SPIDERS (The SPectroscopic IDentification of eROSITA Sources) is a programme dedicated to the homogeneous and complete spectroscopic follow-up of X-ray active galactic nuclei and galaxy clusters over a large area (similar to 7500 deg(2)) of the extragalactic sky. SPIDERS is part of the Sloan Digital Sky Survey (SDSS)-IV project, together with the Extended Baryon Oscillation Spectroscopic Survey and the Time-Domain Spectroscopic Survey. This paper describes the largest project within SPIDERS before the launch of eROSITA: an optical spectroscopic survey of X-ray-selected, massive (similar to 10(14)-10(15) M-circle dot) galaxy clusters discovered in ROSAT and XMM-Newton imaging. The immediate aim is to determine precise (Delta(z) similar to 0.001) redshifts for 4000-5000 of these systems out to z similar to 0.6. The scientific goal of the program is precision cosmology, using clusters as probes of large-scale structure in the expanding Universe. We present the cluster samples, target selection algorithms and observation strategies. We demonstrate the efficiency of selecting targets using a combination of SDSS imaging data, a robust red-sequence finder and a dedicated prioritization scheme. We describe a set of algorithms and work-flow developed to collate spectra and assign cluster membership, and to deliver catalogues of spectroscopically confirmed clusters. We discuss the relevance of line-of-sight velocity dispersion estimators for the richer systems. We illustrate our techniques by constructing a catalogue of 230 spectroscopically validated clusters (0.031 < z < 0.658), found in pilot observations. We discuss two potential science applications of the SPIDERS sample: the study of the X-ray luminosity-velocity dispersion (L-X-sigma) relation and the building of stacked phase-space diagrams.

Cosmic voids are usually identified in spectroscopic galaxy surveys, where 3D information about the large-scale structure of the Universe is available. Although an increasing amount of photometric data is being produced, its potential for void studies is limited since photometric redshifts induce line-of-sight position errors of >= 50 Mpc h(-1)which can render many voids undetectable. We present a new void finder designed for photometric surveys, validate it using simulations, and apply it to the high-quality photo-z redMaGiC galaxy sample of the DES Science Verification data. The algorithm works by projecting galaxies into 2D slices and finding voids in the smoothed 2D galaxy density field of the slice. Fixing the line-of-sight size of the slices to be at least twice the photo-z scatter, the number of voids found in simulated spectroscopic and photometric galaxy catalogues is within 20 per cent for all transverse void sizes, and indistinguishable for the largest voids (R-v >= 70 Mpc h(-1)). The positions, radii, and projected galaxy profiles of photometric voids also accurately match the spectroscopic void sample. Applying the algorithm to the DES-SV data in the redshift range 0.2 < z < 0.8, we identify 87 voids with comoving radii spanning the range 18-120 Mpc h(-1), and carry out a stacked weak lensing measurement. With a significance of 4.4 sigma, the lensing measurement confirms that the voids are truly underdense in the matter field and hence not a product of Poisson noise, tracer density effects or systematics in the data. It also demonstrates, for the first time in real data, the viability of void lensing studies in photometric surveys.

This thesis explores the expansive world of General Relativity, and its role to play in modern cosmology and quantum field theory. We begin with a pedagogical approach to relativity, in particular, highlighting upon the ambiguity that arises with the conventions used in different textbooks. A brief introduction to tensor calculus has also been provided in the appendix. The preliminary chapters are also complimented with examples of numerical relativity via simulation. We then move on to discuss examples of non-linear systems, and their exact solutions. Such systems will be analogous to those we shall encounter later, upon considering scalar field theories as a means of modelling dark energy. We shall introduce the axion as our highly motivated dark matter candidate, since this will ultimately determine the behaviour of the scalar field. Coupled to a scaling factor across the spatial domain, it is found that this scalar field will ultimately determine the evolution of our universe. The key result of this thesis has been the possibility to screen both the cosmological constant, and flatness of the universe, to within observable parameters. These results will be explicitly derived from first principles. Also included is a tentative approach to holographic theory, in which strongly correlated systems may be modelled within the asymptotic domain of Anti-de Sitter (AdS) space. Ultimately, our aspirations are to bridge the gap with condensed matter theory, in particular with the publications included within the latter appendices. These publications discuss graphene as a revolutionary new material, for inclusion in both transistor-based and optoelectronic devices.

