Accurate mass estimates for galaxy groups and clusters are important for understanding the evolution of matter within the universe. In this thesis we first discuss methods of mass estimation currently used within the literature, and introduce a set of scaling relations for mass estimation in cases where literature methods are not applicable. We find that methods based on group richness provide the best (i.e. lowest scatter) mass estimator. Secondly, we study the impact of feedback on hot group gas for a sample of optically selected groups. We refine the group selection using their dynamical state, identified through substructure in the distribution of member galaxies. We find this sample to be underluminous compared to an X-ray selected sample. Furthermore, with two groups showing high 2σ lower limits on entropy, the population of high entropy groups predicted by hydrodynamical simulations may have been detected. Finally, we combine measures of dynamical state and mass estimation scaling relations to understand how the presence of substructure can impact upon the ability to reliably estimate group and cluster masses. We find that substructure introduced through poor group identification has the largest effect on the quality of the final mass estimates.

The formation and evolution of galaxies is an interesting subject to study because it incorporates astrophysics from all scales, from the initial perturbations in the early universe creating the large scale structures that produce galaxies, right down to the evolution of stellar populations and their manipulation of the host galaxy. Simulations of galaxy formation allow us to test the various physical recipes against that which is observed in order to build a true and proper picture of what is happening in the real universe. L-Galaxies is a semi-analytic model of galaxy formation built on top of the merger trees from the Millennium dark matter simulation, and is constrained to match certain key observations at low redshift by applying a Monte Carlo Markov Chain (MCMC) method to constrain the free parameters. In using the model to make high redshift predictions of the stellar mass function, UV luminosity function and star formation rate distribution function we found that the model starts to deviate from observational constraints at the highest redshifts, particularly in high mass galaxies. In the case of the UV luminosity function, this is because the current dust model is calibrated at low redshift and lacks sophistication in that it only depends on the cold gas mass and the density of metals. To improve on this we implement a physically motivated dust model that traces the formation of dust from stellar sources, such as in the stellar winds of AGB stars and in the supernovae remnants of massive stars, the growth of dust inside molecular clouds, and the destruction of dust due to supernovae explosions. The model is fully integrated into L-Galaxies such that the evolution of dust is included in all the recipes relevant to the formation and evolution of galaxies, including: star formation; radiative feedback; cooling and reheating; and both major and minor mergers. Our results show a good fit to observations of the dust mass in galaxies both in the local universe and out to high redshift and we note a similar conclusion as in the literature that dust growth inside molecular clouds is not only necessary but the dominant source of the dust mass in these galaxies. However, stellar sources of dust can not be neglected as molecular clouds must first be seeded by dust grains in order for accretion to occur. This could be important in the very early universe, perhaps for the first galaxies that will hopefully be observed by JWST in the future, because these galaxies may not have had sufficient time to seed their molecular clouds and as such the dust produced by these stellar sources would be important for calculating the galaxies true observed luminosity. We finish by discussing the limitations of the model and discuss areas for possible improvement as well as the next steps in using this to better predict the luminosity of galaxies in future models.

This thesis explores possible solutions to the dwarf galaxy problem. This is a discrepancy between the number of dwarf galaxies we observe, and the number predicted from cosmological computer simulations. Simulations predict around ten times more dwarf galaxy satellites than are currently observed. I have investigated two possible solutions: dark galaxies and the low surface brightness universe. Dark galaxies are dark matter halos which contain gas, but few or no stars, hence are optically dark. As part of the Arecibo Galaxy Environment Survey I surveyed the neutral hydrogen gas around the nearby galaxy M33. I found 32 gas clouds, 11 of which are new detections. Amongst these there was one particularly interesting cloud. AGESM33-32 is ring shaped and larger than M33 itself, if at the same distance. It has a velocity width which is similar to the velocity dispersion of gas in a disk galaxy, as well as having a clear velocity gradient across it which may be due to rotation. The fact that it also currently has no observed associated stars means it is a dark galaxy candidate. Optically, dwarf galaxies may be out there, but too faint for us to detect. This means that with newer, deeper, images we may be able to unveil a large, low surface brightness, population of dwarf galaxies. However, the question remains as to how these can be distinguished from background galaxies. I have used Next Generation Virgo Survey (NGVS) data to carry out photometry on 852 Virgo galaxies in four bands. I also measured the photometric properties of galaxies on a background (non-cluster) NGVS frame. I discovered that a combination of colour magnitude and surface brightness information could be used to identify cluster dwarf galaxies from background field galaxies. The most effective method is to use the surface brightness-magnitude relation.

