This Thesis is written in three parts. The first part describes the analytic calculation of the unequal-time correlator of cosmic strings and superstrings. The first efficient constraint analysis of all string and superstring network parameters is performed. By studying the effect of cosmic strings on the cosmic microwave background (CMB) radiation it is discovered that cosmic strings must make up a vanishingly small proportion of the energy density of the universe. The constraints on string network parameters are all skewed toward reducing the magnitude of energy density arising from strings. Also in this Part, a better comprehension of the unconnected segment model (USM) was gained. In particular, a greater understanding of the string scaling parameter $L_f$ was garnered, as well as finding the reason why the USM tends to provide greater power than simulations of Nambu-Goto cosmic strings. The second part contains a detailed description of statistical cosmology and how differences between parameter constraints from different data sets can lead to misleading quantification of discordance. The majority of this part describes different methods of quantifying differences between probability distributions and how these can be interpreted. In particular, using the most up-to-date data possible, differences between parameter constraints using the CMB and probes of large scale structure (LSS) in the universe can be measured. With current data the discordance can be interpreted as a low level of disagreement, but the application of prior ranges on well known parameters can force the tension to be greater. Using data from earlier work, this issue is considered in greater detail, with extensions to the accepted LCDM model added to test if the discordance can be alleviated. These extensions include the addition of active or sterile neutrinos and even ad-hoc changes to the primordial power spectrum. Although there are slight hints that these may help, when considering only the new data it might be unwise to believe that the discordance between parameter distributions from different data sets exists to a degree where the modifications are necessary. Finally, application of deep learning to astrophysical observations is discussed. Using neural networks to learn about specific problems is de rigueur and their use in astronomy and cosmology is a promising field of study. In particular, applying raw data to neural networks can often outperform, or add enhanced features, to what is possible with current, non-empirical feature detection. The classification of supernovae from their light curves can be achieved using a specific machine learning architecture called a recurrent neural network (RNN). Using the raw data from supernova light curves, the RNN is able to learn about features in sequences which can be used to classify types of supernova. Although a large training set is needed to perform as well as current techniques, one major advantage the RNN method has is the possibility of early detection. Rather than needing the entire light curve to perform statistical fits to categorise the supernova type, relatively little information from the early observation data is needed to classify using the RNN. Installing RNN on machinery for observation would save a vast amount of time by early classification since only supernovae of interest can be concentrated on.

The analysis of nuclear flares is unfortunately difficult. Contamination of supernova surveys by active galactic nuclei (AGN) variability, and the difficulty in detecting transients in the high surface brightness nuclei of galaxies, has led to many surveys avoiding nuclear transients entirely. Even in cases where transients are detected, their identification and classification remains complex, with many possible progenitor pathways, overlapping models and wide ranges of observed properties to explain. Here I consider a sample of these events, placing them in the wider context of transient astronomy. The detection of a class of relativistic tidal disruption flare, thought to be the capture and disruption of Sun-like stars that also powers a moderately relativistic jet, has prompted the search for more of these events. Within this work I analyse the properties of one such candidate, Swift J1112.2-8238, conforming its extragalactic origin and showing it came from a galaxy at a redshift of z = 0.89. Its high energy and optical properties are consistent with the previous candidates and its position, close to the centre of a likely star-forming host, continues to support the tidal disruption flare origin of these events. The rates of these events suggest that only a small fraction of tidal disruption flares launch similar jets. Prompted by these findings, I proposed and obtained medium resolution spectroscopy and radio observations of the source, and analysed high-resolution HST imaging to further constrain the position of the transient within its host. The HST imaging shows that the host has a complex morphology, perhaps due to an interaction with another galaxy, with the transient loosely consistent with the centre of compact bulge-like component. I confirm the host's redshift and determine its nature as a star-forming galaxy. Radio emission detected coming from the host, that is too luminous to be associated with star-formation, shows evidence of variability, suggesting that it is associated with the transient flare and thus is perhaps confirmation of the jetted nature of the event. In almost all respects, Swift J1112.2-8238 remains an excellent candidate relativistic tidal disruption flare. Finally, I analyse HST imaging of a number of flares with unusual properties. I greatly improve the astrometric tie of ASASSN14ae and ASASSN14li to the nuclear regions of their hosts and show that their properties are still most consistent with a tidal disruption flare origin. In the case of CSS100217 and ASASSN15lh however, the “accepted" classification of their origins as superluminous supernovae appears to be at odds with their host galaxies, with ASASSN15lh in particular coming from a massive host with minimal star-formation. I show that CSS100217 has undergone a significant drop in apparent quiescent-level emission following the flare, indicating the possibility that the flare may have directly impacted, or been caused by, a change in the accretion of the known AGN. I consider the possibility that both flares could be associated with unusual tidal disruption flares or AGN variability, though the current observations make it difficult to make strong claims about either flare's true origins.