In this Thesis I explore aspects of dark radiation and its role in String Phenomenology. Dark radiation is any additional hidden type of relativistic matter present in the Universe today, conventionally labelled as an "excess effective number of neutrino species", &Delta; N_eff. It provides a powerful test of hitherto untested theoretical models based on fundamental theories such as String Theory. I begin by considering dark radiation in the LARGE Volume Scenario, a phenomenologically viable class of string compactifications. First I review how the minimal setup slightly overproduces axionic dark radiation via modulus decay. I then demonstrate that loop corrections to the main competing visible-sector decay process have a negligible effect and are unable to alleviate the tension with observations. In the following chapter I explore fibred extensions of the LARGE Volume Scenario. The predictions for &Delta; N_eff are qualitatively different: in particular, models with a sequestered visible sector on D3 branes at a singularity are swamped by massless axions and decisively ruled out. I then consider TeV-scale supersymmetry in a model with anisotropic modulus stabilisation. If the Standard Model is realised on D7 branes wrapping the small volume cycle a hierarchy of soft terms is generated, which may have applications to natural supersymmetry. The final chapter takes a different approach and investigates the proposition that dark radiation, in the form of a Cosmic Axion Background, could explain the long-standing soft X-ray excess from galaxy clusters. I show for the Coma cluster that the morphology of the excess can be reproduced by axion-photon conversion in the intracluster magnetic field, provided the field is allowed to have more structure on smaller scales than typically assumed based on Faraday rotation data. This explanation requires an inverse axion-photon coupling M &sim; 10¹¹ - 10¹² GeV and a mean axion energy (E_CAB) &sim; 50 - 250 eV.

We present the Magellan/FIRE detection of highly ionized C IV lambda 1550 and O III]lambda 1666 in a deep infrared spectrum of the z = 6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously known Ly alpha. No corresponding emission is detected at the expected location of He II lambda 1640. The upper limit on He II, paired with detection of O III] and C IV, constrains possible ionization scenarios. Production of C IV and O III] requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (similar to 0.5 Ryd). As a pure recombination line, He II emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher He II flux. The conclusions based on ratios of metal line detections to helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general z = 2-3 population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.

No description available.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Pietermaritzburg, 2015 / This thesis considers economy and cosmology in the Iron Age of KwaZulu-Natal. It draws on models derived from anthropological and historical analyses of precolonial agriculturists in southern Africa and applies these to archaeological data. Critics argue that anthropological approaches in archaeology are not conducive to the creation of a socially dynamic past. In contrast, I believe that their potential is considerable. The models targeted, principally Huffman’s Central Cattle Pattern, obviously represent socially dynamic relationships. This is clear if we look at lower-level models: Ngubane’s analysis of Zulu sickness and healing, which reveals fracture lines and tensions within the homestead, and Hammond-Tooke’s observation that the Nguni and Sotho pollution systems are variations related to the specifics of marriage and settlement. Ngubane’s analysis couples neatly with Guy’s identification of the ‘history-making’ principle—the struggle for the accumulation, creation and control of human productive and reproductive capacity— that gave Iron Age societies their dynamism. It is an engagement that firmly integrates systems of symbolism and belief with economy. Throughout this study I focus on the expression of this dynamic principle in cosmology and material culture. Consideration of pollution concepts in the Early Iron Age showed that the high exchange value of women created extensive lateral alliance networks as cattle moved as bridewealth from one homestead to another. The system worked against a concern for male agnatic continuity and so generated considerable structural tension within society, which was expressed in material culture. My focus on fish remains in Iron Age sites generated an ‘ethnography’ and political history of fishing where none had existed previously. It established a cultural logic that explained the avoidance of fish eating in some societies, and its adoption and significance in others. The approach combined with Kopytoff ’s frontier model revealed two key findings. First, the marginal category, amalala, originated at the Early and Late Iron Age interface. Secondly, the Zulu kingdom emerged from a dynastic shift in a complex of chiefdoms around the Babanango plateau, with the Zulu leadership usurping Khumalo authority. An analysis of Nguni rainmaking, and of the record of interaction between huntergatherers and agriculturists, revealed no evidence that hunter-gatherers made rain for agriculturists until the late nineteenth century. This work marked their final tragedy, their loss of independent life as the colonial world closed in about them.

