published_or_final_version / abstract / toc / Computer Science and Information Systems / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

Theory of cosmic structure formation outlines how stars, galaxies, clusters of galaxies, and large-scale structures formed out of primordial density fluctuations. It presents us a picture of cosmic mass assembly, and places strong constraints on cosmological model. Both observations and theories suggest that structures formation follows a "bottom up" process, in which small, low-mass component form first, and gradually develop into larger, more massive systems. This dissertation focuses on three crucial stages of cosmic structure formation: first generation stars, quasar host galaxies and the large-scale galaxy overdensities. In Chapter 1, I present an overview of structure formation, acquainting readers with a general picture from first object in the Universe to large-scale structures at later epochs. In Chapter 2 and Chapter 3, I derive strong constraints to the star formation rates (SFRs) of very massive Population III (Pop III) stars in two high redshift galaxies at z = 7. By probing the He II emission lines for both galaxies, I conclude that the contributions of very massive Pop III stars to total the SFRs are less than 3%. In Chapter 4, I move to more massive systems, quasar host galaxies at z ~ 3. Using damped Lyman alpha absorption systems as natural coronagraphs, I report that rest-frame far-UV emission of quasar host galaxy correlates strongly with quasar luminosity. This result suggests a co-evolution of supermassive black holes and their host galaxies. In Chapter 5, I develop a novel method for searching the most massive protoclusters at z = 2-3, by utilizing intergalactic Lyman alpha absorption. My investigations suggest that large intergalactic Lyman alpha absorption systems effectively trace the most overdense regions at large scale of ~ 15 h⁻¹ Mpc. In Chapter 6, I present our imaging observations of an extreme galaxy overdensity (protocluster) BOSS1441+4000, which is discovered using the techniques developed in Chapter 5. Furthermore, I report an intergalactic-scale Lyman alpha nebula detected at the density peak of BOSS1441+4000. This discovery, together with previously discovered Slug nebula, provide us a first look of intergalactic medium in emission in the early Universe. In the Chapter 7, I give a summary of this dissertation and discuss several future prospects.

Dense cluster environment influences the properties of galaxies and their evolution. In order to understand this environmental effect and how it evolves with time, we study the infrared (IR) properties of galaxies in three rich clusters. The IR luminosities provide us with extinction-free measurements of the star formation rates (SFRs) of these cluster galaxies. We find a strong evolution in the IR luminosity function (LF) of two z ∼ 0:8 clusters when compared to two local clusters. The evolution rate of the IR LF found in these clusters is consistent with the evolution in field IR LFs. The similar evolution rate found in very different environments favors some internal mechanism, e.g., the gradual consumption of the gas fuel in galaxies, as being responsible for much of the star formation evolution. The mass-normalized integrated SFRs within 0.5R₂₀₀ of these clusters also shows an evolution trend, ∝ (1 + z)5. But this evolution has large scatter and may be affected by the mass selection effect of the sample. In the dense cluster core regions (r < 0.3 Mpc), we find evidence for enhanced SFR suppression. A substantial fraction of members in MS 1054-03 (z ∼ 0.8) are still forming stars actively. This cannot be explained by the scenario where the cluster is only passively accreting star-forming galaxies from the surrounding field, after which their star formation is quenched quickly. We also study the extended IR emission from the intracluster dust (ICD) in A2029. We only find weak signals at 24 and 70 μm and obtain upper limits for the ICD emission.

