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In this era of high-precision cosmology we are able to measure and predict properties of the large-scale structure of our Universe to a fine degree. However we still lack a clear and tested understanding of the effects of the large-scale environments on galaxies and their host halos. This thesis focuses on bettering our understanding of this issue by investigating the dependence of galaxies and halos on their location within the cosmic web. An algorithm based on the tidal tensor prescription is developed and applied to the MDR1 1 (h-1Gpc)3 dark matter simulation to classify the geometric environment of every location in the simulated volume as one of the four components of the cosmic web; voids, sheets, filaments and knots. Conditional halo mass functions are extracted to investigate the influence of tidal forces on the abundances and mass distribution of dark matter halos. A theoretical framework based on Gaussian statistics is presented and used to derive predictions for halo abundances in different geometric environments. The Gaussian theory predicts no coupling of tidal forces and, hence, that the halo mass function is independent of geometric environment for a given local mass density. It is shown that the halo mass functions extracted from the simulation are fully consistent with this picture. It is then shown how this method of classifying geometric environments can be extended to observational datasets. The Galaxy And Mass Assembly (GAMA) spectroscopic redshift survey, with its wide field and high completeness, is excellently suited to this study. The geometric environments of the three equatorial GAMA fields are classified, following a thorough analysis of the additional uncertainties introduced when moving to observational datasets. Additionally, the geometric environments of the GAMA galaxies and groups are classified, allowing the influence of the cosmic web on large-scale structure to be investigated. Both the galaxy luminosity function and the group mass function within the observed cosmic web are studied and no evidence of a direct impact of the web is seen. It is found that all modulations can be fully attributed to the indirect dependence of these properties on the local matter overdensity. Whilst these results indicate that there is no strong dependence of the scalar properties of large-scale structure on geometric environment, the final investigation of this thesis presents an attempt to look in more detail at the environmental dependence of stellar properties by investigating stellar-formation histories within the cosmic web.

This thesis is concerned with developing new methods for performing weak gravitational lensing with the aim of addressing specific systematic effects in weak lensing surveys. The first of these effects is the multiplicative biases which arise as a result of isotropic smearing. This smearing may be due to atmospheric seeing or an instrumental PSF. Isotropic smearing circularizes a galaxy image and leads to a systematic under-estimate of the modulus of the observed ellipticity. The orientation of the observed galaxy is, however, unaffected. We exploit this property by formulating a weak lensing shear estimator that requires measurements of galaxy position angles only, thereby avoiding the contribution from this systematic. We demonstrate the method on simulations and the CFHTLenS data by reconstructing convergence maps and comparing the results with the standard full ellipticity based approach. We show that the difference between the reconstructed maps for the two approaches is consistent with noise in all of the tests performed. We then apply the technique to the GREAT3 challenge data using three distinct methods to measure the position angles of the galaxies. For all three methods, we find that the position angle-only approach yields shear estimates with a performance comparable with current well established shape based techniques. The second effect addressed arises from the intrinsic alignment of the source galaxies. This alignment mimics a shear signal, and hence biases estimates of the shear. To mitigate this effect, we develop three shear estimators that include polarization information from radio observations as a tracer of a galaxy’s intrinsic orientation. In addition to the shear estimator, we also develop estimators for the intrinsic alignment signal. We test these estimators by successfully reconstructing the shear and intrinsic alignment auto and cross-power spectra across three overlapping redshift bins.

In this thesis, detailed studies of three different types of binary central stars withinplanetary nebulae (bCSPNe) are presented, with the aim of investigating the effectssuch a range of binary systems has on the morphology and kinematics of the surroundingnebulae, as well as discussing what the implication is for the interaction betweenthe stars themselves. A close binary, an intermediate period binary, and a compactbinary system are examined. The close binary PN HaTr 4 is the first system to be studied via detailed spatiokinematicalanalysis and modelling, and it is one of few known to contain a postcommonenvelope (CE) central star system. CE evolution is believed to play an importantrole in the shaping of PNe, but the exact nature of this role is yet to be understood. High spatial and spectral resolution spectroscopy is presented alongside deep narrowbandimagery to derive the three-dimensional morphology of HaTr 4. The nebula isfound to display an extended ovoid morphology with an enhanced equatorial regionconsistent with a toroidal waist - a feature believed to be typical amongst PNe withpost-CE central stars. The nebular symmetry axis is found to lie perpendicular to theorbital plane of the central binary, concordant with the idea that the formation andevolution of HaTr 4 has been strongly influenced by its central binary. Next, PN LoTr 1 is studied using a combination of spectra and photometry, andis thought to contain an intermediate-period binary central star system (P = 100–1500d). Here, we confirm the binary nature of the central star of LoTr 1, consisting of aK1 III star and a hot white dwarf (WD). The nebula of LoTr 1 presents a very differentmorphology than that of other seemingly similar bCSPNe possessing barium stars,A70 and WeBo 1 (included in this study for direct comparison), which may be anindication of a difference in their mass-transfer episodes. There is no evidence ofbarium enhancement in the K1 III companion, but it is shown to have a rotation periodof 6.4 d which is most likely a sign of mass accretion. Such a system represents arare opportunity to further the investigation into the formation of barium stars andintermediate period, post-asymptotic giant branch (AGB) systems. Finally, the first analysis of an object from the new POPIPlaN catalogue of PNe ispresented. Photometry of the central star system of PN G033.8+01.5 showed it to bea compact binary of P = 0.1268 d, consisting of a cool M2 V main sequence star anda hot WD surrounded by a very asymmetric PN. The very short orbital period leadsto the possibility of PN G033.8+01.5 being a cataclysmic variable candidate, which inturn leads to the question of whether the visible nebulous material is that of a true PNor if it is the remnant of an outburst.

