Details of two solid iron magnetic spectrographs, incorporating neon flash tubes, with m.d.m.'s. of 300 GeV/o and 1950 GeV/c are presented, as are the results on the µ(^+)/µ(^-)ratio in the zenith angular ranges 77.º5 - 90º , and 82.5º - 90º based on samples of 10832 particles and 2167 particles respectively, obtained with these instruments. In the energy region where comparison with the work of other authors can be made agreement obtains. The overall results confirm, in general, previous values below 200 GeV but do not show any great evidence for a sharp minimum in the ratio in the region of 50 - 100 G-eV as reported by some authors. At higher energies the present results do not show a rapidly increasing ratio, as was expected from an extrapolation of earlier measurements. A review of all published measurements on the µ(^+)/µ(^-) ratio is presented, which suggests an approximately constant ratio from E-3 - 10(^3) GeV, of 1,20 - 1.25. In an effort to give a theoretical interpretation of the results it is shown that pion production within the framework of a statistical model based on our knowledge of nucleon interactions at high energies will not account for the experimental data. The effect of, and evidence for, the production of kaons, subject to certain assumptions, is investigated and it is concluded that, under the assumptions considered, the adopted value of the K/π ratio is not sufficient to account for the data in the region Eµ= 10 - 50 G-eV. The effect of other dynamical models such as the isobar model and the O.P,3. model are considered. It is argued that the production of T = 1/2 isobars with a slowly decreasing cross-section, combined with the other features of nucleon interactions as observed at accelerator energies, is sufficient to account for the present experimental position when the errors in the experimental data are taken into account, but none of several proposed models for particle production in high energy interactions can be uniquely selected on the basis of the results.

A study has been made of the characteristics of neon flash tubes with a view to their use in cosmic ray work, and for machine experiments. In particular their low efficiency when operated at high repetition rates has been investigated, and this is thought to be due to fields set up by charge deposited on the glass as a result of the discharge. The fields are shown to decay with a time constant consistent with conduction of electrons over the glass surface to neutralise positive ions deposited on the other side of the tube. A long term effect has also been encountered, and is thought to be due to fields caused by electrons trapped in the glass. The discharge laechanism has been examined and shown to be, in most cases, a combination of streamer and Townsend breakdown. The most probable method of propagation down the tube is that several discharges are initiated along the tube by photoemission from the walls caused by photons from the initial and successive avalanches. When the field due to charge separation effectively backs off the applied field, the discharge is quenched. A random walk method has been developed to solve the equation of diffusion and drift for electrons in a flash tube, taking formative distance into account, and hence the fields built up during the experiment have been estimated, and compared with values calculated from the decay constant obtained from the resistivity of the glass and the capacitance of the tube. Some methods of overcoming the clearing field effect, namely increased surface conductivity, and the use of a bipolar ringing pulse, have been successfully tried, with a view to making the efficiency of flash tubes independent of repetition. rate.

