44 research outputs found
Hydrodynamic Simulations of Cosmological Galaxy Merger Trees
No full textIn this thesis we use cosmological merger trees and semi-analytic models of galaxy formation to provide the initial conditions for multi-merger hydrodynamic simulations. In this way we exploit the advantages of merger simulations (high resolution and detailed treatment of the gas physics) and semi-analytic models (cosmological background and low computational cost), and integrate them to create a novel tool. This approach allows us to study the evolution of various galaxy properties with an improved treatment of the gas components, including, for the first time, the hot gaseous halo from which gas cools and accretes onto the central disc. Using a suite of minor merger simulations we find that disc thickening is reduced relative to the collisionless case through the absorption of kinetic impact energy by the gas. In a following series of major merger simulations, we show that adding the hot gas component is a key ingredient in order to reproduce several observed properties of elliptical galaxies, like the abundance of fast rotators. Moreover, the presence of a gaseous halo reduces the starburst efficiency. We then focus on the effects of multiple concurrent mergers, which we found to be cosmologically more common than sequences of isolated binary mergers. For this, we investigate, whether accreted satellite stars can be distinguished kinematically from stars formed 'in situ' in the central galaxy, and find that this is only possible for a fraction of the disrupted satellites. Our simulations to date indicate that the combination of a detailed treatment of gas physics, high resolution, and a cosmological background, brings numerical simulations in better agreement with observations. Overall, the novel tool developed in this thesis will be very useful for pursuing a number of questions pertaining to the transformation of galaxy internal structure by mergers and accretion
Hydropolitics and Conflict Management in Transboundary River Basins
No full textThis thesis set out to develop methodologies that promote cooperation, peace and development instead of conflict and violence in transboundary water resources management. In particular, its objectives were the following: o To examine and understand the complexity of water systems and water conflict management o To investigate factors affecting resilience of water treaties o To develop methods for equitable water allocation in transboundary river basins using bankruptcy theory o To develop a risk-based Multi Attribute Decision Making method (RB-MADM) to select, rank and/or classify alternatives among a set of available options in negotiation and decision making processes over shared natural resources o To demonstrate the applicability of the developed methods in practice through some real cases: the Euphrates and Tigris Rivers and the Caspian Sea. This thesis starts with an in-depth discussion of the complexity of transboundary water systems and the nexus between resource scarcity and resource conflict. Water systems are complex coupled human-natural systems. This complexity can be exacerbated when water is shared between two or more riparian countries. Shared water resources can be a source of both conflict and cooperation among riparian countries. Conflict resolution in a complex system is also a complex task that cannot be managed by means of scientific methods or modelling approaches only. This thesis focuses on conflict resolution in transboundary river basins. Rather than looking at the issues just from a technical point of view, the research endeavours to recognize the complexity in conflict resolution in transboundary river basins. The nexus between resource scarcity and resource conflict is analyzed in terms of several schools of thought, including the Neo-Malthusians, Cornucopians, and the Distributionists. Furthermore, the missing link between water scarcity, as a main issue in international water resources management, and water crisis and conflict is examined. The evidence suggests that water scarcity can be both a cause and the result of conflict. Water systems have been affected by various human and natural perturbations and changes, such as global environmental and climate changes that are at least partly humanly induced. These disturbances can potentially affect conflict and cooperation between riparian countries. Water treaties should be resilient to cope with these changes and disturbances. Chapter three raises questions concerning main factors affecting non-resilience of water treaties as well as some reasons increasing the difficulty of implementation of treaties. Additional insights into these factors can help to enhance their resilience and reduce the risk of conflict among riparian countries. Among several factors affecting hydropolitical relationships, water quantity is the perhaps most conflictive issue. The principle of “equitable and reasonable utilization” is the main normative principle referred to for allocating internationally shared water resources. Despite this, a serious limitation of international water laws and conventions is that they have not introduced a generally accepted allocation mechanism to achieve an equitable and reasonable water allocation. Therefore, equitable water allocation among riparian countries is one of the main critical issues in water conflict management. Using bankruptcy theory, as a form of cooperative game theory method, novel flexible allocation methods have been developed in chapters four and five. The proposed rules have been applied to the Euphrates and Tigris Rivers, shared between Turkey, Syria, and Iraq, to illustrate the application of the rule to a real conflictive allocation problem. The results reveal that the proposed approaches may facilitate negotiation and have a great potential to help solving conflict and dispute over river resources allocation problems in transboundary river basins. A risk-based Multi Attribute Decision Making method was developed to support consensus-based decision making over several allocation mechanisms. This method was applied to help establish the legal status of the Caspian Sea, which is shared among five states: Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan. To date, these nations have been unable to reach an allocation agreement. We apply a methodology to propose the most appropriate solution among a set of options under different risk attitudes of the states. The analyses show that the overall ranking of options varies even when the states slightly change their risk attitudes from pessimistic to fairly pessimistic.Water ManagementCivil Engineering and Geoscience
Recoiling supermassive black hole escape velocities from dark matter haloes
No full textInternational audienceWe simulate recoiling black hole trajectories from z = 20 to z = 0 in dark matter haloes, quantifying how parameter choices affect escape velocities. These choices include the strength of dynamical friction, the presence of stars and gas, the accelerating expansion of the Universe (Hubble acceleration), host halo accretion and motion, and seed black hole mass. Lambda cold dark matter halo accretion increases escape velocities by up to 0.6 dex and significantly shortens return time-scales compared to non-accreting cases. Other parameters change orbit damping rates but have subdominant effects on escape velocities; dynamical friction is weak at halo escape velocities, even for extreme parameter values. We present formulae for black hole escape velocities as a function of host halo mass and redshift. Finally, we discuss how these findings affect black hole mass assembly as well as minimum stellar and halo masses necessary to retain supermassive black holes