Clusters of galaxies provide a unique opportunity to simultaneously study cosmology through low scatter scaling relations and the complex baryonic physics that occurs in cluster cores. As such it is of key importance to quantify the effects of the various physical processes that drive cluster evolution. In this thesis a sample of 30 clusters from the Millennium Gas Simulation, of masses 10^14/h Solar Masses < M200 < 10^15/h Solar Masses, were selected and run at a higher resolution using the re-simulation technique, using a modified version of Gadget-2, an N-body SPH code. Each cluster was run multiple times with increasing levels of sub-grid physics in order to separate the different effects that govern cluster evolution. The models implemented starting with non-radiative (NR), simulations then added cooling and star formation (CSF), supernova feedback (SFB) and AGN feedback model (AGN) respectively. In order to best match observations a study of supernova and AGN feedback parameters was conducted. The sample of clusters were also used to quantify the magnitude of biases created when observing clusters, in an attempt to classify the accuracy of these measurements of clusters. Additionally, the effects of the biases were also included in the estimation of the cluster mass using hydrostatic equilibrium. The best match to the observed gas, star and baryon fractions, scaling relations and gas profiles was found when powerful supernova feedback was included, which heats gas particles to 10^7K, and an AGN model whose heating temperature scales with the final virial temperature of the cluster, so that particles in a 10^14/h Solar masses and 10^15/h Solar Masses cluster are heated to 10^8K and 10^8.5 K respectively. Outside the core, this model successfully matches all the observed profiles and scaling relations excluding the spectrascopic-like temperature. The core region is simulated with come success, with pressures matching those observed but gas that is too cool and dense, resulting in an inability to reproduce the non cool core entropy profiles. Cold dense gas is more heavily weighted in the spectrascopic-like temperature, allowing significant contributions from gas in substructures and cold dense clumps of gas that are un-ascociated with any substructures and seems to be an artificial construct of SPH. When this gas is removed using the method outlines in \cite{Roncarelli2006}, temperatures outside the core match observations, but the core region is still too dense and cool. Clearly this core region requires more complex physics, possibly through implementation of an improved SPH code or more complex sub-grid physics such as that associated with the AGN feedback. The bias profiles also exhibit a similar sensitivity to the cool dense gas clumps, having a profound effect on the observed profiles and creating significant scatter in the mass estimated using hydrostatic equilibrium. Removing this cold dense gas using the Roncarelli method results in reduced biases and hydrostatic mass estimates closer to the true values. The resulting scaling relations and profiles including the effects of biases differ from those without the biases, but not significantly. It is clear that biases can affect the observed profiles and scaling relations, but this effect is minimised by excluding the coldest densest gas. As the choice of how much gas is removed is somewhat arbitrary, it is clear that further work in this field would require better SPH implementations that do not produce the erroneous dense gas clumps and the generation of mock observations using the simulated data.

This thesis analyses and proposes solutions for the following PNe mysteries: 1. The missing PNe population. 100 new objects were spectroscopically observed and analysed using the INT Photometric Halpha Survey (IPHAS). 32 are PNe, 13 are possible PNe and the rest are HII regions, SNR and unclear objects. The new PNe were within the inner parts of the galactic plane. The IPHAS survey has been the deepest survey done in Halpha, and revealed objects never seen before. 2. The mixed chemistry in Galactic Bulge PNe. These GBPNe are the end-products of low-mass stars (and therefore expected to be O-rich) and belong to a metal rich population. These PNe present dual-dust chemistry, with the presence of PAHs (carbon-rich) and crystalline silicates (oxygen-rich) in their spectra. This thesis analysed 40 Galactic Bulge PNe and found a strong correlation between intensity of the PAH bands and morphology, notably a massive torus. This torus provides the dense irradiated environment needed to form the PAHs. Follow-up observations using VISIR revealed a dense dusty tori in 11 PNe. PAHs are present in the outer edges of these tori, whereas ionised material is located within the dusty tori.3. The 3He problem. Using the VLA we observed three PNe, upper-limits of their 3He abundances were estimated. The resulting evolution of 3He is only consistent with the values determined in pre-solar material and the ISM, if the PNe observed represent only 4% of the population of low-mass stars, and the remaining 96% have undergone enhanced 3He depletion.