A significant amount of activity in the Universe is obscured by dust, produced in the final phases of stellar evolution and in the detonation of Type II supernovae. Re-processed radiation from starlight is emitted from this dust at infrared wavelengths, and this must be taken into consideration when performing surveys of star formation (and nuclear activity) in order to form an unbiased picture of galaxy evolution. It is also clear that the star formation histories of galaxies are significantly modified by their local environment, the outcome of which is the characteristic galaxy populations observed in rich clusters and in the field in the local Universe. In this thesis I examine galaxy evolution in the context of environment from z ~ 0.5 to 2 ~ 3, paying attention to obscured activity revealed by observations in the rest-frame infrared. A mid-infrared (24μm) survey of two intermediate redshift clusters reveals a population of luminous infrared galaxies (LIRGs) which are missed in optical surveys (or significantly underestimated in terms of their star formation rates). Despite there being a large difference between the number of LIRGs detected in the two clusters (likely due to varying global cluster properties controlling the survival of starbursts in the cluster environment), these could be a potentially important population of galaxies. Their large star formation rates mean that they could evolve into local passive S0s by the present day. Although the S0s must be assembled after z ~ 0.5, local clusters are also dominated by massive elliptical galaxies which are mostly already in place by z ~ 0.5, and therefore must have assembled their stellar mass at much higher redshifts (z ≥3). At z = 3.1 I examine the nature of extreme activity in a rich, primitive environment - an example of a progenitor of a rich cluster of galaxies, and therefore the likely site of formation of local massive ellipticals. A number of giant (100 kpc-scale) Lyman-α emission-line nebulae (LABs) in the SA 22 protocluster contain bright submillimeter (850μm) galaxies (SMGs). Their extremely luminous rest-frame far-infrared emission suggest very high star formation rates and/or nuclear activity. Given that a large fraction of LABs seem to contain these active galaxies, it is plausible to link LABs' formation with feedback events such as superwind outflows from starburst regions. Indeed, a weak correlation between the SMGs' bolometric luminosity and the LABs' Lya luminosities appears to suggest that SMGs are powering these extended haloes. Although feedback from active galaxies appears to be important at early times, it remains a significant factor in galaxy-environment symbiosis at all epochs. The most profound effect a galaxy can have on its surroundings is to impart energy to the surrounding medium. In clusters, this is important for preventing the cooling of baryons and therefore the truncation of star formation. I investigate the environments of four low-power (L(_1.4GHz) ≤ 10(^25) WHz (^-1)) radio galaxies in the Subaru-XMM-Newton Deep Field at z ~ 0.5. The environments are all found to be moderately rich groups, and at least one shows evidence that it is in a stage of cluster assembly via sub-group merging. The conclusion is that the radio loud active galactic nuclei are triggered by galaxy-galaxy interactions within sub-groups, prior to cluster virialisation. These radio galaxies are destined to become brightest cluster galaxies, providing a low-power, but high-duty cycle feedback on gas in high-density regions at low redshift - necessary to suppress star formation in massive ellipticals at z ~ 0. The hostility of clusters to star formation (or at least the observation that it is suppressed in the highest density regions of the local Universe) is thought to be in part responsible for the rapid decline in the global volume averaged star formation rate (SFRD) since 2 ~ 1. Tracking the evolution of the SFRD beyond z ~ 1 is hard, because optical tracers (e.g. Ha) used in the local Universe become redshifted into the near-infrared, and up until recently the cameras suitable for large surveys have not been available. I have performed the largest ever near-infrared narrowband blank field survey for Hα emission at z = 2.23. Understanding the evolution of the SFRD before its decline to the present day is essential if we are to find the 'epoch' of galaxy formation. I present the Hα luminosity function and measure the SFRD at this epoch, finding little evolution in the time between z = 1.3 and z = 2.23. This is consistent with a flattening of the SFRD, indicating that this is the peak era of star formation in the Universe, before the gradual suppression of activity during the build up of groups and clusters to the present day.

Black holes are undoubtedly the most fascinating and exciting objects in the Universe, capturing the imagination of scientists and film writers alike. It has been a long standing objective of the scientific community to understand how these objects work and whether the black holes seen in the centers of galaxies, including our own, show analogous physics to those seen in stellar mass binary systems in other parts of the galaxy. In this body of work I aim to introduce the reader to the many broad facets of the subject to which the first 4 chapters are dedicated. These explain the mechanisms allowing black holes to be seen, i.e. luminous accretion and reprocessing of radiation, the environments black holes are found in and the effect this can have on observations. The 5 chapters following the introduction are the papers that I have published from my studies in this field as I try to address the outstanding problems which present obstacles to our understanding of unified black hole accretion. I hope that you, the reader will find this work compelling in nature and an enjoyable insight into one of the greatest mysteries of the Universe.

