This work is focused on a search for strong gravitational lenses in early-type galaxies (ETGs). The total number of samples is 4,706 galaxies encompassing a magnitude range 15.0 < i < 18.0 and colour 3.5 < (u-r) < 5.0. Two databases were employed as the source of K-band images (UKIDSS Large Area Survey) and g, r, i images (SDSS). All samples were fitted to a Sersic component and automatically processed using GALFIT (Peng et al. 2002; Peng et al. 2010) inside a Python script (Appendix A). The first classification generated 259 galaxies which are seen as single galaxies in their K-band images. These galaxies were then reclassified based on image contouring in g, r, i, and K filters and therefore resulted in three categories of samples: Sample A (99 galaxies), Sample B (96 galaxies), and Sample C (64 galaxies).

The Magellanic System at a distance of 50 kpc from the Milky Way (MW), is a prime target in the study of stellar populations, star formation histories and galactic dynamics in low metallicity environments. The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) have been observed in great depth, however there has been somewhat less interest in the Magellanic Bridge connecting the two and only more recently has the interest surged in the Magellanic Stream, which trails the Clouds between them and the MW. The Magellanic Bridge has a known younger stellar population dating back to Irwin’s observations (Irwin et al., 1990), only more recently has an older population been confirmed in the Bridge by Bagheri et al. (2013) and No¨el et al. (2013), while the Magellanic Stream is known to contain gas only with no stellar component. The estimated ages of the Bridge and Stream are 200 Myr (Bekki, 2007) and 2 Gyr (Diaz and Bekki, 2012) respectively, with the postulated Bridge formation from a tidal interaction between the Clouds. The formation of the Stream is less well understood with different models using varying assumptions and parameters such as Besla et al. (2012) and Nidever et al. (2010), including possibilities that the Clouds were historically bound or un-bound, and that the MW may or may not have been involved in the Stream formation. The work in this thesis makes use of different methods of removing the Galactic foreground population in the direction of the Magellanic Bridge and Stream to create cleaned catalogues of these regions. Various methods of analysis are applied to the cleaned catalogues in this work to identify stellar populations in the Bridge and Stream and density variations in the Bridge, including the production of CMDs and two-colour diagrams, fitting isochrones to the observational data, creating stellar density maps and studying spatial variations. This work contains the first published confirmation that the Bridge contains an older population of stars from public catalogues, which is supported with observations of the older population in recent deeper surveys, and confirmed with spectroscopic follow up observations. The young population has ages within the age of the Bridge ( 200 − 500 Myr) and are likely to have formed in-situ, in regions of high density gas closest to the SMC. The number of young blue objects in the Bridge tiles is greater towards the SMC and decreases towards the LMC. Populations identified here reach ages up to 3 Gyr are likely to have been drawn into the Bridge from the Clouds at formation. The key results from this work are that an older Bridge stellar population has been identified and confirmed, indicating that stars as well as gas were drawn into the Bridge at its creation. The fact that the younger population has the highest density away from the main concentration of hydrogen show that the gas within the Bridge has been displaced by ram pressure, most likely due to the Clouds moving through the Galactic halo. Less concrete results in this work reveal a puzzling populetion of objects within the Magellanic Stream, which could be stellar in nature and with follow up work, could be the first observation of Stream objects. This work contributes to our understanding of the interaction between the LMC and SMC via the Stellar populations between them.

The overall aim of this thesis is to investigate the environments of AGN, in particular, the density of galaxies in the environments of radio-loud and radio-quiet AGN. This determines whether AGN trace dense environments at high redshifts and whether the environments are important in addressing the problem of radio-loud dichotomy. I extend my research by investigating whether star-formation evolves differently in high-redshift AGN environments compared to the field. I begin by investigating the environments of 169 AGN using Spitzer data at z ∼ 1. I investigate the source density of star-forming galaxies in the environments of radio galaxies, radio-loud quasars and radio- quiet quasars. I do not find any significant overdensity of star-forming galaxies in these environments, although I find tentative evidence for a diff erence in the colours of galaxies in the radio galaxy environments compared to the quasar and field environments. I next use VIDEO data to investigate the environments of the quasars out to z ∼ 3. Firstly, I use a training sample of QSOs and galaxies, which trains a neural network to detect QSOs in the VIDEO data. I detect 274 possible QSOs in the VIDEO data using this method. I am able to determine that the efficiency of the neural network clas- sification is 95 per cent using the training sample. I compare these results to a colour selection method, which detects 88 QSOs in the VIDEO data, and find that the neural network is able to detect ∼ 80 per cent of the colour selected QSOs at Ks = 21. I then investigate the source overdensity using a radial analysis on the environments of the VIDEO QSOs. I find a significant overdensity of galaxies in the environments of the whole QSO sample and in the environments of the radio-loud quasars compared to the radio-quiet quasars. I extend the density analysis by using a second density measure, called the spatial clustering amplitude technique, to compare the environments of the quasars with their radio luminosities, absolute magnitudes and redshifts. I do not fi any significant correlations between environmental density and radio luminosity, absolute magnitude or redshift for the QSOs. I extend this research to investigate the type of galaxies found in the AGN environments. However, I do not find any significant differences between the type of galaxies found in the QSO environments and the background field.

