Broad absorption line quasars show broad blue shifted absorption indicative of high velocity outflowing material. The mechanism responsible for accelerating these outflows is a matter of much debate. In this thesis I investigate the possibility that radiative line driving is responsible for driving the outflows in some objects. Various metrics have been proposed to compile homogeneous samples of BALQSOs. Using a new hybrid algorithm we have compiled a sample of 3552 BALQSOs representing approximately 12.5% of the quasars for which SDSS spectra (from DR5) covering the C IV broad absorption line region are available. From this BALQSO catalogue we have selected the largest sample of objects exhibiting evidence for radiative line driving (the ghost of Ly-α), a feature observed in the BAL troughs of around 1% of BALQSOs. Using this sample we test the criteria purported necessary for ghost-formation in order to confirm whether the identified feature is in fact a ghost. This investigation suggests that the feature observed in the majority of objects arises from the superposition of multiple absorption components, which mimic the appearance of a ghost and unrelated to interactions between N V and Ly-α. Follow up spectroscopic observations of some of the ghost candidate objects taken using the William Herschel Telescope and the Gemini North telescope are presented. The reduction of these data is described in detail. The criteria for ghost formation are tested on the new spectra with similar results. One object whose absorption has undergone significant changes is identified and photo-ionisation models used to provide constraints on the physical conditions within the absorber. These models suggest that the changes are due to an increase in the ionisation parameter and not due to changes in the gas covering fraction. The location and mass outflow rate in this object is consistent with previous estimates in similar systems. The work presented within this thesis does not preclude radiative line driving as a mechanism responsible for high velocity quasar outflows. It does however suggest that the ghost of Ly-α, previously considered the strongest evidence for radiative line driving is rarely observed.

Cosmography is the study of the shape, size, disposition and other properties of the large-scale components of the physical universe. The following survey assembles and discusses available Chinese material on this subject from early times up to the rise of the T'ang dynasty in A.D. 618, after which Chinese interest in the topic seems to have diminished. Part I discusses evidence from texts dating before 250 B.C. A cosmography involving a number of mythical elements is thus reconstituted; heaven is a solid vault over a flat square earth. Geographical speculations about the existence of several continents are discussed. Part II describes the Kai t'ien theory according to the Chou pei, a book possibly compiled in the first century B.C.; an umbrella-like heaven rotates over a similarly shaped earth. The dimensions of this universe are linked to astronomical observations made with a gnomon. Parts III, IV and V follow the general discussion of cosmography from 250 B.C. to the close of the survey. The Kai t'ien theory was repeatedly criticised on empirical grounds, and by about 100 A.D. had been replaced by the Hun t'ien theory, which involved a spherical heaven rotating about an inclined axis and enclosing a flat earth. Within the context of this scheme there was much discussion of points of detail, including the cause of the luminosity of the heavenly bodies and the mechanism of eclipses. Considerable efforts were made to establish the dimensions of the Hun t'ien universe, but were vitiated by lack of an adequate geometry. A number of other original but less important theories were proposed. It appears that the ancient Chinese never conceived of the earth as spherical; implications of this are discussed in the Introduction.

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We study the large-scale dynamics of the interstellar medium in barred galaxies. The interest in doing this is two-fold. On the one hand, the hydrodynamic flow is the source of many interesting physical phenomena, such as shocks and spiral arms. On the other hand, it is a powerful tool to constrain the characteristics of individual galaxies. Approximately half of this thesis is devoted to understanding the general characteristics of the gas dynamics. There are two key ingredients. The first is ballistic closed orbits. We investigate the connection between closed orbits and the full hydrodynamic flow. We show how they also form the basis to explain bar-driven spiral arms. The second key ingredient is shocks. These can arise at the transition between different families of closed orbits, or when the spiral arms become too strong. Under certain conditions, shocks become hydrodynamically unstable and produce density fluctuations that may have an observational counterpart in the form of asymmetries in the Milky Way molecular gas distribution. We also explore systematically how the gas flow is affected by a change in the parameters that characterise the bar, such as its length, strength and pattern speed. Through the study of the gas dynamics, inferences can be made about the structure of galaxies. We have compared our models with observations of the Milky Way. First, we revisit and refine the Binney et al (1991) model, one of the most successful models for the gas flow in the Galaxy. Then, we present new models that while preserving the good properties of the previous model can also correct its main shortcomings. These models can qualitatively account for most of the observational signatures of the Galactic bar and allow us to constrain its characteristics, such as its pattern speed, length and strength. However, we show that is difficult to find a model that accounts for all the important observational features simultaneously due to the high dimensionality of parameter space involved. We argue that automatic fitting method are necessary. To this end, we develop a new quantitative method to fit Milky Way longitude-velocity diagrams that is based on feature matching.