In this thesis, we study how dark energy may be constrained by measurements of large-scale clustering in future galaxy surveys, and through the imprint of a time-varying large-scale potential on the CMB (the integrated Sachs-Wolfe effect). We use semi-analytical galaxy formation models implemented in N-body simulations to build mock galaxy catalogues which are well suited for use in conjunction with large photometric surveys. Using mock catalogues, we predict that Pan-STARRS1 will be able to detect 10^8 galaxies in all its 5 photometric bands. We investigate the photo-z performance of the mock survey, which is a crucial factor when measuring large-scale structure statistics from a survey of this kind. We find using Pan-STARRS1 alone, photo-z accuracy of dz/(1+z) ~ 0.06 is achievable. The accuracy can be improved by combining near infrared photometry or by choosing a red galaxy sample. We implemented the photo-z errors to investigate their influence on the detectability of dark energy in the survey via the baryonic acoustic oscillations. We explore the challenges of using the ISW effect as a constraint on dark energy. In particular, we investigate the effect of non-linear gravitational evolution, using N-body simulations. We have quantified the non-linear contribution to the ISW measurements in terms of both its power spectrum and its cross-correlation with large-scale structure. We have discovered that the non-linear ISW effect is more important relative to the linear ISW effect at larger scales at higher redshifts. This draws attention to the need to accurately model the non-linear effect when using galaxy samples to do ISW tomography at high redshifts. We have developed a ray-tracing method for constructing full sky maps of the ISW effect including its non-linear aspect. Using these maps, we have developed a complete picture of the non-linear ISW effect in void and cluster regions. We quantify the total contribution of the linear and non-linear ISW effects to reported abnormal CMB cold spots and discuss possible confusion with the Sunyaev-Zel'dovich effect.

This Thesis examines the formation and evolution of early-type elliptical and S0 galaxies. In order to shed light on the key processes which lead to the formation of elliptical and S0 galaxies a range of techniques have been employed. The kinematics and stellar populations of the major and minor axes of the nearby edge-on S0 galaxy NGC 3115 have been examined through deep Gemini-GMOS longslit spectroscopy. The behaviour of the radial profiles of the line strength indices is found to be well explained by a simple model where NGC 3115 is composed of two components; an older classical spheroidal component, which displays a negative (decreases outwards) metallicity gradient and is enhanced in α-elements relative to the solar value together with a younger, constant metallicity disc which displays close to solar [а/Ғе]. The kinematics and stellar populations of the integrated light of NGC 3115 are compared to those previously found for its globular duster (GC) system. The GCs are found to rotate in a manner consistent with the galaxy as a whole and the red GC sub-population is found to have ages, metallicities and [a/Fe] abundance indistinguishable from those displayed by the spheroid of NGC 3115 at 2 effective radii. This study provides strong support to the theory that the GC systems of galaxies are closely linked to galaxy spheroid formation. The formation and evolution of so galaxies is further examined through the study of 18 edge-on S0 galaxies observed in a manner identical to that presented for NGC 3115. These galaxies are all found to display significant stellar discs and to present regular disc kinematics. In several cases signatures of kinematic substructure such as inner discs or bars are found. Ionised gas emission is observed at a low level in 72% of the sample; the kinematics of this ionised gas is highly variable, with examples of regular disc-like kinematics, confused counter-rotation and streams not aligned with any axis observed. The kinematic data has been utilised to produce a B-band Tully-Fisher relation (TFR) for S0 galaxies. The S0 TFR is observed to be offset to fainter magnitudes from the TFR for local spirals, an observation which can crudely be explained by the formation of S0s through the truncation of star formation (and subsequent disc fading) in normal spiral galaxies. The offset from the spiral TFR for each galaxy is shown to correlate well with both disc and central age, in the correct sense, and with the correct magnitude, such that the large observed scatter in the S0 TFR can be explained as being due to the different times at which the progenitor spirals ceased forming stars and hence the different amounts they have faded up to the present. Multi-object spectroscopic observations of the GC system of the shell elliptical NGC3923 are presented, as well as a novel technique for examining the spectra of the integrated light of galaxies at the same time as observing their GC system. The observations are used to demonstrate that neither the integrated light nor the GC system of NGC 3923 shows evidence for significant rotation. The stellar populations of the red GCs are again found to be consistent in age, metallicity and [a/Fe] with that displayed by the integrated light of the spheroid of NGC 3923 at large radii (> 2R(_e)). The velocity dispersion profile of the integrated light and GC system of NGC 3923 are both observed to be flat at larger radii, a finding indicative of the presence of significant amounts of Dark Matter. Using the GC kinematic data, a simple spherical isotropic dynamical model is presented which demonstrates that a significant increase in mass-to-light (M/L) ratio is required to explain the observed constant velocity dispersion profile. The M/L profile determined is in good agreement with that measured independently from X-ray observations.

