By performing 542 high resolution N-body+SPH simulations of Milky-Way like galaxies, galaxy groups and clusters of galaxies, we investigate the effect of feedback by anactive galactic nucleus (AGN) on the evolution of these objects and the black holes theyhost. We first analyse the role of AGN within isolated Milky-Way like galaxies and majormergers between them, utilising a selection of methods from the literature, additionallyincluding supernova feedback. We find that AGN and supernova feedback are largelyindependent, and that although current AGN models are highly susceptible to numericaleffects, the temperature to which gas is heated is important for determining the impacton the host. Having investigated the effects on galaxy-scales, we then apply the existingAGN models to idealised equilibrium groups and clusters of galaxies and identify the keycomponents of an AGN model which is active in more massive systems. By simulatingmassive objects we learn of how AGN behave in a radically different regime where theyare thought to perform a maintenance role and support the cluster against radiative cooling. Simulations of existing AGN models find that they do not provide sufficient heatingto the wider cluster environment to prevent catastrophic cooling, whilst over-heating thegalaxy groups. Finally, we develop and introduce an anisotropic feedback method whichsimulates jet AGN feedback, and a hybrid model which includes both jet and quasar feedback. Such descriptions better match observed phenomenology as they produce spatiallyconcentrated feedback which penetrates its surroundings more effectively, producing jetsand bubbles in the cluster gas. Through this work we have found that the inclusion ofAGN is a crucial, if challenging, part of a modern galaxy evolution simulation, which ishighly sensitive to the modelling of feedback.

The large scale structure of the universe has been shown to be a powerful probe of cosmology and fundamental physics. In order for future surveys to continue to improve and constrain our model of the Universe we rely on predictions from N-body simulations regarding the cosmological matter density field. First, we examine the effect of baryons on the cluster mass function in an effort to ameliorate the apparent tension in cosmological parameters determined from the cosmic microwave background anisotropies and cluster number counts in the Planck survey. It is shown that ignoring baryonic depletion causes a decrement in the number of galaxy clusters of a fixed mass and an artificial shift in the inferred cosmological parameters. While this effect is not sufficient to completely resolve the tension in the Planck collaboration (2014) results, it will be an important factor in measurements from future cluster surveys. In order to investigate the effects of modified gravity on the matter power spectrum we present an f(R) gravity extension to the massively parallel N-body GADGET-2 code. After validating our multigrid enabled Newton-Raphson-Gauss-Seidel relax- ation solver we investigate two different f(R) models, namely the designer and Hu & Sawicki (2007) models. We find that the designer model power spectrum is well ap- proximated by the models of He et al. (2014) and He, Li & Hawken (2015) to 0.2 per cent. Furthermore, the power spectrum found when using the exact functional form of the Hu & Sawicki (2007) model is in excellent agreement with the power-law approximation. Finally, we have performed simulations with the Hu & Sawicki (2007) model with n > 1. It is shown that increasing n decreases the overall amplitude of the fractional power spectrum. Future measurements of large scale structure should beable to constrain both n and f_R0.

Observations of the Cosmic Microwave Background (CMB) radiation are an extremely important tool for understanding the Universe. The next generation of CMB experiments will attempt to measure the polarisation signal. In particular, the detection of B-mode polarisation, which is mainly generated by gravitational waves from the very early Universe, would provide a strong indicator for the energy level of inflation. However, due to the relative weakness of this signal, and the fact that there exists a much stronger E-mode signal, detecting B-modes polarisation poses several technical challenges. In particular, the standard method of CMB analysis, the pseudo-C_l method, is insufficient for the purposes of analysing B-mode polarisation, because of the difficulties of separating E and B mode polarisation on a cut sky. However, an alternate method, the pure-C_l method, has recently been outlined, which removes the effect of E-B mixing by subtracting out ambiguous modes. In order to test this method, I have written code to implement my own version of the algorithm, as well as the ordinary pseudo-C_l method. I have then used this code to compare the two methods both with and without E-modes, thereby demonstrating that the pure-C_l method does not suffer from issues with E-B mixing. However, I also find that, due to the effects of subtracting out ambiguous modes, the performance of the pure-C_l method is degraded on large scales, by a factor which is dependent on the length of the mask boundary. I have, therefore, investigated this effect. I find that, in general, the issues with this method are limited to only the very largest scales, and they are unlikely to cause significant problems for most masks. However, due to the nature of point source masks, these are particularly susceptible to such effects. Therefore, I have considered this case more carefully. Here, I find that there is indeed a significant effect up to multipoles of l ~ 50, and I have discovered that there is a simple relationship between the additional error caused by a point source mask and the number of sources. Due to the rapidly rising (in l 2 C_l) nature of the point-source power-spectrum, this issue can probably be avoided by only using the point-source mask for high-l measurements and, thus, the effect of point sources on the pure-C_l method will probably be limited. The exception to this, however, is the case where an exceptionally large number of sources must be masked out. In this case, the requirement to apodise the mask in order to implement the method will result in the analysis breaking down. Additionally, I have also looked at several other aspects of B-mode detection. I used various optimal error formulae, in conjunction with my pure-C_l code, to attempt to determine the optimal scan strategy for a given set of parameters, with a particular focus on the QUIJOTE experiment. Ultimately, it was found that this experiment in its original form would only be able to detect values of r of around 0.2, although increasing the number of beams would improve this slightly. Finally, I have outlined a novel null test that is designed to detect systematic errors in CMB experiments using polarisation position angles. I have showed that this method will be able to detect shear-like systematic errors at a level of less than 1%.