This thesis aims to understand more about the developmental histories of galaxies and their internal components by studying the wavelength dependence of their spatial structure. I use a large sample of low-redshift galaxies with optical–near-IR imaging from the GAMA survey, which have been fitted with Sersic and Sersic + exponential functions in nine wavebands simultaneously, using software developed by the MegaMorph project. The first section of this thesis examines how the sizes and radial profiles of galaxies vary with wavelength. To quantify the wavelength dependence of effective radius I use the ratio, R, of measurements in two restframe bands. The dependence of Sersic index on wavelength, N, is computed correspondingly. I show that accounting for different redshift and luminosity selections partly reconciles variations between several recent studies. Dividing galaxies by visual morphology confirms the behaviour inferred using morphological proxies, although our quantitative measurements allow me to study larger and fainter samples. I then demonstrate that varying dust opacity and disc inclination can account for features of the joint distribution of R and N for late-type galaxies. However, dust does not appear to explain the highest values of R and N. The bulge-disc nature of galaxies must also contribute to the wavelength-dependence of their structure. The second section of this thesis studies radial colour gradients across the galaxy population. I use the multi-wavelength information provided by MegaMorph analysis of galaxy light profiles to calculate intrinsic colour gradients, and divide into six subsamples split by overall Sersic index (n) and galaxy colour. I find a bimodality in the colour gradients of high- and low-n galaxies in all wavebands which varies with overall galaxy luminosity. Global trends in colour gradients therefore result from combining the contrasting behaviour of a number of different galaxy populations. The ubiquity of strong negative colour gradients supports the picture of inside-out growth through gas accretion for blue, low-n galaxies, and through dry minor mergers for red, high-n galaxies. An exception is the blue high-n population which has properties indicative of dissipative major mergers. In the third section of this thesis I apply bulge-disc decompositions to my sample of galaxies, in order to discover the structural origin of the wavelength dependences found in the previous two chapters. I find that most galaxies with a substantial disc, even those with no discernible bulge, display a high value of N. The increase in Sersic index to longer wavelengths is therefore intrinsic to discs, apparently resulting from radial variations in stellar population and/or dust reddening. Similarly, low values of R (< 1) are found to be ubiquitous, implying an element of universality in galaxy colour gradients. I also study how bulge and disc colour distributions vary with galaxy type. I find that, rather than all bulges being red and all discs being blue in absolute terms, both components become redder for galaxies with redder total colours. I even observe that bulges in bluer galaxies are typically bluer than discs in red galaxies, and that bulges and discs are closer in colour for fainter galaxies. Trends in total colour are therefore not solely due to the colour or flux dominance of the bulge or disc.

In this thesis we use both observations and modelling to explore the gas content of galaxies. We use the L-Galaxies semi-analytic model to simultaneously match the Hɪ and stellar mass properties of model galaxies to observations using Markov Chain Monte Carlo methods. We add the observed Hɪ mass function as an extra model constraint and successfully match the Hɪ and stellar mass functions. However, the fit to the star formation properties has been weakened compared to without the Hɪ constraint. We suggest that this problem may be partially resolved by forming stars out of only H2 gas instead of the total cold gas. The environment in which a galaxy resides can affect its evolution. We use the counts in a fixed size cylinder method to estimate 3 environment measures for the GAMA survey. We use density and edge corrections to allow us to calculate estimates for every galaxy out to z = 0.4 in our flux limited sample. We then use these estimates to examine the effect of environment on the luminosity and stellar mass functions. Using Hɪ observations of the groups and galaxies in the ALFALFA and GAMA surveys we calculate Hɪ masses using the stacking technique. The use of the stacking technique has allowed us to exploit survey data that would not otherwise be possible. We stack galaxies in halo mass bins and calculate the Hɪ to halo mass fraction as a function of halo mass. We see a steady decline in the Hɪ fraction as we move to higher mass halos. These are the highest density environments where there is less cold gas. Combining this fraction with the halo mass function we are able to calculate a lower limit value for ΩHɪ of 1.8 ± 0.39 x 10-4h-¹.

Evolution of galaxies is one of the most important topics in astronomy to understand how the universe has been evolving. In particular, galaxy groups are important because they are the observable equivalent of dark matter (DM) haloes, and thus offer a direct insight into the physics that has occurred in the DM haloes in the Universe up to the present day. Isolated galaxies are crucial for studying intrinsic and secular processes able to affect the structure, morphology, and dynamics of galaxies for obtaining clear relationships and correlations to be confronted with the model predictions. The main goal of this work is to characterize the GAMA G3Cv1 galaxy groups catalogue and the UNAM-KIAS catalogue of isolated galaxies by one of the most important statistical studies, the galaxy Luminosity Function (LF), that helps to constrain the models of formation and evolution of galaxies. LFs have been estimated for galaxies in groups and isolated galaxies. The LF for groups has been characterized by the physical properties of the groups (mass and velocity dispersion), the photometry (colour), the morphological type and eleven wavelengths from the far infra-red to the ultra violet. The LF estimated for the isolated galaxies is characterized by morphology and the colour in the five SDSS bands. The results obtained constrain more effectively the formation and evolution models of the universe than previous samples. The differences between both catalogues are presented in the conclusions. Additionally, the galaxy morphology is one of the no well understood problems in the galaxy evolution process to support the hierarchical model of formation of large objects. In this work, a classification based on the colour and concentration of light was considered. However, due to the low resolution of the images, the confidence of this classification was only ~60%.