Numerical simulations present a fundamental building block of our modern theoretical understanding of the Universe. As such the work in this thesis is primarily concerned with understanding fundamental differences that lie between the different hydrodynamic schemes. In chapter 3 I outline the optimisations I make to the FLASH code to enable larger simulations to be run. These include developing and testing a new FFT gravity solver. With these complete, in chapter 4 I present results from a collaborative code comparison project in which we test a series of different hydrodynamics codes against a suite of demanding test problems with astrophysical relevance. As the problems have known solutions, we can quantify their performance and are able to develop a resolution criteria which allows for the two different types to be reliably compared. In chapter 5 we develop an analytic model for ram pressure stripping of the hot gaseous haloes of galaxies in groups and clusters. We test the model against a suite of hydrodynamic simulations in the SPH GADGET-2 code. To ensure that the spurious suppression of hydrodynamic instabilities by SPH codes does not bias our results, I compare our findings to those obtained with the FLASH AMR code and find excellent agreement. Chapter 6 presents work in which we unambiguously determine the origin of the difference between the entropy cores formed in AMR and SPH codes. By running mergers of model clusters we are able to systematically explore the various proposed mechanisms and determine that turbulent mixing generates the higher entropy cores within AMR codes but is suppressed in SPH codes. The startling differences between the two hydrodynamic schemes presented in chapter 6 leads me to investigate their affect upon different sub-grid physical recipes. In chapter 7 I present the implementation of a sub-grid star formation recipe in FLASH and find strong differences in the way the two codes model pressure laws. I extend the work in chapter 8 by implementing a kinetic supernova feedback mechanism in FLASH and contrasting it with the results from the GADGET-2 code. I find that AMR codes dissipate energy much more efficiently than in SPH codes.

As it is well established that almost all galaxies host a central supermassive black hole (SMBH), it is natural to ask, how many of these SMBHs are actively accreting? Studies aimed at answering this question can better define the fraction of nearby galaxies that harbour active galactic nuclei (AGNs), provide constraints on the growth of local SMBHs, and further our understanding towards the connection between AGN activity and galaxy evolution. In the first part of this thesis, we use high-quality mid-infrared (IR) spectroscopy to investigate the ubiquity of AGN activity in a complete volume-limited sample of nearby galaxies to D<15 Mpc. We present analyses based on the detection of high-excitation emission lines to unambiguously identify AGNs in even the most heavily dust-obscured and gas-rich galaxies. We find that almost half of the AGN population are not identified in large-scale optical surveys, most likely due to strong circumnuclear star-formation activity and/or extinction through the host galaxy. In the second part of this thesis, we use sensitive hard X-ray (2-10 keV) and mid-IR constraints to calculate bolometric luminosities of these D<15 Mpc AGNs and combine these luminosities with well-constrained SMBH masses to estimate relative mass accretion rates. We use these data to calculate the volume-average SMBH growth rate of galaxies in the local Universe and find that AGNs hosting SMBHs with M_BH ~ 10^6-10^7 M_o are dominated by optically unidentified AGNs. These relatively small SMBHs are acquiring a significant proportion of their mass in the present-day and are amongst the most rapidly growing in the local Universe. In the third part of this thesis, we using [NeV]3427A as an unambiguous indicator of AGN activity to assess the incidence of AGNs in a statistically significant galaxy sample derived from the Sloan Digital Sky Survey (SDSS). On the basis of [NeV], we find strong empirical evidence that the theoretical maximum starburst limit provides a good identification threshold for AGNs. However, we find that only ~27% of Seyfert galaxies have strong detections of [NeV]3427A in their optical spectroscopy. Using spectral stacking analyses we determine that the lack of high-ionisation emission line signatures are due to poor spectral sensitivity and/or high levels of dust extinction. In the final part of this thesis, we use a suite of mid-IR, optical and X-ray luminosity indicators to search for Compton-thick AGNs at z~0.03-0.2, a region of parameter space which is currently poorly constrained by deep narrow-field X-ray surveys. We use the XMM-Newton serendipity survey and the SDSS to select a sample of 14 candidate Compton-thick AGNs. On the basis of the optical [OIII], mid-IR [OIV] and 6um AGN continuum luminosities we conservatively find that the X-ray emission in almost half of our sample appear to be attenuated by a factor >15, i.e., they are likely to be obscured by Compton-thick material with N_H > 1.5 x 10^24 cm^-2. Under the reasonable assumption that our 14 AGNs are representative of the overall X-ray undetected AGN population in the SDSS-XMM parent sample, we estimate that >20% of the optical Type-2 AGN population are likely to be obscured by Compton-thick material. These Compton-thick AGNs identified in our sample harbour some of the most rapidly growing SMBHs in the nearby Universe. Overall, the techniques and results presented in this thesis have arguably provided the most complete and unambiguous census of AGN activity in nearby galaxies to date.