The habitat diversity of the fungus Nectria haematococca MPVI has been shown to be due in part to conditionally dispensable (CD) chromosomes that carry habitat-defining genes. From a biological perspective, the CD chromosomes are analogous to plasmids that possess genes that determine the habitats of plant-associated bacteria. This study establishes that the N. haematococca CD chromosome that contains the genes for Pea Pathogenicity (PEP cluster) also carries genes for the utilization of homoserine, an amino acid found in pea root exudates. Competition studies presented here demonstrate that an isolate that lacks the PEP cluster, but carries a portion of the CD chromosome containing the homoserine utilization (HUT) genes, is more competitive in the pea rhizosphere than an isolate without the CD chromosome. Further competition studies show that both the PDA1 and PDA6 CD chromosomes confer a competitive advantage in the rhizosphere of soybean, whereas only the PDA6 CD chromosome confers a competitive advantage in the rhizospheres of tomato and alfalfa, and only the PDA1 CD chromosome confers a competitive advantage in the rhizosphere of pea. These studies suggest the presence of genes on the PDA6 and PDA1 CD chromosomes that enhance the ability of N. haematococca to expand its habitat and support the idea that fungal CD chromosomes are analogous to host-specifying plasmids in plant-associated bacteria. Transformation, insertional mutagenesis, and bioinformatics were used to identify a cluster of five genes on the PDA1 CD chromosome that was responsible for the HUT phenotype in N. haematococca. One of the genes was found only in N. haematococca, another was a fungal transcription factor, and the other three had homologs involved in the synthesis of the amino acids methionine, threonine, and isoleucine, in which homoserine is an intermediate. Competition experiments that compared isolates with or without the HUT cluster showed that the HUT cluster is responsible for increased competitive ability of HUT+ N. haematococca isolates in the rhizosphere of pea. This study establishes that homoserine utilization can be a rhizosphere competency trait for N. haematococca and, to our knowledge, is the first example of a rhizosphere competency trait identified in a fungus.

Die zweiphasige Stichprobe zur Stratifizierung ist ein effizientes Inventurverfahren, das seine Praxistauglichkeit in verschiedenen Waldinventuren unter Beweis stellen konnte. Dennoch sind weitere Effizienzsteigerungen wünschenswert. In der vorliegenden Arbeit werden verschiedene Ansätze die Effektivität dieses Verfahrens zu steigern separat vorgestellt, in Fallstudien mit Daten der Niedersächsischen Betriebsinventur getestet und diskutiert. Der erste Ansatz (Kapitel 2) beschäftigt sich mit der Anwendung der zweiphasigen Stichprobe zur Stratifizierung in Wiederholungsinventuren. In einem Zusammengesetzten Schätzer werden Daten eines aktuellen mit Simulationsergebnissen des vorhergehenden Inventurdurchgangs kombiniert. Dabei kann der Stichprobenumfang der aktuellen Inventur verringert werden, während die Daten aller Inventurpunkte des vorherigen Durchgangs für Simulationen genutzt werden. Zwar kann ein solcher Schätzer konstruiert werden, jedoch lässt die Fallstudie darauf schließen, dass keine, oder zumindest keine ausreichende, Effizienzsteigerung erzielt werden kann. Erklärt werden kann dies durch die großen Unterschiede zwischen den aktuellen Inventurergebnissen aus den reduzierten Inventuren und den prognostizierten Volumina aus den Simulationen. Eine Erhöhung der Effizienz dieses Verfahrens könnte nur durch Weiterentwicklungen der Waldwachstumsmodelle möglich werden. In Wiederholungsinventuren kann jedoch eine höhere Effizienzsteigerung mit einem dreiphasigen Verfahren erreicht werden, das die zweiphasige Stichprobe mit der zwei\-phasigen Regressionsstichprobe kombiniert (Kapitel 3). Mittelwert- und Varianzschätzer, die auf dem sogenannten infinite population approach in der ersten Phase beruhen, werden präsentiert. Genutzt werden dabei die Korrelationen zwischen den aktuellen Inventurergebnissen und den Wachstumssimulationen auf der Basis des vorherigen Inventurdurchgangs. Statt der Simulationsergebnisse können auch einfach die Ergebnisse des vorherigen Inventurdurchgangs zur Berechnung der Korrelationen genutzt werden. Allerdings führt die Nutzung der Simulationsergebnisse als Regressor in den meisten Fällen zu besseren Ergebnissen. Bei verringertem Stichprobenumfang der Folgeinventur und damit einhergehendem Präzisionsverlust, ist die Effizienz des dreiphasigen Verfahrens höher als die des klassischen zweiphasigen Verfahrens. Die Nutzung der Vorinventur in Form eines stratenweisen Regressionsschätzers hat sich damit als erfolgreich und gegenüber dem zusammengesetzten Schätzer als deutlich überlegen gezeigt. Als weiterer Ansatz wird die Erweiterung der zweisphasigen Stichprobe zur Stratifizierung um eine geclusterte Unterstichprobe zu einem dreiphasigen Design vorgestellt (Kapitel 4). Sowohl für den Ratio-to-Size- als auch für den unverzerrten Ansatz werden entsprechende Mittelwert- und Varianzschätzer präsentiert. Verglichen mit dem zweiphasigen Verfahren, führt dieses dreiphasige Design in der Fallstudie zu keiner Effizienzsteigerung. Gründe hierfür können in der vergleichsweise kleinen Größe der Forstämter und der hohen Stichprobendichte der Niedersächsischen Betriebsinventur gesehen werden. Sinnvolle Anwendungen dieses Verfahrens sind aber möglicherweise unter anderen Erschließungsbedingungen in Großgebieten denkbar. In einer weiteren Fallstudie wird versucht existierende Probepunkte in Clustern von homogener Größe zusammenzufassen (Kapitel 5). Eine solche Zusammenfassung soll der Optimierung der Wegzeiten bei der Aufnahme von Inventurpunkten dienen. Dazu werden sieben verschiedene Methoden getestet und deren Ergebnisse miteinander verglichen. Durch einen Vergleich mit optimierten Richtwert-Lösungen wird zudem die Qualität dieser Lösungen evaluiert. Es zeigt sich, dass drei Algorithmen des Vehicle Routing Problems gut dazu geeignet sind, Cluster von homogener Größe zu erstellen. Nicht empfohlen werden kann dagegen die Verwendung von drei anderen Cluster-Algorithmen, sowie die Nutzung von Bewirtschaftungseinheiten als Cluster, da diese Methoden zu Clustern von sehr heterogener Größe führen.