In this thesis I use X-ray observations to study the cores and extended structures of radio-loud AGN, to determine their structure, accretion properties and the impact they have on their surroundings. I use new Chandra data and archival XMM-Newton observations ofMarkarian 6 to look for evidence of emission from shocked gas around the external radio bubbles, using spatially resolved regions in Chandra and spectral analysis of the XMM data. The results show that the bubbles in Mrk 6 are indeed driving a shock into the halo of the host galaxy, with a Mach number of 3.9. I also find that the spectrum of the AGN has a variable absorbing column, which changes from 8 × 1021 atoms cm−2 to 3 × 1023 atoms cm−2 on short timescales (2-6 years). This is probably caused by a clump of gas close to the central AGN, passing in front of us at the moment of the observation. Using new and archival Chandra observations of the Circinus galaxy, I match them to pre-existing radio, infrared and optical data to study the kpc-scale emission. As for Mrk 6, I find that the radio bubbles in Circinus are driving a shock into the interstellar medium of the host galaxy, with Mach numbers M 2.7–3.6 and M 2.8–5.3 for the W and E shells respectively. Comparing the results with those we previously obtained for Centaurus A, NGC 3801 and Mrk 6, I show that the total energy in the lobes (thermal+kinetic) scales approximately with the radio power of the parent AGN. The spatial coincidence between the X-ray and edge-brightened radio emission in Circinus resembles the morphology of some SNR shocks, a parallel that has been expected for AGN, but has never been observed before. I investigate what underlying mechanisms both types of systems may have in common, arguing that, in Circinus, the edge-brightening in the shells may be accounted for by a B field enhancement caused by shock compression, but do not preclude some local particle acceleration. I also carry out a systematic study of the X-ray emission from the cores in the 0.02 < z < 0.7 2Jy sample, using Chandra and XMM-Newton observations. I combine the results with the mid-IR, optical emission line and radio luminosities, and compare them with those of the 3CRR sources, to show that the low-excitation objects in our sample show all the signs of radiatively inefficient accretion. I study the effect of the jet-related emission on the various luminosities, confirming that it is the main source of soft X-ray emission for our sources. I also find strong correlations between the accretion-related luminosities, and identify several sources whose optical classification is incompatible with their accretion properties. I derive the bolometric and jet kinetic luminosities for the sample and find a difference in the total Eddington rate between the low and high-excitation populations, with the former peaking at 1 per cent and the latter at 20 per cent Eddington. There is, however, an overlap between the two, indicating that a simple Eddington switch may not be possible. The apparent independence of jet kinetic power and radiative luminosity in the highexcitation population in our plots allows us to test the hypothesis in which jet production and radiatively efficient accretion are in fact independent processes that can coexist in high-excitation objects.

I present low-resolution (Dl 6 A° ) follow-up spectroscopy of 370 Ha emitters (12 . r . 17) identified with IPHAS, in a 100 deg2 wide section of the Galactic plane that is located between ` = (120 ; 140 ) and b = ( 1 ; +4 ). Classical Be stars are found to be the most numerous group of the observed targets ( 60%). Sixty-eight classical Be stars have also been observed at higher spectral resolution (Dl 2 4 A° ) and S/N ratio, which allows spectral typing to an estimated precision of 1 sub-type. Colour excesses were measured via spectral energy distribution fitting of flux-calibrated data. I took care to remove the circumstellar contribution to the measured colour excess, using an established scaling to the Ha equivalent widths. In doing so, this method of correction was re-evaluated and modified to better suit the data at hand. Spectroscopic parallaxes were measured constraining the luminosity class via estimates of distances to main sequence A/F stars, which are found within a few arcminutes of each classical Be star on the sky. In order to probe the structure of the outer Galactic disc, I studied the spatial distribution of 63 out of 248 classical Be stars identified. Their cumulative distribution function with respect to the distance is statistically compatible both with a smooth exponential density profile and with a simple spiral arms representation. The distribution of reddenings of classical Be stars is compared with estimates of the total Galactic reddening along their sightlines. It is expected that the measured reddenings match the integrated Galactic values, for distant stars located outside the Galactic dust layer, or they are smaller than the asymptotic values if the stars are less distant. The outcome meets expectations, and lends support to the conclusion that the measured reddenings are determined to a precision of 10%. The sample of 248 objects doubles the number of known classical Be stars in this part of the Galactic plane. Unlike the pre-existing bright sample, the new objects are seen at large distances, between 2 – 8 kpc with typical E(B V) 0:9. Only four stars are members of known clusters. Ten classical Be stars are proposed to be well beyond the putative Outer Arm, at distances larger than 8 kpc. The large sample of stars, which has been identified here, is the result of a successful selection and analysis of classical Be stars that is offered for more exploitation in future. The proposition is that GAIA observations will use the present sample of classical Be stars as a new tracer of the Galactic disc.