No description available.

In this work a comprehensive analysis is made of the galaxy composition and dynamics of galaxy groups. An extensive investigation into the changing nature of the Schechter luminosity function (LF) as a function of group size (calculated in multiple ways) is carried out, with the finding that the least massive, numerically poorest and least luminous systems possess a markedly repressed faint end. The projected 2D shape of groups is analysed carefully, taking into account the distortions created by a range of effects, and there is strong evidence that the smallest groups possess an underlying oblate (frisbee) 3D shape, in contrast to more massive systems that display significantly prolate (rugby ball) distributions. The radial profiles of groups are used to identify the key regions in which the group environment strongly influences galaxy morphology, with convincing evidence that the central scale-radius determines the LF of the full range of group scales.

This thesis is concerned with the study of two inter-related topics: the existence and propertiesof horizons, boundaries and barriers in cosmological models and the information that an observer in a certain model may, in principle, gain about his universe. Accordingly, it deals first with concepts of uncertainty and indeterminacy in cosmology (Chapter I).In Chapter II, sections (i), (ii), (iii), (vi) and (vii) introduce and summarise earlier work on the subject of event and particle horizons in homogeneous and isotropic world-models; the remainder of the chapter discusses certain features and develops various problems which arise from this, during the course of which is introduced the new notion of the degenerate (invariant) horizon. Chapter III is concerned with the "Milne-type" boundary and discusses the boundary of distance by parallax. It is shown that the boundary in Milne's model is a degenerate particle horizon. The behaviour of observables in the neighbourhood of an event horizon or a particle horizon is examined for five expanding model universes of the Robertson-Walker type, and for their duals, obtained by time-reversal; the results are demonstrated diagram-matically (Chapter IV). This examination paves the way for, and finds application in, Chapter V which investigates information theory in cosmological models and studies in particular the rate of flow, and hence the rate of loss, of information in the models considered; the influence of the existence or otherwise of horizons is explicitly demonstrated. The two remaining chapters (VI, VII) investigate the nature of the singularity at r = 2m in the Schwarzschild space-time, in the Finkelstein spacetime obtained by transformation from Schwarzschild's metric, and also in the space-time obtained by time-reversal of the Finkelstein metric. By studying the amount of information which an observer in the region r > 2m may in principle receive from r < 2m from light signals or probes, it is shown (Chapter VI) that the surface r = 2m in the Schwarzschild space-time is a barrier which is a degenerate event horizon for such an observer. Chapter VI concludes by considering Darwin's work on the manifestation of the singularity in the presence of a neighbouring star and of a star-field background. In Chapter VII, it is shown that the barrier at r = 2m in the Finkelstein space-time is not a degenerate event horizon for the observer in r > 2m, but that, in contrast, it is a degenerate event horizon in the time-reversed case. This topic is completed by an investigation of the transformations concerned to discover to what extent they are valid and to demonstrate how this difference arises.

The work reported in this thesis falls conveniently into two parts. The first two chapters deal with interstellar while the final two chapters deal with intergalactic material. The problem of the formation of molecules (principally H2) at the surfaces of interstellar dust grains is considered in Chapter I. It is shown that the amounts of formed are sufficient to maintain the interstellar gas at 100&deg;K. The results obtained for CH, CH show that the mechanism considered does not lead to absurd results under the conditions assumed. The importance of at high densities is also discussed. Chapter II deals with a theory of star formation in a medium composed of randomly moving cloudlets or floccules. An outline is given of a recent paper by McCrea who shows that this idea leads to a satisfactory account of the formation of the solar system and removes the angular momentum difficulty of other star formation theories. The collisions of floccules are considered in detail and a theory of the gravitational capture of the fragments of collision is outlined. The expansion of a fully ionised gas into avacuum is discussed in Chapter III. This problem is connected with a recent theory of galaxy formation. The expansion of a monatomic gas and Lagrange's Ballistic problem are first considered. The latter leads to a new model for a freely expanding gas. It is then shown that, if the fully ionised gas remains neutral, by suitably defining the sound speed the equations for a fully ionised gas may be reduced to those for a monatomic gas. Finally in Chapter IV, Hoyle's theory of the origin of the angular velocities of galaxies is reviewed. A specific model based on this theory is discussed. It is shorn that some features of galaxy rotation may be accounted for in terms of this theory.