From discs to bulges: Effect of mergers on the morphology of galaxies
No full textWe study the effect of mergers on the morphology of galaxies by means of the simulated merger tree approach first proposed by Moster et al. This method combines N-body cosmological simulations and semi-analytic techniques to extract realistic initial conditions for galaxy mergers. These are then evolved using high-resolution hydrodynamical simulations, which include dark matter, stars, cold gas in the disc and hot gas in the halo. We show that the satellite mass accretion is not as effective as previously thought, as there is substantial stellar stripping before the final merger. The fraction of stellar disc mass transferred to the bulge is quite low, even in the case of a major merger, mainly due to the dispersion of part of the stellar disc mass into the halo. We confirm the findings of Hopkins et al., that a gas-rich disc is able to survive major mergers more efficiently. The enhanced star formation associated with the merger is not localized to the bulge of galaxy, but a substantial fraction takes place in the disc too. The inclusion of the hot gas reservoir in the galaxy model contributes to reducing the efficiency of bulge formation. Overall, our findings suggest that mergers are not as efficient as previously thought in transforming discs into bulges. This possibly alleviates some of the tensions between observations of bulgeless galaxies and the hierarchical scenario for structure formation
Un descubrimiento fortuito: notas de Christian David Ginsburg y dos códices históricos
No full textThe author purchased a copy of Christian David Ginsburg’s The Massorah (1880-1905) and it contained thousands of mysterious handwritten numbers. It was determined by a handwriting analysis that these notes belonged to Ginsburg himself, and that he was analyzing the Masoretic material of two privately-owned codices. The first, MS Holkham 2, has been attributed to the scribe Yisrael Hasofer from Toledo, ca. 1200-1250. The second, the Schottenstein Spanish Bible, is of Sephardic origin from ca. 1300s. Both of these codices reference the now-lost Masoretic Bibles known as the Muggah and Hilleli codices.El autor adquirió una copia de The Massorah de Christian David Ginsburg (1880–1905) y esta tenía miles de números misteriosos escritos a mano. A través de un análisis de la letra se pudo determinar que estos apuntes eran del propio Ginsburg y que este había estado analizando el material masorético de dos códices en colecciones privadas. Primeramente, MS Holkham 2 ha sido atribuido al escribano Yisrael Hasofer de Toledo, ca. 1200–1250. El segundo, la Biblia Schottenstein española, es de origen sefardí de ca. 1300. Ambos códices hacen referencia a las Biblias masoréticas conocidas como los códices de Muggah y Halleli (Translator: Luis A. Guzmán Valerio)
Dalfampridine in Nonarteritic Anterior Ischemic Optic Neuropathy (NAION)
No full textDalfampridine improves walking speed in MS patients. There is some evidence of visual improvement in MS and improved motor function in early stroke studies. We hypothesized that patients with chronic stable deficits after NAION will have improved visual function with the administration of dalfampridine. Therefore, we performed a pilot clinical trial to determine whether this was true
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Clustering constraints on the relative sizes of central and satellite galaxies
No full textWe empirically constrain how galaxy size relates to halo virial radius using new measurements of the size- and stellar mass-dependent clustering of galaxies in the Sloan Digital Sky Survey. We find that small galaxies cluster much more strongly than large galaxies of the same stellar mass. The magnitude of this clustering difference increases on small scales, and decreases with increasing stellar mass. Using forward-modelling techniques implemented in Halotools, we test an empirical model in which present-day galaxy size is proportional to the size of the virial radius at the time the halo reached its maximum mass. This simple model reproduces the observed size dependence of galaxy clustering in striking detail. The success of this model provides strong support for the conclusion that satellite galaxies have smaller sizes relative to central galaxies of the same halo mass. Our findings indicate that satellite size is set prior to the time of infall, and that a remarkably simple, linear size–virial radius relation emerges from the complex physics regulating galaxy size. We make quantitative predictions for future measurements of galaxy–galaxy lensing, including dependence upon size, scale, and stellar mass, and provide a scaling relation of the ratio of mean sizes of satellites and central galaxies as a function of their halo mass that can be used to calibrate hydrodynamical simulations and semi-analytic models
Fine aggregate resources in the greater Cape Town area
No full textIncludes bibliographical references.Fine aggregate resources in the Greater Cape Town area have historically been in abundant supply. This supply has been typically comprised of the extensive natural building sand deposits that are scattered throughout the region. However, over the past decade increasing concerns have been raised regarding their long-term availability. This concern has arisen from reports suggesting the diminishing reserves of naturally occurring fine aggregates, which have historically dominated the market, and are beginning to near the end of their reserve life. The research undertaken in this thesis was to assess the plausibility of this concern and ascertain an understanding as to the future supply of resources in the region. In order to address the issues raised, a comprehensive understanding of fine aggregates, their properties and their source are required. This was developed in a literature review, whereby this knowledge provided an understanding as to the type of material that is preferable for fine aggregate use. The findings highlights the importance of using a resource that possesses properties, which are most appropriate for its specific use. In terms of concrete, this is achieved through using materials which possess the following preferred properties: (1) round particle shape and smooth surface texture, (2) a wide range of particle sizes that are well distributed, and (3) a mineralogical composition that is relatively inert, thus is not deleterious in concrete. Additionally, a resource must be economically viable, and be exploited in a manner such that the development aims to safeguard the environment for the benefit of current and future generations. These concepts and principles were then applied to the subsequent study into the fine aggregate resources located within the greater Cape Town region