The overall purpose of this thesis work has been to further our understanding of the growth and removal of inclusions in gas- and induction-stirred ladles. The primary focus has been on alumina inclusions. Growth mechanisms were studied using data from fundamental mathematical models of gas- and induction-stirred ladles. The results showed the turbulence mechanism to be the most dominant in alumina inclusion growth. The dynamic growth and removal of inclusions in a gas-stirred ladle was studied using mathematical modelling. The model results showed concentration gradients of inclusions. The effect was most obvious in the steel flow past the removal sites: top slag, ladle refractory, and gas plume (bubble flotation). A new removal model was developed for large spherical caps bubbles. In order to verify the predicted concentration gradients for the size population of inclusions, three experiments were carried out in production. The sampling equipment enabled sampling at five different positions and different locations at the same time. The results showed that concentration gradients of inclusions do exist both in induction-stirred and gas-stirred ladles. A theoretical analysis showed that the drag force on the inclusions to be the dominating force and that therefore inclusions follow the fluid flow. The cluster behaviour of alumina inclusions were examined on steel samples taken in an industrial-scale deoxidation experiment in a ladle. The samples were examined by microscope and the results used to study cluster growth. It was found that there was rapid cluster growth due to collision during stirring and that at the end of the deoxidation experiment a majority of the small inclusions were bound in clusters. The cluster growth data determined using the microscopic results were compared with predicted cluster-growth data. A method was developed for converting the experimental data observed per unit area into data given per unit volume and vice versa. An expression for the collision diameter of the cluster was also developed. The results showed that the predicted cluster growth agreed well with the microscopic observations for the assumptions made in the growth model.

The data from Super Dual Auroral Radar Network (SuperDARN) radar and Cluster was used to investigate whether the processes observed by SuperDARN radar in the highlatitude ionosphere leads to the processes that take place in the magnetosphere. SuperDARN is a global network of high-frequency coherent-scatter radars operating in the northern and southern hemispheres and these radars are capable of observing backscatter from ionospheric irregularities. The Tsyganenko T96 field model was used to map the footprint of the Cluster spacecraft into the ionosphere. The SuperDARN radar data analysis software was used to generate time series, fi eld of view and convection plots. The data from Syowa South, Syowa East SuperDARN radars, Advance Composition Explorer (ACE) and Cluster spacecrafts was used in this study. Two events were found and discussed, an event on 09 August 2002 and another event on 30 September 2002. The SuperDARN radar and Cluster data for these events were compared. When a change in electric field was observed by Cluster, SuperDARN radar observed a change in the convection flow. There was a time delay between the change in the electric fi eld and the change in convection flow. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2010.