Since the discovery of radial migration a galaxy can no longer be considered to be a collection of annuli evolving in isolation. Previous works have considered the implications of migration on properties of disc galaxies, but most of these efforts concentrate on the Solar neighbourhood, where observational comparisons are at their best. Only a small number of studies have investigated the effect on the outer regions of galaxies. Using N-body+SPH simulations, I consider the effect of radial migration on the outskirts of disc galaxies. I demonstrate that when falling through a gas rich cluster environment, a galaxy can evolve from a type II profile to a type I profile with little increase in the radial velocity dispersions. Instead the cluster environment induces more spirals when compared to the same galaxy evolving in isolation, driving radial migration of stars into the outer disc and explaining the relative abundance of type I galaxies in cluster environments. I demonstrate that during the transformations of the profiles the galaxy evolves from a spiral to a lenticular galaxy and becomes redder. This significantly alters the position of the galaxy in the colour-mass plane, transitioning from the blue sequence to the green valley. Furthermore, these changes occur rapidly after the onset of ram pressure stripping. Finally I consider the effects of migration in disc galaxies with strong warps. I find that the warp remains isolated from the mixing effect of migration and thus forms a tight relation between age and metallicity. Stars forming in the warp settle into the disc, where they migrate across all radii, imprinting the warp AMR over the flattened, broadened disc AMR.

Assuming the redshifts of quasars are cosmological, their distribution in space is investigated by means of new test, the luminosity- volume test (Chapter I), which is shown to be far more powerful than the magnitude-redshift test. In most relativistic cosmological models, strong * evolutionary factors must be influencing the distribution of quasars in space, and possible explanations are discussed qualitatively and quantitatively. The similar behaviour of the stronger radio-galaxies strengthens the link between the two classes of source. Less direct information about more remote parts of space is obtained by the interpretation of the radio source-counts (Chapter III) and the integrated radio and X-ray background intensities (Chapter IV). Assumptions have to be made about the physical properties of sources, particularly the magnetic field, and a number of different models for the structure of radio-sources are discussed in Chapter II. Evidence is presented against the idea that the radio-emitting components are ejected from the parent galaxies at relativistic speeds. Models are considered in which relativistic electrons are generated continuously in the radiating components.

This work uses multiwavelength observations of nearby galaxies to explore the relationship between star formation and the interstellar medium in galaxies of various sizes and morphology. Galaxies in the Herschel Reference Survey are divided into barred and unbarred spirals to test for differences in dust temperature, dust mass, star formation rate, farinfrared luminosity, NUV-r colour and stellar mass between the two populations. The only significant observed difference is with stellar mass, where barred spirals are generally less massive. I suggest this is due to the speed of bar creation depending on galaxy mass, although this is counter to some previous observations. Trends with Hubble-type and environment are consistent with previous work. The resolved star formation law is studied in the two largest extragalactic sources in the local group, Andromeda (M31) and the Triangulum (M33). The two are measured to have global star formation rates (SFR) of 0.25M⊙ yr−1 and 0.16M⊙ yr−1 respectively using far-ultraviolet and 24 mm emission as star formation tracers. M33 has a higher mean surface density of star formation, as expected as it is later type than M31, and a higher star formation efficiency. Both galaxies appear consistent with the globally averaged SFR and gas surface density of normal spirals studied in previous work, with M31 at the low end in terms of SFR. When looking at smaller scales, both galaxies show evidence of saturation of neutral monatomic hydrogen at §Gas = 10M⊙ pc−2 when looking at the star formation law with total gas. They also appear to follow close to linear star formation laws with molecular gas only, consistent with previous work on resolved galaxies. M31 shows evidence of a sub-linear star formation law with molecular gas, indicating that star formation efficiency is lower in the highest density regions. Testing the relationship in M31 on different pixel scales does not effect the measured Kennicutt-Schmidt index, as has been suggested in previous work. M33 shows a significant portion of the galaxy has a relatively high SFR surface density, but little molecular gas as traced by CO. I suggest this could be evidence of CO-free molecular hydrogen in these regions.