The Cosmic Infra-red Background (CIB) has a similar energy density to that at UV/optical wavelengths, implying that a significant proportion of star formation over the history of the Universe has been obscured by dust. We investigate the dusty star-forming galaxies responsible for the CIB. For this, we use the latest version of the hierarchical galaxy formation model, GALFORM, which is embedded within the Lambda cold dark matter cosmological paradigm. To compute far-IR (FIR) galaxy spectral energy distributions (SEDs), a simple model for the absorption and re-emission of radiation by interstellar dust is used. Recent interferometric observations have highlighted that the coarse angular resolution of single-dish telescopes used for FIR imaging surveys can blend the emission of multiple galaxies into a single source. Simulating single-dish imaging we show that the model can reproduce the difference between the observed interferometric and single-dish derived sub-millimetre number counts. Additionally, we make the prediction that the blended galaxies are typically physically unassociated. The simulated imaging is also used to show that the clustering of single-dish sources is boosted with respect to the underlying galaxy population. We term this `blending bias', and show that it can lead to the dark matter halo masses of FIR-bright galaxies being significantly overestimated. These galaxies are predicted to reside in halos of masses 10^11.5-10^12 h^-1 Msol, and taking the blending bias into account yields a good level of agreement with halo masses inferred from observed clustering. This is also the halo mass range that produces the bulk of the CIB, as it represents the halos most conducive to star formation in the model. We show that the model can predict the observed average FIR SEDs of main sequence galaxies to a remarkable degree of accuracy over the redshift range 0.5 < z < 4. However, a shortcoming of the simple dust model is that it cannot make accurate predictions for mid-IR emission. To address this, we couple GALFORM with the spectrophotometric code GRASIL to compute UV-to-mm SEDs, which we use to make predictions for future James Webb Space Telescope galaxy surveys.

Post-starburst galaxies are a rare class of objects with unusual spectroscopic characteristics. Previous studies have shown that the presence of strong Balmer absorption lines in the spectra of these galaxies are consistent with a recent bust of star formation; however, the cause of such events remains unclear. Their environment and disturbed morphology suggest that many of them are likely remnants of gas-rich mergers of galaxies with comparable masses and models of galaxy mergers support that claim; however, some studies disagree and the origin of these curious systems remains an open debate. Post-starburst galaxies are also often regarded as a plausible transition channel between the blue continuously star-forming and quiescent red galaxies, commonly observed in the local Universe. This is supported by models of merger- driven starbursts, which cause structural transformation of galaxies consistent with evolution towards the red population, and can ultimately lead to quenching of star formation in the merger remnant; however, observational evidence for this scenario remains elusive. In my study I aimed to place further constraints on the role of galaxy mergers in triggering starbursts in local galaxies and to investigate whether the post-starburst galaxies are indeed in transition between the star-forming and passive phases of galaxy evolution, by the analysis of the morphology and structure of galaxies with spectroscopic signatures of a recent starburst. An important difference between this work and many previous studies is the post-starburst sample selection. Traditionally, post-starburst galaxies are selected to have completely quenched their star-formation; here, I also considered those which show spectroscopic sign of residual star-formation, selected using a method introduced by Wild et al. (2007). I have also followed a different approach to quantifying the morphology of galaxies, by means of an automated method for detecting visual post-merger signatures in galaxies, introduced as a part of this work. My analysis suggests that major mergers play a significant role in inducing starbursts in local Universe, but their significance declines as a function of the stellar mass of the galaxies. I also found that post-starburst galaxies have different structural properties in the low- and high-mass regimes and concluded that they are likely to be transitioning between the blue and red galaxy populations through a multi-stage scenario where both major and minor mergers are at play.

This thesis presents an investigation of the X-ray properties of four nearby galaxies, NGC 4151, NGC 7582, Mrk 3 and NGC 4945, all of which harbour a bright, heavily obscured Seyfert nucleus. It explores the implications of the results from these sources for heavily obscured active galactic nuclei (AGN) in general. The work focuses largely on data from the XMM-Newton X-ray satellite, concentrating initially on hard X-ray spectra from the EPIC CCD cameras. The analysis emphasises the complex nature of the X- ray absorption in NGC 4151 and highlights the massive impact that absorption has on the observed X-ray spectrum and variability of all four sources. The range of column densities exhibited (1023 cm-2 to >1024 cm-2) encompasses both Compton-thin, continuum-dominated sources (such as NGC 4151) and Compton-thick, reflection-dominated sources (e.g. Mrk 3 and NGC 4945). The often-subtle role that Compton reflection plays in the X-ray spectrum and variability of these sources is discussed in de tail. The source spectra, in each case, contain an intrinsically narrow, neutral, iron Ka line emission and a sharp iron K edge, adding to the growing body of evidence which suggests that iron reprocessing features are ubiquitous constituents of the hard X-ray spectra of AGN. The analysis of NGC 4945 demonstrates the enormous impact that a nuclear starburst can have on the X-ray properties of AGN. A preliminary analysis of the soft X-ray spectrum of NGC 4151, revealed by the high spectral resolution XMM-Newton RGS instruments, shows the spectrum to be completely dominated by line emission and radiative recombination continua from hydrogen-like and helium-like ions. It is argued that the soft emission originates in photoionized and photoexcited gas located in X-ray/optical ionization cones. The observational findings presented in this thesis are all generally consistent with the unified AGN picture, in which the major differences between the sources are produced by different orientations with respect to our line-of-sight.