The turbulent atmosphere has two detrimental effects in astronomy. The phase aberration induced by the turbulence broaden the point spread function (PSF) and limits the resolution for imaging. If there is strong turbulence high in the atmosphere then these phase aberration propagate and develop into intensity fluctuations (scintillation). This thesis describes three novel instruments related to these problems associated with atmospheric turbulence. The first is an optical turbulence profiler to measure the turbulence strength and its position within the atmospheric surface layer in real-time. The instrument is a development of the slope detection and ranging (SLODAR) method. Results from the prototype at Paranal Observatory are discussed. An instrument to improve the PSF for imaging is also discussed. The instrument works by adaptively blocking the telescope pupil to remove areas which are the most out of phase from the mean. This acts to flatten the wavefront and can therefore be used after an adaptive optics system as an additional clean up, or stand alone on a telescope as a relatively affordable and easy way to improve the PSF. The third instrument reduces the scintillation noise for high precision fast photometry. Simulation results show that it is possible to reduce the scintillation noise to a level where the measurements are photon noise dominated.

This thesis studies galactic stellar haloes built up through the tidal disruption of accreted dwarf galaxies. Numerical simulations are used to explore this process in the context of the Cold Dark Matter model of cosmological structure formation. We predict the properties of stellar structures that the next generation of surveys may discover in the Milky Way halo and the haloes of other nearby galaxies. We present six simulations based on the Aquarius project, a suite of high resolution N-body simulations of individual dark matter haloes in a fully cosmological setting. We tag subsets of particles in these simulations with stellar populations predicted by the GALFORM semi-analytic model of galaxy formation. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between redshifts 1 and 7 from less than 5 significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals (100-100 million Solar masses). In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses (0.1-1 billion Solar masses) and density/surface-brightness profiles of the stellar haloes (from 10 to 100 kpc) are consistent with expectations from the Milky Way and M31. Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. The central regions (within 10 kpc) of our haloes are highly prolate (c/a approx. 0.3), although we find one example of a massive accreted thick disc. Metallicity gradients in our haloes are typically significant only where the halo is built from a small number of satellites. We contrast the ages and metallicities of halo stars with surviving satellites, finding broad agreement with recent observations. We examine these simulations from the perspective of an observer located at the position of the Sun. We discuss the apparent smoothness of the halo relative to simple 3D star counts derived from photometric tomography. We then describe a simple correlation function statistic that quantifies the amount of spatial and kinematic substructure in the distant stellar halo. We test this statistic with the simulations we have developed, and find that it can distinguish between a range of realistic alternatives for the global structure of the stellar halo. We show that current observational data from pencil beam surveys of approximately 100 tracer stars (such as the Spaghetti Survey) are not sufficient to constrain the degree of structure in the Milky Way halo with this statistic. Larger area surveys with more than 1000 tracer stars (such BHB stars in the Sloan Digital Sky Survey) provide much tighter constraints on comparisons between CDM models and the Milky Way. Finally, we explore the kinematic structure of accreted stellar haloes in the CDM model. We demonstrate that multicomponent haloes like those of the Milky Way and M31 arise naturally through the accretion of stars from tidally disrupted satellite galaxies. Accreted haloes can reproduce the gross properties of the velocity ellipsoid measured in the Solar neighbourhood, although they can be far from dynamical equilibrium and have complex anisotropy profiles. In particular, halo stars do not trace the dark matter velocity distribution up to the escape velocity in the Solar neighbourhood. This suggests that mass estimates of the Milky Way based on related kinematic measurements may deviate significantly from the true mass, if the stellar halo is built largely though accretion.