The first direct detection of dark matter is anticipated in coming years by one of a range of experimental strategies. Because the identity of dark matter remains unknown, the strategy that will be successful in this one cannot say. However beneath this fundamental particle physics uncertainty lies another uncertainty with regard to the structure of the dark matter halo of the Milky Way that must be confronted when interpreting data from terrestrial experiments. However these astrophysical uncertainties might only be resolved with the very same experiments; in fact, directly detecting dark matter represents the only way to probe the ultralocal structure of the halo. This thesis explores the impact of astrophysical uncertainties on the particle physics goals of dark matter detection but also the extent to which we might in the future be able to resolve those uncertainties. The discussion is framed around the detection of three types of particle, two of which are dark matter candidates: weakly interacting massive particles (WIMPs), neutrinos and axions. In the case of WIMPs I consider how upcoming directionally sensitive experiments can be used to probe the full 3-dimensional velocity distribution to learn about dark matter substructure. A range of model dependent and independent statistical approaches are tested under various astrophysical benchmarks. I also explore prospects for WIMP direct detection when faced with the ultimate neutrino background, as expected in the next generation of experiment. In this eventuality the uncertainties in the neutrino flux are essential in predicting the WIMP models inaccessible due to the background. However the same is true of astrophysical uncertainties. Once astrophysical uncertainties are accounted for the neutrino floor limit is raised in cross section by up to an order of magnitude and the accuracy of any potential WIMP particle measurement is greatly increased. Addressing these concerns, I demonstrate how one should go about subtracting the neutrino background. This involves a return to directional detection. I find that even non-ideal circumstances, the neutrino and WIMP signals can be distinguished and the neutrino floor overcome. Finally in the context of axions, I discuss the prospects for microwave cavity haloscope experiments to perform "axion astronomy". Haloscopes measure the direct conversion of axions into photons and hence can make potentially much finer measurements of the dark matter halo compared with WIMPs. I develop a technique to extract astrophysical parameters, such as the halo velocity dispersion and laboratory velocity, as well as learn about properties of substructure from tidal streams and axion miniclusters. I show that a level of precision can be achieved in relatively short duration haloscope experiments that can match or improve upon that of astronomical observations.

No description available.

The evolution of galaxy cluster counts is a powerful probe of several fundamental cosmological parameters. A number of recent studies using this probe have claimed tension with the cosmology preferred by the analysis of the Planck primary CMB data, in the sense that there are fewer clusters observed than predicted based on the primary CMB cosmology. One possible resolution to this problem is systematic errors in the absolute halo mass calibration in cluster studies, which is required to convert the standard theoretical prediction (the halo mass function) into counts as a function of the observable (e.g., X-ray luminosity, Sunyaev-Zel'dovich flux, optical richness). Here I propose an alternative strategy, which is to directly compare predicted and observed cluster counts as a function of the one-dimensional velocity dispersion of the cluster galaxies. I show that the velocity dispersion of groups/clusters can be theoretically predicted as robustly as mass but, unlike mass, it can also be directly observed, thus circumventing the main systematic bias in traditional cluster counts studies. With the aid of the BAHAMAS suite of cosmological hydrodynamical simulations, I demonstrate the potential of the velocity dispersion counts for discriminating even similar ΛCDM models. Then, I compare the abundance of groups in the GAMA survey to the predictions from BAHAMAS to constrain the values of several cosmological parameters. Additionally, I investigate the role of active galactic nuclei (AGN) in galaxy evolution. The color bimodality of galaxy populations roughly divides galaxies into two groups: blue, star-forming galaxies, and red, quiescent galaxies. One theory that explains how high-mass, red, non-star-forming galaxies maintain this condition is the duty cycle hypothesis. This hypothesis invokes AGN feedback from low luminosity radio-loud AGN (LERGs) to deposit mechanical heating into the intergalactic medium, thus preventing star formation. I test this hypothesis by comparing the half-light radii of quiescent elliptical galaxies with LERG host galaxies using a large multi-wavelength sample from two surveys, UKIDSS/UDS, and ULTRAVISTA/COSMOS. The radius distribution of the two groups are similar, thus providing evidence for the duty cycle hypothesis. I also check the star formation activity of the LERGs. For the duty cycle to hold, LERGs should reside within non-star-forming galaxies. However, I find that a subset of LERGs appear to be dusty star forming galaxies.