This thesis is concerned with how to extract cosmological information from observations of weak gravitational lensing - the modification of observed galaxy images due to gravitational lensing by the large-scale structure of the Universe. Firstly, we are concerned with how we can use all possible observa- tional probes of weak lensing to squeeze out as much cosmological information as possible from future surveys. Up until now, the tra- ditional approach to cosmological weak lensing analyses have focused on the distortion of shapes of distant galaxies measured across the sky - cosmic shear. However, shearing of galaxy shapes is only half the picture - weak lensing also magnifies the sizes and fluxes of observed objects and this lensing magnification field contains the same cosmo- logical information as the cosmic shear field, whilst being subject to a different set of systematic effects. As such, weak lensing magnifica- tion is an exciting complement to cosmic shear and a holistic approach to weak lensing, combining shear and magnification, promises tighter constraints on cosmology, better control of systematics, and more ro- bust science at the end-of-the-day. We develop the theoretical and statistical formalism for performing a cosmological weak lensing anal- ysis using shape, size and flux information together and demonstrate that significant information gains and synergies can be expected from the addition of this new lensing observable - cosmic magnification. Secondly, we are interested in how we can use the statistics of the lensing fields to constrain cosmology via an analysis that is prin- cipled in its propagation of uncertainties, optimal in its use of the full information-content of the data, and exact under clearly stated and well understood model assumptions. We introduce a totally fresh perspective on weak lensing data analysis - Bayesian hierarchical modelling (BHM) - that promises to achieve all of these goals. The BHM approach provides a general framework for analysing weak lensing data that accounts for the full statistical interdependency of all model components in the weak lensing analysis pipeline, allowing information to flow freely from (in principle) raw pixel and photo- metric data through to cosmological inferences. We develop efficient Bayesian sampling schemes that explore the joint posterior of the shear maps and power spectra (and cosmological parameters) from a catalogue of estimated shapes and redshifts. We demonstrate that these algorithms bring the benefits of the Bayesian approach whilst being computationally practical for current and future surveys, and are readily extendable to extract information beyond the two-point statistics of the lensing fields or to incorporate the full weak lensing pipeline in a global principled analysis, presenting significant advan- tages over traditional estimator-based methods. We apply the newly developed Bayesian hierarchical approaches to the current state-of- the-art cosmic shear data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), constraining cosmological parameters and models from weak lensing using Bayesian hierarchical inference - the first application to weak lensing data.

When a subdominant light scalar field ends slow roll during inflation, but well after the Hubble exit of the pivot scales, it may determine the cosmological perturbations. This thesis investigates how such a scalar field, the spectator, may leave its impact on the Cosmic Microwave Background (CMB) radiation and be consequently constrained. We first introduce the observables of the CMB, namely the power spectrum Pζ , spectral index ns and its running dns/d ln k, the non-Gaussianities fNL, gNL and τNL, and the lack of isocurvature and polarization modes. Based on these studies, we derive the cosmological predictions for the spectator scenario, revealing its consistency with the CMB for inflection point potentials, hyperbolic tangent potentials, and those with a sudden phase transition. In the end, we utilize the spectator scenario to explain the CMB power asymmetry, with a brief tachyonic fast-roll phase.

The broad line region (BLR) in active galactic nuclei (AGN) is at a distance from the central power source that is not currently resolved by observations. Thus the exact structure and kinematics of the BLR are largely unknown and have to be inferred by indirect methods and a considerable degree of uncertainty remains. This thesis will study one such area of uncertainty: Is the BLR axially symmetric or spherically symmetric A model is developed for the distribution of the broad emission line widths and it is shown that the best fit parameters support an axisymmetric BLR. A similar model for the X-ray spectral index distribution also shows evidence for an orientation dependent ax. Solving for the inclination angle for a sample of 116 Mgii line widths from the RIXOS sample show that the orientations of the complete sample are not random as expected. Rather they are random at each luminosity up to some cut-off angle which itself increases with luminosity. This reflects an increase in opening angle of the torus in unified models of AGN - i.e. the angle beyond which broad lines are no longer observed and the object appears as a type 2 rather than a type 1 AGN. This change in opening angle can also be used to explain the observed anti-correlation between FWHMh and ax without the requirement of an orientation dependent spectral index. Finally, it is also shown how changes in the line width distribution with redshift, z, are sensitive to different values of H0 and q0 and that it is possible to use this to set limits on H0 and q0 with a relatively small observational sample at z = 2.5.