La population de galaxies poussiéreuses ayant un fort taux de formation stellaire (Dusty Star Forming Galaxies, DSFGs) joue un rôle très important dans l’histoire de l’univers, avec des taux de formation d’étoiles allant de quelques centaines à quelques milliers de masses solaires par an. Les sondages infrarouges, comme ceux entrepris à l’aide du satellite Herschel, nous offrent l’opportunité de recenser de manière approfondie ces DSFGs jusqu’à de grands décalages spectraux. Cependant, jusqu’à présent seul un petit nombre de DSFG détecté par Herschel ont été confirmés pour être à des décalages spectraux supérieurs à 4. Les modèles de formation et d’évolution des galaxies stipulent généralement que la population de DSFG à z > 4 sont les progéniteurs des galaxies elliptiques, observées dans les amas les plus massifs de l’univers local. L’abondance des DSFGs à z > 4 se révèle donc être décisive pour contraindre ces modèles ainsi que pour vérifier notre compréhension globale de l’univers lointain. Le premier objectif de mon travail de recherche est d’identifier les candidates galaxies à z > 4 détectées sur des champs les plus larges possibles observés par Herschel et l’instrument SPIRE et d’examiner les propriétés statistiques de celles-ci. A cette fin, j’ai créé un nouvel algorithme de sélection dans le but d’augmenter substantiellement le nombre de candidates et de comprendre leur nature. / Over the last few decades, great progress has been made in our understanding of the star formation history of the Universe. With the discovery of distant, dusty star-forming galaxies (DSFGs) it has become apparent that observing at rest-frame UV and optical wavelengths is insufficient as a large fraction of the star formation is dust obscured. Thanks to the extensive observational studies carried out during the last two decades, we learn that DSFGs have a redshift peak at z ∼ 2, matching the cosmic time where galaxies have formed most of their young, massive stars. However, it remains extremely challenging to use the Herschel space observatory for identifying a tail extending towards much higher redshifts (z > 4). As a result, until recently only a small number of infrared-selected DSFGs at z > 4 were known, most of them strongly gravitationally lensed. One of the main goals of this Thesis is to assemble candidate z > 4 galaxies detected in a large area survey observed by Herschel-SPIRE and to examine the statistical properties and environments of these systems.

Identifier la nature de la matière sombre est l'un des plus grands problèmes de la physique contemporaine. Si la matière sombre est constituée de particules, on peut espérer la détecter, directement ou indirectement, grâce à des expériences terrestres ou spatiales. Prédire les résultats de ces expériences, ou les interpréter en cas de détection, nécessite une compréhension profonde de la structuration de la matière sombre dans notre Galaxie. En partant de considérations issues de la physique des particules et de la cosmologie, je construits un modèle du halo de matière sombre Galactique contraint dynamiquement qui incorpore une description détaillée des ses inhomogénéités. L'impact des ces inhomogénéités sur les recherches utilisant le rayonnement cosmique est ensuite analysé en détails. J'étudie également une méthode permettant de prédire la distribution dans l'espace des phases des particules de matière sombre, et discute sa possible application aux recherches de matière sombre. Cet outil est ensuite appliqué aux recherches utilisant les électrons et positrons cosmiques, et de nouvelles contraintes très fortes sont obtenues sur les modèles microscopiques de matière sombre. / Understanding the nature of dark matter is one of the greatest challenges of modern physics. If dark matter is made of particles, we can hope to detect it, directly or indirectly, using Earth-based or spatial experiments. Make predictions for the outcome of these experiments, or interpret the results in case of a detection, requires a deep understanding of the structuring of dark matter in our Galaxy. Starting from particle physics and cosmological considerations, I built a dynamically constrained model of the Galactic dark halo including a detailed description of its inhomogeneities. The impact of these inhomogeneities on searches with cosmic rays is then analysed in details. I also study a method allowing to predict the phase-space distribution of dark matter particles, and discuss its possible application to dark matter searches. This method is then applied to searches with cosmic-ray electrons and positrons, and new very stringent constraints are obtained on microscopic models of dark matter.