Galaxy clusters are the most recent of cosmological structures to have formed by the present time in the currently favoured hierarchical scenario of structure formation and are widely regarded as powerful probes of cosmology and galaxy formation physics alike. Over the past few years, it became increasingly clear that precision cluster cosmology requires the development of detailed, realistic theoretical models of galaxy clusters and the confrontation of synthetic surveys generated using these models with observations. This motivates a campaign of large cosmological hydrodynamical simulations, with plausible 'sub-grid' prescriptions for the relevant galaxy formation physics. This thesis presents a new suite of large-volume cosmological hydrodynamical simulations called cosmo-OWLS. They form an extension to the Overwhelmingly Large Simulations (OWLS) project, and have been designed to help improve our understanding of cluster astrophysics and the non-linear structure formation, which are now the limiting systematic errors when using clusters as cosmological probes. Starting from identical initial conditions in either the Planck or WMAP7 cosmologies, the most important 'sub-grid' physics, including feedback from supernovae and active galactic nuclei (AGN) has been systematically varied. Via the production of synthetic surveys of the simulations and comparisons with observations, the realism of these state-of-the-art models was explored. At the same time, the simulations were shown to providea valuable tool for interpreting the observational data, as well as powerful means for testing commonly-employed methods for estimating, for example, cluster masses and determining survey selection functions, which are crucial for cluster cosmology. The properties of the simulated galaxy groups and clusters were first compared to a wide range of observational data,such as x-ray luminosity and temperature, gas mass fractions, entropy and density profiles, Sunyaev-Zel'dovich flux, I-band mass-to-light ratio, dominance of the brightest cluster galaxy, and central massive black hole (BH) masses, by producing synthetic observations and mimicking observational analysis techniques. These comparisons demonstrated that some AGN feedback models can produce a realistic population of galaxy groups and clusters, broadly reproducing both the median trend and, for the first time, the scatter in physical properties over approximately two decades in mass (¹³M⊙≲500≲10¹⁵M⊙) and 1.5 decades in radius (0.05≲500≲1.5). However, in other models, the AGN feedback is too violent (even though they reproduce the observed BH scaling relations), implying calibration of the models is required. The production of realistic populations of simulated groups and clusters, as well as models that bracket the observations, opens the door to the creation of synthetic surveys for assisting the astrophysical and cosmological interpretation of cluster surveys, as well as quantifying the impact of selection effects. A study of the scatter and evolution of the hot gas properties of the populations of galaxy groups and clusters, such as X-ray luminosity and temperature, gas mass and Sunyaev-Zel'dovich flux, as a function of the important non-gravitational physics of galaxy formation was then conducted. The median relations and the scatter about them are reasonably well-modelled by evolving broken power-laws. The non-radiative model and the model that neglects AGN feedback are consistent with having selfsimilar mass slopes, whereas the mass slopes of the AGN feedback models deviate significantly from the self-similar expectation. Self-similar evolution, which is widely adopted in current cosmological studies, was also found to break down when efficient feedback is included. The log-normal scatter varies mildly with mass, is relatively insensitive to non-gravitational physics, but shows a moderately strong decreasing trend with increasing redshift. The X-ray luminosity has a significantly larger scatter than all the other hot gas proxies examined. It is thus the poorest one, while the 'best' one is the mean X-ray temperature. Synthetic Sunyaev-Zel'dovich observations, generated using a 'multi-purpose' light cone software package developed during the thesis, were used to check the veracity of some of the results reported by the Planck collaboration at the end of 2012. Taken at face value, their results seem to favour a close to self-similar scaling relation between the Sunyaev-Zel'dovich flux and total mass all the way down to individual galaxy haloes, which is in contradiction with X-ray and absorption lines observation. The matched filter used by the Planck collaboration recovers fluxes which are biased increasingly high as feedback intensity increases. Two likely causes for the bias, i.e. confusion and deviations from the universal pressure profiles were investigated. Confusion was found to have a negligible effect when the signal is averaged over a large number of systems. Instead a shape mismatch (in terms of pressure profiles) was identified as being mostly responsible for the bias. Finally, synthetic X-ray observations, generated using a combination of the developed light cone software and of the XMM-Newton simulator and processed with the detection pipeline of the XXL survey, were used to start quantifying the selection function of the XXL survey. Preliminary results suggest that: (i) XXL is only able to find a very small fraction of the galaxy group population, (ii) the survey is best at finding lowmass clusters (14.0≲log₁₀[M₅₀₀(M⊙)]≲14.5) at z ≲ 0:75, and (iii) the detection pipeline misses a few very massive, very extended, nearby systems.