Investing in cluster formation or encouraging companies to network in regional strategic networks is a common strategy used by municipalities to promote regional growth in peripheral regions. Previous research has investigated the significance of creating regional advantages by building clusters and regional networks, but researchers have not provided much insight into the problems facing the project management trying to implement such collaboration. In my thesis I describe and analyze a network project in order to shed light upon some of the complications that such a collaboration project might entail. My theoretical framework of analysis rests upon the concept of social capital, a concept that investigates the value that social contacts might incur. I have studied a designed network situated in the Swedish municipality of Söderhamn called Firsam. After the closure of the telecommunications factory of Ericsson/Emerson and the military airbase F15 Söderhamn lost 10 % of its local employment in 2004.The need for regional growth programmes therefore became dire. The companies that prior to the closure worked in close collaboration with the Ericsson/Emerson factory were also looking for new revenue streams to compensate for their loss of business. Collaboration with the local manufacturing companies to create innovative projects and to take on joint tenders seemed to be a perfect solution to the problems facing them and the municipality. In this spirit a regional strategic network called Firsam (Företag i regional samverkan) was initiated. I analyze the Firsam project using two different aspects of the concept social capital:”bonding” and”bridging”. The bonding form of social capital is associated with small and homogeneous groups that build prerequisites for long-term collaboration by forming close contacts and building trust. The bridging form of social capital creates an open stance towards social relations that enables new contacts to be formed outside one’s own socially established context. The bonding form of social capital provides prerequisites for close collaboration but can also result in close-mindedness and over-embeddedness in one’s own social context. Building bridging connections outside one’s own social context might encourage innovative thinking and spur entrepreneurship. The somewhat fleeting connections that are associated with the bridging form of social capital might on the other hand make it difficult to cultivate a common sense of trust within an existing group. These different manifestations of social capital create a paradox that might be hard to handle in the design of a regional strategic network. Is it best to support already existing network structures and impose the risk of creating a less innovative environment, or should members from outside the established social context be included in the network design to encourage innovative thinking? There are both positive and negative effects associated with either strategy. I shed light upon this paradox by analyzing the regional strategic network of Firsam.

Giant Polyoxometalates : Dynamic Structure and Reactivity. In this thesis different methods were used to study several systems in which giant polyoxometalates are involved. The validity of the theoretical methods applied and the results obtained were always contrasted with the experimental evidence provided by the groups of Prof. Achim Müller (Bielefeld) and Prof. Marcella Bonchio (Padova). In certain cases the theoretical results provided an explanation for experimental observations and in other cases they had allowed prediction as main objective.Chapter IV presents the results regarding encapsulated water molecules inside the giant polyoxometalate cavities. Chapter V presents the results of the studies about ion pairing on polyoxometalates. Chapters VI and VII presents the results of two different polyoxometalates and their catalytic activity. Finally Chapter VIII presents a theoretical study on the effect of counterions on the growth of giant uranium polyperoxometalates. / Polioxometal·lats Gegants: Estructura Dinàmica i Reactivitat.En la present tesi diferents mètodes han estat utilitzats per estudiar diversos sistemes on els polyoxometalats gegants juguen un paper fonamental. La validesa dels mètodes computacionals utilitzats i dels resultats obtinguts han estat sempre contrastats amb les evidencies experimentals facilitades pels grups del Prof. Achim Müller (Bielefeld) i Prof. Marcella Bonchio (Padova). En alguns casos els resultats computacionals han permès explicar les observacions experimentals i en d'altres han permet fer prediccions com a principal objectiu.En el capítol IV es presenten els resultats referents a l'estructura de l'aigua encapsulada en les cavitats presents a polioxometal·lats gegants. El capítol V presenta els resultats dels estudis sobre ion pairing en polioxometal·lats. Els capítols VI i VII presenten els resultats de dos sistemes diferents i la seva activitat com a catalitzadors. Finalment, el capítol VIII presenta un estudi teòric sobre l'efecte dels contraions en el creixement de poliperoxometal·lats gegants d'urani.