We examine resonant scattering of Lyα (Lyman-alpha) photons in the neutral hydrogen Intergalactic Medium (IGM) at high redshift. Lyα scattering plays a key role in the 21cm emission/absorption against the Cosmic Microwave Background via the 'Wouthuysen-Field Effect' Knowledge of the strength of Lyα scattering induced by the first sources will constitute a significant step in predicting and understanding the eventual observations of the impact of these objects on the signal from the IGM during the Epoch of Reionisation (EoR), using planned facilities such as the Square Kilometre Array. A quantitative analysis of the scattering rate requires formulation and solution of the radiative transfer equation for the system. We consider radiative transfer of Lyα scattering far from a source in the homogeneous expanding IGM for photons that free stream until scattering in the blue wing of the local line profile: we describe an approximation that ignores spatial diffusion of photons and assumes a locally homogeneous scattering medium, allowing the calculation of simple analytic solutions to examine the dependence of the local scattering rate on various physical effects such as local expansion/contraction, and extend our approach to determine time-dependent solutions. The more complex problem of Lyα photons scattering in both frequency and space within a spherically symmetric medium is subsequently solved for several test problems using both Monte Carlo methods and a method based on the ray and moment radiative transfer equations following an approach due to Mihalas et al. [1975, 1976]. We examine local Lyα scattering around a continuum source in the homogeneous expanding IGM using both methods and compare our results with an analytic solution in the zero-temperature diffusion regime derived from an analogous solution for a monochromatic source found by Loeb and Rybicki [1999]. Our results are used to make estimates of the resulting size of the spherical region around the source that is rendered detectable via 21cm observations against the CMB background. We also examine cases with different density and velocity profiles and determine the effect on the scattering rate.

The first stars, galaxies and black holes in the universe produced large quantities of ionising UV radiation; forming H II regions in the neutral gas before the Epoch of Reionisation (EoR). These ionisation fronts will have come into contact with overdensities in the surrounding Intergalactic Medium (IGM), including haloes which were in the process of collapse. Previous studies have shown that the feedback processes on these secondary haloes can either disrupt the gas, or induce further cooling from the formation of molecular hydrogen. The ionising source will eventually die and create a defunct H II region, which expands into the surrounding neutral IGM. Minihaloes at the edge of these defunct H II regions are particularly susceptible to positive feedback due to not having been photoevaporated like their counterparts further inside the ionised volume. In this thesis, numerical simulations of minihaloes at the edges of H II regions formed by the first luminous objects before the EoR are presented. A methodology of including secondary ionisations from high energy photons is also implemented into the radiation hydrodynamical code ZEUS-RT. The interaction of differing spectral index sources with minihaloes including secondary ionisation is discussed. The secondary ionisations show the greatest effect for hard spectral sources with a large fraction of high energy photons; where a decrease in photoheating and an increase in ionisation rate is found at the outer reaches of the ionisation front (I-front). The increased ionisation rate lowers the optical depth of the gas for subsequent photons, and thus reduces the trapping of I-fronts in high densities found in the minihalo cores. The ratio of the free electron fraction to the temperature in the core of the minihaloes is found to constrain the resulting evolution. A high ratio is correlated with the creation of molecular hydrogen, which can then induce further cooling and the collapse of the system.A large parameter suite of 3780 two-dimensional minihalo models utilising radiative hydrodynamical simulations with 12 species and a coupled reaction network, including H2 cooling, HD cooling, Lyman-Werner radiation and secondary ionisation is performed. The parameter space includes: the spectral index representing different sources such as quasars or galaxies, the mass of the minihaloes from 105 - 106 Mʘ, the redshift of ionisation from z ~ 10 - 30, and other factors which denote the position of the minihalo relative to the boundary of the H II region. Minihaloes with average core densities of n0 = 2 - 10 cm-3 show almost unanimous positive feedback, while the majority of minihaloes under this average density are disrupted. Minihaloes with core densities above this value generally would have collapsed in isolation anyway, but are found to not be delayed by the I-front. The H2 fraction in the minihalo gas is also increased in models with no blowout by factors between 2 - 100 times that of an isolated minihalo. This is especially significant for lower redshift, z ≤ 15, minihaloes. Finally, a parameter suite of larger 106 - 107 Mʘ minihaloes results in the creation of self-gravitating clumps of gas moving out of the dark matter potential. The gas core is compressed by the I-front, enriched with molecular hydrogen, and ejected by the pressure front after the source dies. These "baryon bullets" could be progenitors of primordial globular clusters found in the haloes of galaxies today. Properties such as old stellar populations and the lack of dark matter haloes can be explained by this radiative ejection method. The dynamical nature of these interacting systems and diversity of subsequent evolution suggest that minihaloes less than 108 Mʘ are important in the early formation history of the universe, and in determining the constraining parameters of the EoR. The feedback mechanisms investigated, and secondary ionisation physics, should be included in astrophysical simulations and analytical calculations determining the evolution of the universe at this critical epoch.