The motivation of this thesis was the study of radio-loud, active galaxies. These galaxies house relativistic jets at their centres, powered by accretion onto a super massive black hole. The focus was on the optical flux and polarised emission produced by these powerful jets. An automated pipeline was developed to reduce data from the Liverpool Telescope Ringo2 and Ringo3 polarimeters. As part of this work, the Ringo3 instrumental polarisation and depolarisation were characterised by repeated observations of standard stars. The Ringo2 and Ringo3 optical polarimetry and photometry of a sample of 20 gamma-ray bright blazars were combined with Fermi gamma-ray space telescope data and were used to explore possible correlations and thus probe the emission sites in the jet. We found that optical and gamma-ray fluxes had strong, positive correlations. This suggests that the dominant source of optical and gamma-ray emission is from shared emission regions. If the Inverse Compton model is adopted to explain the gamma-ray emission (i.e. upscattering of photons by relativistic electrons), this correlation suggests that synchrotron self-Compton emission processes are occurring in the jet, along with inverse Compton upscattering from nearby electrons (rather than those outside the jet). The gamma-ray flux and optical degree of polarisation were not significantly correlated. The optical flux and degree of polarisation were weakly positively correlated (with correlations that did not improve with an introduced lag). Both of these results imply that there is no large scale highly ordered magnetic field in the region where the gamma-ray emission originates. We found that the maximum degree of polarisation differs depending on the location of the source's synchrotron-peak. This may be a result of the viewing angle of the observer with respect to the jet. This suggests that the majority of optical polarisation is produced in shocked regions within the jet, downstream of the main emission region. We found that the degree of polarisation was lower during a period of polarisation angle rotation compared with a period of non-rotation. This implies that the downstream magnetic field structure is either helical or compressed in a direction transverse to that of the jet. Consistent with other work, our Ringo3 colour analysis showed that, with the exception of one source, flat spectrum radio quasars had a `redder' when brighter property. This suggests that when the source is more luminous, the jet (i.e. non-thermal) emission dominates over the thermal emission from the accretion disk (which is powerful in FSRQs). We found that BL~Lacs had a `bluer' when brighter behaviour, suggesting that the brighter emission may come from more energetic photons within the jet. We presented data from our long-term, multi-colour, blazar monitoring campaign. We found that all but one source had a `redder' polarisation when the polarisation was higher. This implies that the highest polarisation is associated with higher densities of lower energy particles in the jet. Well-sampled, regular cadence data is very important for the effective study and interpretation of blazars. This is particularly crucial for the interpretation of the position angle rotations, which can afford information about the electric vector angle (and hence the magnetic field angle). In this work, we presented the design of a new multicolour polarimeter, MOPTOP. The optical components in MOPTOP allow as much of the light from the source to be exploited as possible by replacing the rotating Polaroid (from the Ringo polarimeter design) with a rotating half-wave plate and beam splitter. MOPTOP's design minimises exposure times, allowing more frequent observations and a better sampling of data. A densely sampled monitoring program that is not interrupted by periods of sunlight would be highly desirable for the study of blazar jets.

This thesis investigates the impact of photoionisation on gas clouds of various scales in our Galaxy. On kiloparsec scales, the origin of the diffuse ionised gas (DIG) has been studied using Monte Carlo photoionisation simulations in static simulations of the interstellar medium (ISM). Low density pathways through the gas allow photons to propagate, producing the DIG. The emission in this gas indicates that the temperature increases as the density of the gas decreases, suggesting a heating mechanism in the gas that dominates over photoionisation. This has been investigated with the inclusion of an approximation to cosmic ray heating, which is able to reproduce the observed line ratios of [NII]/Hα and [SII]/Hα. It has been suggested that the Hα emission in this gas may not be a result of in situ emission, rather, it results from dust scattering of Hα photons from HII regions. Dust scattering simulations find that 20% of the Hα intensity observed in the diffuse gas is a result of scattering, leaving in situ ionisation as the primary producer of the DIG. The dynamical impact of photoionisation in small-scale gas clouds, as well as larger scale diffuse gas, is now recognised as important in our understanding of star forming regions. A new code has been developed using a combination of hydrodynamical and Monte Carlo photoionisation methods. Tests of the validity of this code are presented, showing that this method accurately reproduces benchmark tests. This method has been used to investigate the lifecycle and x-ray observations of ultra-compact HII (UCHII) regions with the inclusion of stellar wind and photoionisation feedback. These simulations find that x-ray emission is not produced in high enough intensity to be observed until after the UCHII phase, when stellar winds begin to drive the expansion.

