is also constructed for a sample of star-forming galaxies at z = 0:05 - 0:3, probing their mass and size evolution. First, using single-dish observations of multiple locations within the nearby spiral galaxy NGC 6946, extensive CO ladders are generated. The molecular line ratios reveal a large variety of physical conditions across the molecular gas complexes, depending primarily on the presence of current or recent star formation, itself compared with that in the centre of the galaxy and other galaxies. Second, interferometric observations of CO and high density molecular tracers in the nearby edge-on early-type galaxies NGC 4710 and NGC 5866 are presented. The gas kinematics reveals that the galaxies are barred, with most of the gas contained within a nuclear disc and a distinct inner ring. Using the molecular line ratios to probe the physical conditions of the gas, the nuclear discs appear to have a more diffuse and hotter molecular medium than the inner rings, with more embedded dense clumps. This suggests that the conditions in the nuclear discs are similar to those in photo-dissociation regions, with intense UV radiation from young stars and few cosmic rays. Indeed, the observed molecular line ratios are also intermediate between those of spiral galaxies and starbursts, with even milder star formation in the inner rings. Third, homogeneously measuring the line widths in the CO spectra of star-forming disc galaxies at z = 0:05 - 0:3, their Ks-band CO Tully-Fisher relation is constructed. A comparison to local star-forming galaxy TFRs from the literature provides mild evidence that our sample galaxies are ≈ 0:89 mag brighter than local ones at a given rotational velocity, a result entirely consistent with our stellar mass TFR, suggesting that our sample galaxies are more massive than local ones by ≈ 0:35 dex. While they deserve further scrutiny, we suspect that these results are due to our sample galaxies being more heavily star-forming (and thus brighter at a given mass) than the comparison sample galaxies.

This thesis presents a detailed study of the impact of radio-AGN on star formation and the interstellar medium (ISM) of galaxies across cosmic time. To do this, this thesis uses far-IR/sub-mm data from the Herschel Space Observatory. I create a well-selected sample of 1599 radio sources using the NRAO VLA Sky Survey (NVSS) and Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) data in combination with the UKIRT Infrared Deep Sky Survey - Large Area Survey (UKIDSS - LAS) data. I find that the far-IR luminosities and dust temperatures of radio galaxies are lower in comparison to those of non-radio-detected galaxies. This luminosity deficit grows with increasing stellar mass. I argue that the reasons for these differences is probably due to indirect radio-AGN feedback, i.e. radio jets mechanically heat the halo-environment, preventing external sources of cold gas from entering the host and forming stars. The far-IR luminosity and dust temperature is found to decrease as a function of radio source size. I find the most likely explanation for this is jet-induced star formation while the jets are confined to the ISM. Finally, a method for identifying reliable high-z, star-bursting radio sources in the H-ATLAS is described with which statistically significant studies of radio-jet induced star formation may be undertaken.

The recent space-based exoplanet transit searches, CoRoT and Kepler, have revolutionised the field of stellar variability. In this thesis I exploit the public Kepler data to characterise stellar variability, and study rotation periods. For the study of stellar variability it is a complicated but necessary process to remove instrumental systematics while maintaining intrinsic stellar signal. I was involved in the development of a new correction method for systematics, denoted ARC (Astrophysically Robust Correction). This method relies on the removal of a set of basis functions that are determined to be present in small amounts across many light curves. Using the first month of Kepler data, corrected with the ARC method, I studied the variability properties of main sequence stars as a function of fundamental stellar parameters. I find that the fraction of stars with variability greater than that of the Sun is 60%, and confirm the trend of increasing variability with decreasing effective temperatures. I show tentative evidence that the more active stars have lower proper motions and may be located closer to the galactic plane. I also investigate the frequency content of the variability, showing that there exist significant differences in the nature of variability between spectral types, with a trend towards longer periods at later spectral types. In order to exploit the full potential of the Kepler data for stellar rotation period measurement, I developed a novel method of period detection for use on star spot modulated light curves. Standard approaches to period detection are based on Fourier decomposition or least-squares fitting of sinusoidal models. However, typical stellar light curves are neither sinusoidal nor strictly periodic. Therefore, I developed an algorithm for period detection based on the autocorrelation function (ACF) of the light curve. Because the ACF measures only the degree of self-similarity of the light curve at a given time lag, the period remains detectable even when the amplitude and phase of the photometric modulation evolve significantly. I applied the ACF method for the sample of M-dwarfs observed during the first 10 months of the Kepler mission, and detected rotation periods in 1570, ranging from 0.37-69.7 days. The rotation period distribution is clearly bimodal, with peaks at ~19 and ~33 days, hinting at two distinct waves of star formation. These two peaks form two distinct sequences in period-temperature space, with the period decreasing with increasing temperature. In a natural continuation to this work I applied measured periods for 1000 stars in each of the F, G and K-dwarf sets observed by Kepler, and combined these with the M-dwarf results. The trend of increasing rotation period with increasing mass is clear throughout, as the observations fall along a wide by distinct sequence. Comparison to the rotational isochrones of Barnes (2007) show an overall agreement, although the dataset, which I believe is the largest set of rotation period measurements for main sequence stars, shows addition detail, not captured by the gyrochronology relations. This includes a dip in the rotation period distribution at ~0.6 M⊙ and a steep increase in period for the M-dwarfs. I also applied the ACF method to the Kepler exoplanet candidate host stars and used the results to search for evidence of tidal interaction between the star and planet. I show that for the majority of exoplanet host stars, spin-orbit interaction will not have affected the stellar rotation period, permitting the application of gyrochronology for age determination. A comparison of the host stars with a sample of field stars selected to match their temperature and magnitude distribution also indicates no significant difference in the period or amplitude distributions of the two sets. The only notable variation is the lack of planets around the very fast rotators across all spectral types.