Several hundred million years after the Big Bang, the Epoch of Reionisation(EoR) started as the photons from the first objects ionised neutral baryons in the Universe. The observations such as the Gunn-Peterson troughs in quasar absorption spectra and the linear polarisation of the cosmic microwave background (CMB) impose strong constraints on reionisation models of the EoR. Recent data provide the rest-frame ultraviolet luminosity of galaxies up to redshift 10. However, the observation of star formations in low mass galaxies is still not practicable. Their star formations are expected to be suppressed by the increase of ionised baryons and greatly affect the reionisation models. We develop a flexible pipeline which utilises the Munich Semi-Analytic Model of galaxy formation, L-Galaxies, and a semi-numerical modelling of cosmic reionisation. This combination allows us to create a self-consistent reionisation simulation in computational models of galaxy formation. We use this pipeline on a high resolution cosmological Nbody simulation to produce the redshift evolution of the star forming galaxies during the EoR. Comparisons of the properties of mock galaxies and the growth of ionised hydrogen bubbles suggest that the reionisation history heavily depends on the suppression models used in the modeling of dwarf galaxy formation. During this research, some numerical flaws of merger tree generation algorithms were identified. We investigated the origins of these problems and present suggestions for solving them.

Inflation is currently the most promising paradigm of the Early Universe. The simple paradigm involves a single canonical scalar field minimally coupled to gravity slowly rolling down a potential. In this thesis, we discuss an extension to the simple paradigm, multifield inflation, in which inflation is driven by more than one scalar field. Unlike in the single field paradigm, isocurvature perturbations could be non-vanishing and source curvature perturbation on superhorizon scales. Analytic model predictions during the slow-roll regime in some classes of multifield inflation models have been worked out in the literature. However, curvature perturbation may continue to evolve after slow-roll as isocurvature perturbations are not necessarily exhausted when inflation ends. In this thesis, by using the δ N formalism, we investigate the effects of perturbative reheating on the curvature perturbation and related observables in multifield models. By considering various two-field models, we demonstrate that the subsequent (p)reheating evolution is significant and must be taken into account even for perturbative reheating. How the model predictions evolve during reheating is a model dependent question, implying that models of multifield inflation cannot be compared to observations directly without specifying how reheating takes place. We also discuss a different class of two-field models, conformal inflation, which is locally scale invariant. Universal behaviour emerges as a critical phenomenon near the enhanced SO(1,1) or shift symmetry point, leading to model independent predictions. Going beyond the original model proposed by Kallosh and Linde, we show that this universal behaviour extends to more generalised models involving higher order derivatives for slow-roll potential driven inflation.

Structure in the Universe formed from tiny density perturbations that grew into the complex cosmic web we see today. At the nodes of this web we find galaxy clusters - the largest gravitationally bound objects ever observed. Their abundance and their properties provide an insight into the evolution of the Universe, so they are important probes of cosmology. Knowledge of their mass is critical for cosmology, but as most of this mass is in the form of dark matter it is a complex measurement, motivating interest in scaling relations between other observables and mass. In this thesis I use a sample of galaxy clusters with high quality multiwavelength observations to investigate these cluster observables and their scaling relations with mass. I find that the easily measured near-infrared and optical cluster luminosities tightly scale with mass, making them promising tools for cosmology with future wide field surveys. I also parameterise scaling relations of observables across a wide range of the electromagnetic spectrum, finding the results to be consistent with self-similarity, in which clusters are formed from a single spherical collapse driven by gravity, and with a closed box picture, in which clusters maintain their baryon budget.

This thesis covers initial work to increase the total number of galaxy clusters with strong lensing features and, therefore, lensing derived masses. These clusters also possess other multi-wavelength measurements that can be, and were, used for comparison. The work included an investigation into what additional information Dressler Shectman analysis of the incomplete redshift cluster member galaxy samples produced as a result of prioritising mask slit placement on potential strong lensing images. With the test blind on areas centred around the BCG, only a qualitative link was found, providing possible assistance with mass model parametisation. From the sample of eight clusters reduced, five had strong lensing features, of which, A3084 had an unusual and rare lensing configuration. Analysis of A3084 with relatively shallow survey data revealed a cluster with the largest cluster-scale halo centred on the intra-cluster gas and not the BCG: these two were offset from one another. The BCG had a compact DM halo coincident with it, yielding a high substructure fraction of f\(_{sub}\)= 0.73±0.13 which, along with smooth X-ray surface brightness contours, bi-modal cluster galaxy redshift histogram and luminosity map, provided evidence that the interpretation of the cluster had suffered a cluster-cluster merger. From the redshift reduction, five new strong lensing models were produced following similar methods to the ones used for A3084. With the addition of a cluster from Paraficz et al. (2012), this increased the initial sample size from 17 to 23, providing a ∼ 35% in- crease, with all clusters having other multi-wavelength measurements. Following previous literature comparisons a fit of M SL2D (R < 250 kpc) against T\(_X\) was carried out, but large scatter was observed in this. A possible link was found between the residuals of the fit with that of the clusters’ BCG ellipticity after a cut was made of m\(_{12}\) ≥ 1 for luminosity gap to remove potentially disturbed clusters.