This thesis presents the first population study of supernova remnants (SNRs) in one whole galaxy – the Large Magellanic Cloud (LMC) on submm and FIR wavelengths. The first part is about the dust production in supernovae (SNe) and SNRs, based on several observations of SN1987A on mm and submm wavelengths that I made using the ATCA and APEX telescopes. SN1987A is found to produce ∼0.7 M⊙ of dust, which is 2 orders of magnitude higher than the masses found in most of other SN/SNR observations. I constrained the spectral energy distribution (SED) of SN1987A, confirming Herschel data using better resolution, but did not manage to resolve the object. These data were used in the preparation of the ALMA observations (Kamenetzky et al. 2013; Indebetouw et al. 2014). The second part of the thesis is the population study of all LMC SNRs using Herschel and Spitzer data, which resulted in the conclusion that SNRs are significant dust destroyers. This conclusion is based on dust mass maps of SNRs and their surroundings which have shown that there is less dust within SNRs than outside. My study shows that a SNR in the LMC removes on average 4–6 M⊙. I conclude that SNRs might not be the main suppliers of dust in galaxies, and that it is possible that other sources of dust production are needed to explain the origin of the dust at high red-shifts. I estimate the mass of sputtered dust from all SNRs in LMC to be ∼373+746 249 M⊙, a dust destruction rate in the LMC of ∼0.037+0.074 −0.025 M⊙ yr−1 due to SNRs and an average lifetime for interstellar dust in the regions close to SNRs of ∼2+4 −1.3 × 107 yr.

Over the last few decades enormous progress has been made in the study of the Universe and we are now entering the age of precision cosmology, with numerous upcoming high precision surveys expected to provide us with an incredible wealth of information. Observational data is usually interpreted once assumptions about the underlying cosmology are made. One of those commonly made assumptions is that the Universe is homogeneous and istropic, which observations seem to indicate is the case on very large scales. We develop a class of exact inhomogeneous solutions to general relativity for dust and a cosmological constant with which we can model a line of sight with arbitrary matter distribution; far away from this line of sight the solutions tend towards a standard homogeneous model of the Universe. This class of solutions is very well suited to model the effects of inhomogeneities along the line of sight on cosmological observations. We find that the effects of the inhomogeneities on the relation between distance and redshift are small if one imposes that the inhomogeneities along the line of sight average to the background density. Using compensated structures of several shapes and sizes, we find the deviations from the distance redshift relation to be below 1%. However, as soon as the lines of sight are not completely compensated larger deviations are found. We investigate this effect further and compare several exact solution to general relativity and perturbative approaches. The results from the three exact solution are very similar and indicate that uncompensated lines of sight can result in distance – redshift relations very different to the homogeneous ones. For small fluctuations we find that the complete linear analysis agrees with the results from exact solution but weak lensing predictions do not. The expansion rates along lines of sight which are not compensated are different than in the background which causes the large deviations in the distance – redshift relation. We find that void regions expand faster than in the background, but can they expand fast enough to explain the observed cosmic acceleration? However, on smaller scales such as galaxies and groups of galaxies this is clearly not the case. With the precision of observations increasing to unprecedented levels, is it still justifiable to make the assumption that the Universe is homogeneous on all scales even though we know that this is not the case on most scales? Much of this thesis is dedicated to this question. To answer this question from the exact solutions point of view we develop an inhomogeneous solution to general relativity for a single fluid with a constant equation of state parameter in the background. Within this solution we investigate the expansion properties of compensated regions and void regions. We find that compensated regions expand as the background and find that void regions do expand faster than the background but cannot cause cosmic acceleration. The physical mechanisms at work during the early Universe are not very well understood yet, but the hope is that through the data provided by future high precision surveys we might be able to constrain some of the theories. In particular constraints on the levels of primordial non-Gaussianity will be a powerful discriminator between theories. Therefore we investigate the growth of matter inhomogeneities to second perturbative order in a concordance cosmology and find the dependence of the density fluctuations on primordial non-Gaussianities. We also show how Newtonian and purely general relativistic non-linear effects enter into the second order density fluctuations. This understanding is essential in extracting information about primordial non-Gaussianties from the distribution of large scale structure today. Lastly, we analysed a proposed way of probing cosmic expansion by using the well studied Alcock-Paczynski effect in the dynamics of galaxy pairs. We studied the dynamics of galaxy pairs in an N-body simulation and found that once several cuts are made on the selection of the galaxy pairs, including isolation criteria, mass cuts and separation cuts, there might be a possibility of using pairs with such properties as cosmic tracers. Modelling of the velocities of the galaxies due their mutual attraction and local densities needs to be done first though to remove systematic errors in the observations.