Na última década, o extraordinário progresso nas observações astronômicas (distâncias com supernovas (SNe Ia), espectros de potência da matéria e da radiação cósmica de fundo (RCF), determinação do brilho de aglomerados de galáxias, etc.) aliado com importantes desenvolvimentos teóricos, transformaram a Cosmologia numa das fronteiras mais excitantes da ciência contemporânea. Nesta tese, diferentes testes observacionais são utilizados para vincular alguns cenários cosmológicos acelerados (com e sem energia escura), todos eles definidos no contexto teórico da Relatividade Geral. Inicialmente, para uma grande classe de modelos com decaimento do vácuo, investigamos os vínculos provenientes da existência de objetos velhos em altos redshifts. No modelo de Chen e Wu generalizado, encontramos que o limite para o parâmetro livre descrevendo a taxa do decaimento do vácuo é 0,21 < n < 0,81. Este resultado descarta o modelo de Chen e Wu original (n=2) e também o modelo de concordância cósmica, LCDM (n=0). Além disso, quando incluímos o fluido bariônico em nossa análise do modelo de Wang e Meng, obtemos para seu parâmetro livre um limite inferior, epsilon > 0,231, um valor em desacordo com estimativas independentes baseadas em SNe Ia, RCF e o brilho de Raios-X de aglomerados. Propusemos também um teste estatístico com base nas idades estimadas para uma amostra de 13 galáxias velhas em altos redshifts. Através de uma análise conjunta envolvendo as idades das galáxias e as oscilações acústicas dos bárions (BAO), vinculamos o valor da constante de Hubble no contexto do modelo LCDM plano. Considerando um tempo de incubação adotado por diferentes autores, obtemos h=0,71±0,04 (1 sigma), um resultado de acordo com observações independentes baseadas em Cefeidas (obtidas com o Hubble Space Telescope) e outras estimativas mais recentes. Outro resultado interessante foi obtido através de uma análise termodinâmica para uma classe de modelos com interação no setor escuro (matéria escura-energia escura). Contrariamente ao que se pensava até então, encontramos que a termodinâmica permite que a matéria escura decaia em energia escura, contanto que ao menos uma das componentes possua um potencial químico não-nulo. Como complemento, mostramos que, para um termo de interação específico, dados de SNe Ia, BAO e RCF favorecem o decaimento da matéria escura com ~ 93% de confiança estatística. Investigamos também o comportamento do redshift de transição em diferentes cosmologias, com e sem energia escura, e mostramos que essa quantidade pode ter uma variação extrema dependendo do modelo cosmológico subjacente. Finalmente, discutimos também um novo modelo cosmológico cuja aceleração em baixos redshifts é determinada pela criação de partículas da matéria escura fria. O modelo representa uma redução do setor escuro, isto é, não tem energia escura, contém apenas um parâmetro livre e satisfaz os vínculos de Supernovas do tipo Ia tão bem quanto o modelo LCDM padrão. / In the last decade, the extraordinary progress of the astronomical observations (distances with supernovas, matter and cosmic background radiation (CBR) power spectrum, X-ray surface brightness of galaxy clusters, etc) associated with important theoretical developments turned Cosmology one of the most exciting frontiers of contemporary science. In this thesis, different observational tests are used to constrain several cosmological accelerating scenarios (with and without dark energy), all of them defined in the theoretical framework of General Relativity. Initially, for a large class of decaying vacuum models, we investigate the constraints provided by the existence of old high redshift objects. In the model proposed by Chen and Wu, we find that the limit for the free parameter describing the decay rate of the vacuum fluid is 0.21 < n < 0.81. This result ruled out the original Chen and Wu model (n = 2) and also the cosmic concordance model, LCDM (n = 0). Further, when we include the baryonic fluid in our analysis of the Wang and Meng model, we find for its free parameter a lower bound, epsilon > 0.231, a value in disagreement with independent estimates based on SNe Ia, CMB (shift parameter) and the X-ray surface brightness of galaxy clusters. We also propose a new cosmological statistical test based on the estimated ages of 13 old high redshift galaxies. By performing a joint analysis involving the ages of the galaxies and the baryon acoustic oscillations (BAO) probe, we constrain the value of the Hubble parameter in the context of the flat LCDM model. For an incubation time adopted by different authors, we find h = 0.71 ± 0.04 (1 sigma), a result in agreement with independent observations based on Cepheids (obtained with the Hubble Space Telescope) and other recent estimations. Another interesting result has been derived from a thermodynamic analysis for a class of models endowed with interaction in the dark sector (dark matter and dark energy). In contrast with some results appearing in the literature, we show that the decaying of cold dark matter into dark energy is not forbidden by thermodynamics, provided that the chemical potential of one component is different from zero. As a complement, we also show (for a specific term describing the interaction) that this kind of decaying is favored by SNe Ia, BAO and CMB data with ~ 93% of statistical confidence. We also investigate in detail the behavior of the transition redshift for different cosmologies (with and without dark energy). It is found that such a quantity may have an extreme variation that depends on the underlying cosmological model. Finally, we also discuss a new cosmological model whose acceleration at low redshifts is determined by the creation of cold dark matter particles. The model represents a reduction of the dark sector, that is, it has no dark energy, contains only one free parameter and satisfies the Supernovae type Ia constraints with the same precision of the standard LCDM model.