Cosmological constraints from galaxy surveys are as accurate as our understanding of the relative distributions of dark matter and galaxies, known as galaxy bias. Weak gravitational lensing is a powerful probe of galaxy bias, since the distortion in the shapes of distant galaxies, called shear, is directly related to the dark matter distribution, which can be compared to the galaxy field. I look at the galaxy clustering amplitude relative to the dark matter field, quantified by the galaxy bias b, as well as the cross-correlation coefficient r, which tells us how correlated the positions of galaxies are with the dark matter. In this thesis I present several techniques to constrain galaxy bias through weak lensing, using both numerical simulations and observational data. The most commonly used method, using aperture statistics, is shown to be subject to serious systematics in the presence of noisy data and scale- and time dependence in the galaxy bias. A local comparison technique is introduced, where the foreground distribution is used to predict the shear in the background, to which it is compared. The technique is tested with simulations, concluding that it requires high quality data. A model fitting approach is proposed, based on the McDonald (2006) galaxy bias model. The two parameters of this model, a large scale bias, b1, and a parameter, b2, that quantifies the scale dependence of the bias, are insufficient in the presence of stochasticity. Therefore, R is introduced as an additional parameter to take this into account. I present galaxy bias constraints for two spectroscopic galaxy samples: the Baryon Oscillations Spectroscopic Survey (BOSS) and the WiggleZ Dark Energy Survey (WiggleZ), applying the traditional aperture method and the model fitting approach to the Red Sequence Cluster Lensing Survey (RCSLenS). Both techniques strongly suggest that galaxies trace mass, but in a complicated way, with differences in scale- and time dependence between the samples considered. The WiggleZ galaxy bias is found to be around b ~ 1:2, depending on redshift and scale, and has a low cross-correlation coefficient of r ~ 0:5 at small scales. The BOSS samples have higher bias with scale dependence around b ~ 2:0 and show no sign of stochasticity, finding r to be close enough to unity to be explained within a deterministic scenario. The observations are in line with previous galaxy bias measurements from lensing data. The thesis incorporates work on the X-ray Luminosity Function (XLF) of galaxy clusters, measured from the Wide Angle ROSAT Pointed Survey (WARPS). Evolution is quantified with a likelihood analysis and I conclude that it is driven by a decreasing number density of high luminosity clusters with redshift, while the bulk of the cluster population remains nearly unchanged out to redshift z ~ 1:1, as expected in a low density Universe. I conclude by investigating the impact of my galaxy bias measurements from BOSS and WiggleZ on the growth rate of structure, as extracted from Redshift Space Distortions (RSD). The imperfect correlation between the galaxy and matter field, as quantified by R and b2, leads to an underestimation of the true growth rate under the assumption of a linear bias. Therefore, in order to constrain galaxy bias and gravity simultaneously, future cosmological redshift surveys require high quality lensing data.

Contemporary cosmology is a vibrant field, with data and observations increasing rapidly. This allows for accurate estimation of the parameters describing our cosmological model. In this thesis we present new research based on two different types of cosmological observations, which probe the universe at multiple epochs. We begin by reviewing the current concordance cosmological paradigm, and the statistical tools used to perform parameter estimation from cosmological data. We highlight the initial conditions in the universe and how they are detectable using the Cosmic Microwave Background radiation. We present the angular power spectrum data from temperature observations made with the Atacama Cosmology Telescope (ACT) and the methods used to estimate the power spectrum from temperature maps of the sky. We then present a cosmological analysis using the ACT data in combination with observations from the Wilkinson Microwave Anisotropy Probe to constrain parameters such as the effective number of relativistic species and the spectral index of the primordial power spectrum, which we constrain to deviate from scale invariance at the 99&percnt; confidence limit. We then use this combined dataset to constrain the primordial power spectrum in a minimally parametric framework, finding no evidence for deviation from a power-law spectrum. Finally we present Bayesian Estimation Applied to Multiple Species, a parameter estimation technique using photometric Type Ia Supernova data to estimate cosmological parameters in the presence of contaminated data. We apply this algorithm to the full season of the Sloan Digital Sky Survey II Supernova Search, and find that the constraints are improved by a factor of three relative to the case where one uses a smaller, spectroscopically confirmed subset of supernovae.