The nature of dark matter and dark energy are currently two of the most important questions in cosmology. In this thesis, we consider studying the dark universe with the redshifts and peculiar velocities of galaxies. In the first half of the thesis, we analyse current peculiar velocity measurements of the bulk flow of our local volume to estimate the underlying dark matter power spectrum. In the second half of the thesis, we consider the prospects for measuring dark matter and dark energy with future galaxy redshift surveys, particularly via redshift space distortions. Fundamentally, bulk flow measurements and redshift space distortions are both sensitive probes of the power spectrum and growth rate of cosmic structure. In the final chapter, we directly compare power spectrum measurements with both methods.

We motivate the importance of understanding the kinematics and dynamics of the Milky Way stellar halo both in unravelling the formation history and evolution of our host Galaxy and in the more general context of galaxy dynamics. We present a cleaned picture of the kinematics of the smooth component of the stellar halo: we develop a method to quantify the average distance error on a sample of stars based on the idea of Schoenrich et al. (2012), but adapted so that it uses velocity information only on average. We use this scheme to construct an analytic distance calibration for Blue Horizontal Branch (BHB) field halo stars in Sloan colours and demonstrate that our calibration is a) more accurate than the ones available and b) unbiased w.r.t. metallicity and colour. We measure the rotation of the smooth component of the stellar halo with a tool-set of four estimators that use either only the l.o.s. velocities or the full 3D motion. From two samples of BHB stars from the Sloan Digital Sky Survey, we favour a non-rotating single halo. We critique conflicting results in the literature based on similar samples and trace back the disagreement (either in the sign of rotation or in the morphology of the halo) to sample contaminations and/or neglect account of the halo geometry. We propose a scheme that generalizes any isotropic spherical model to a model where the potential is axisymmetric and the distribution function is a function of the three actions. The idea is to approximate the Hamiltonian as a function of the actions with a library of quadratic fits to surfaces of constant energy in action space and then make explicit the dependence of the energy on the three actions in the ergodic distribution function. The transparency of the physics implied by the model we achieve, should make it possible to combine our spheroidal models to the f(J)-models of Binney (2010) for the disks and of Pontzen & Governato (2013) for the dark-matter halo, and obtain a complete actions-defined dynamical model of the Milky Way Galaxy.

The centre of the Milky Way, commonly referred to as the Galactic Centre, is roughly that region within 500 pc of the central black hole, Sagittarius A*. Within the innermost parsec around the supermassive black hole Sagittarius A* are more than a hundred massive young stars whose orbits align to form one or possibly two discs. At about 100 pc is a ring containing more than ten million solar masses of molecular gas which could be the origin of some of the most massive star clusters in the Galaxy. I have performed a number of numerical simulations to help us understand how it is that these structures may have been formed. I firstly describe and test an improvement to the smoothed particle hydrodynamics code I used. This improves conservation of energy and momentum in certain situations such as in strong shocks from supernovae, which were to be included in a later chapter. The discs of massive stars around Sagittarius A* are believed to have been born there within fragmenting gaseous discs. This is problematic, as the formation of two stellar discs would require two gaseous counterparts. A method is described of forming multiple discs around a black hole from a single cloud's infall and subsequent tidal destruction. This is due to its prolate shape providing a naturally large distribution in the direction of the angular momentum vectors within the cloud. The resulting discs may then go on to form stars. Energetically, it would appear that a sequence of supernovae could potentially cause a giant molecular cloud to fall inwards towards the central black hole from an originally large orbit around the Galactic Centre. I simulate the impact on a giant molecular cloud of supernovae originating from a massive stellar cluster located a parsec away. Ultimately, the supernovae are found to have little effect. Finally, I simulate the formation of the dense ring of clouds observed in the Central Molec- ular Zone at a distance of about 100 pc from Sgr A*. Infalling gas is shown to be subject to such extreme tidal forces that a single cloud of gas is extended to form a long stream. The ribbon grows to the point that it self-intersects and forms a ring-like structure. Its complexity depends on the orbit of the original cloud. The position-velocity data is compared with observations, and similarities are noted.