Black hole jets and accretion discs are the most extreme objects in modern astrophysics whilst dark energy is undoubtedly the most mysterious. This thesis focuses on understanding these three topics. The majority of this thesis is dedicated to investigating the structure and properties of black hole jets by modelling their emission. I develop an inhomogeneous jet model with a magnetically dominated parabolic accelerating base, transitioning to a slowly decelerating conical jet, with a geometry set by radio observations of M87. This model is able to reproduce the simultaneous multiwavelength spectra of all 38 Fermi blazars with redshifts in unprecendented detail across all wavelengths. I constrain the synchrotron bright region of the jet to occur outside the BLR and dusty torus for FSRQs using the optically thick to thin synchrotron break. At these large distances their inverse-Compton emission originates from scattering CMB photons. I find an approximately linear relation between the jet power and the transition region radius where the jet first comes into equipartition, transitions from parabolic to conical and stops accelerating. The decreasing magnetic field strength and increasing bulk Lorentz factor with jet power are the physical reasons behind the blazar sequence. I calculate the conditions for instability in a thin accretion disc with an α parameter which depends on the magnetic Prandtl number, as suggested by MHD simulations. The global behaviour of the instability induces cyclic flaring in the inner regions of the disc, for parameters appropriate for X-ray binary systems, thereby offering a potential solution to a long standing problem. Finally, I calculate the effect of an interacting quintessence model of dark energy on cosmological observables. I find that a scalar-tensor type interaction in the dark sector results in an observable increase in the matter power spectrum and integrated Sachs-Wolfe effect at horizon scales.

While the progenitors of Type Ia supernovae (SNe Ia) have long been thought to be thermonuclear explosions of white dwarf stars, what triggers the explosion are still a topic of debate. This thesis considers constraints on single-degenerate progenitors of SNe Ia based on the presence of a Roche-lobe filling companion. The ejecta strips material from the companion, that maybe detectable via Hα emission during the nebular phase. Using the full structure from simulations produces line widths are larger than those produced in simple models. The structure formed by the ejecta-companion interaction produce a broken reverse shock that may be visible in X-rays via the Fe Kα line at the age of Tycho's supernova remnant (SNR). If the similar structures in Tycho’s SNR are produced this way then the companion star must have been massive, with M ~ 2 M⊙. Detections of circumstellar material within the supernova provides another way to indirectly probe the companion star. Mass loss through winds or novae are expected to shape the circumsteller medium for single-degenerate progenitors and the velocities, v ~ 100 km s-1 appear to be consistent with recurrent nova shells, a model that is tested by analysing simulations of RS Ophiuchi. Models of RS Ophiuchi can explain the absorption lines seen around the 2006 outburst if the mass loss is 10−6 M⊙ yr^{-1,/sup>. The circumsteller medium is shown to produce in the velocity and relative strengths of the features seen in SN 2006X. However, whether density in the shells is high enough to produce the required recombination timescale and to overcome ionization by γ-rays for shells at 5 × 1016 cm remains uncertain.}

The correspondence of stationary, axisymmetric, asymptotically flat space-times and bundles over a reduced twistor space has been established in four dimensions. The main impediment for an application of this correspondence to examples in higher dimensions is the lack of a higher-dimensional equivalent of the Ernst poten- tial. This thesis will propose such a generalized Ernst potential, point out where the rod structure of the space-time can be found in the twistor picture and thereby provide a procedure for generating solutions to the Einstein field equations in higher dimensions from the rod structure, other asymptotic data, and the requirement of a regular axis. Examples in five dimensions are studied and necessary tools are developed, in particular rules for the transition between different adaptations of the patching matrix and rules for the elimination of conical singularities.