The dark matter paradigm has been a great source of speculation in both the 20th and 21st Centuries. Since its proposed existence in 1933, the mounting evidence has led to this theoretical particle becoming one of the greatest mysteries of modern physics. However, despite its dominant presence in the Universe, little is known about its nature and how it behaves. In this thesis I critically analyse one particular property of dark matter: the self-coupling. The self-interacting dark matter paradigm hypothesises that dark matter is not collisionless as assumed in most cosmological simulations, and in-fact has some probability that it will scatter off itself. Such a self-coupling will resolve many discrepancies that exist between observations and theory, particularly on small, non-linear scales. Moreover, any detection of a self-interaction cross-section will place considerable limitations on the acceptable particle physics models of dark matter and hence has grown to become an important question. In this thesis I develop and implement a method to constrain the self-interaction cross-section of dark matter that exploits continually accreting and merging groups of galaxies as they fall into galaxy clusters. Utilising the ubiquitous nature of accreting substructure, I measure the offsets between dark matter and baryonic gas as they become separated due to their differing interaction properties. Studying this effect over a sample of events, I will be able to make the first ever statistical estimate of the cross-section of dark matter, while averaging over many different unknown merging scenarios. I begin my thesis by deriving an analytical description of sub-halo in-fall, allowing me to constrain dark matter self-interaction models directly from observations. In this study, I find that current archival data should be able to detect a difference in the dynamical behaviour of dark matter and standard model particles at 6σ, and measure the total interaction cross-section σDM/m with 68% confidence limits of ±1 cm2g-1. Having constructed a new method to derive constraints on the cross-section of dark matter I carry out a study into the potential systematics that may affect a measurement. I determine the accuracy of weak gravitational lensing, which is the distortion of light due to intervening mass, as a tool to estimate the positions of substructure in galaxy clusters. I find that the public Lenstool software can measure the position of individual 1:5 x 1013Mʘ peaks with ~ 0:3" systematic bias, as long as they are at least ~ 30" from the cluster centre. Finally, I develop a pipeline that can analyse a sample of inhomogeneous observations from The Hubble Space Telescope and the Chandra X-ray Observatory. By measuring the positions of dark matter, gas and galaxies for 68 individual merging events, from a total of 28 galaxy clusters, I detect a 7:4σ offset between gas and an unobserved dark mass. I make the first ever measurement of cross-section of dark matter from a sample of clusters finding σDM < 0:50cm2/g [95% CL], the best constraints to date. In addition to this I find that the brightest group galaxy in-fact tends to lead the dark matter halo during merging events. Although evidence for the existence of interacting dark matter, I conclude that the astrophysics of the BCG is complicated, and that this apparent directional bias should be considered in all galaxy cluster analyses. Moreover, I show that this technique is easily extendable for future surveys that have larger samples of galaxy clusters, with constraints of σDM < 0:001cm2/g potentially attainable.

Exact solutions to the Einstein field equations are notoriously difficult to find. Most known solutions describe systems with unrealistically high degrees of symmetry. A notable example is the FLRW metric underlying modern cosmology: the universe is assumed to be perfectly homogeneous and isotropic, but in the late universe, this is only true on average and only at large scales. Where an exact solution is not available, discrete gravitational approaches can approximate the system instead. This thesis investigates several cosmological systems using two distinct discrete approaches. Closed, flat, and open ‘lattice universes’ are first considered where matter is distributed as a regular lattice of identical point masses in constant-time hypersurfaces. Lindquist and Wheeler’s Schwarzschild–cell method is applied where the lattice cell around each mass is approximated by a perfectly spherical cell with Schwarzschild space–time inside. The resulting dynamics and cosmological redshifts closely resemble those of the dust-filled FLRW universes, but with certain differences in redshift behaviour attributable to the lattice universe’s lumpiness. The application of Regge calculus to cosmology is considered next. We focus exclusively on the closed models developed by Collins, Williams, and Brewin. Their approach is first applied to a universe where an exact solution is already well-established, the vacuum Λ-FLRW model. The resulting models are found to closely reproduce the dynamics of the continuum model being approximated, though certain constraints on the applicability of the approach are also uncovered. Then using this knowledge, we next model the closed lattice universe. The resulting evolution closely resembles that of the closed dust-filled FLRW universe. Constraints on the placement of the masses in the Regge skeleton are also uncovered. Finally, a ‘lattice universe’ with one perturbed mass is modelled. The evolution is still stable and similar to that of the unperturbed model. The thesis concludes by discussing possible extensions of our work.