73 research outputs found

    Pursuing Parameters for Critical Density Dark Matter Models

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    We present an extensive comparison of models of structure formation with observations, based on linear and quasi-linear theory. We assume a critical matter density, and study both cold dark matter models and cold plus hot dark matter models. We explore a wide range of parameters, by varying the fraction of hot dark matter ΩΜ\Omega_{\nu}, the Hubble parameter hh and the spectral index of density perturbations nn, and allowing for the possibility of gravitational waves from inflation influencing large-angle microwave background anisotropies. New calculations are made of the transfer functions describing the linear power spectrum, with special emphasis on improving the accuracy on short scales where there are strong constraints. For assessing early object formation, the transfer functions are explicitly evaluated at the appropriate redshift. The observations considered are the four-year {\it COBE} observations of microwave background anisotropies, peculiar velocity flows, the galaxy correlation function, and the abundances of galaxy clusters, quasars and damped Lyman alpha systems. Each observation is interpreted in terms of the power spectrum filtered by a top-hat window function. We find that there remains a viable region of parameter space for critical-density models when all the dark matter is cold, though hh must be less than 0.5 before any fit is found and nn significantly below unity is preferred. Once a hot dark matter component is invoked, a wide parameter space is acceptable, including n≃1n\simeq 1. The allowed region is characterized by \Omega_\nu \la 0.35 and 0.60 \la n \la 1.25, at 95 per cent confidence on at least one piece of data. There is no useful lower bound on hh, and for curious combinations of the other parameters it is possible to fit the data with hh as high as 0.65.Comment: 19 pages LaTeX file (uses mn.sty). Figures *not* included due to length. We strongly recommend obtaining the full paper, either by WWW at http://star-www.maps.susx.ac.uk/papers/lsstru_papers.html (UK) or http://www.bartol.udel.edu/~bob/papers (US), or by e-mailing ARL. Final version, to appear MNRAS. Main revision is update to four-year COBE data. Miscellaneous other changes and reference updates. No significant changes to principal conclusion

    The Ontology of the Venetian Halo in its Italian Context

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    This thesis aims to reposition the halo’s status within an artwork through arguing a reassessment of its activity 'as a sign' rather than acceptance of its passivity. This active state is further explored and expanded by a heuristic application of semiotic theory to interrogate its fluctuation between sign/non-sign and its oscillation between a seemingly real status and behaviour juxtaposed with its very consciously artificial “manifestation”. A variety of halo shapes are considered, together with texture contained in and on its surface, and this has revealed the Venetian and Venetan artistic innovation of “glass” and “silk” haloes, through artists’ utilisation of contemporaneous industrial practices and their application to halo appearance. Additionally, extant architectural vocabulary is translated and reformulated into internal halo motifs by Venetian and Venetan artists, further enhancing the halo’s somatic characteristics, contextualized by examination of halo representation in various media in Florence, Rome and Siena, and a consideration of haloes within other, mainly Italian, centres. Additionally, the fugitive and transient qualities of the nimbus are noted, with its mimesis of the dying corporeal body in its fading insubstantiality, a further factor in its inexorably reductive form as increasing realism in art challenges its ontological traits. Textual characters contained within the halo body are also examined in their many forms and languages and their contribution to an intertextual function espoused by the ideologeme. An adjunct to this function is the halo’s propagandist role presented by artists. It will be demonstrated how all these different strands of interpretation are imbricated in the changing theological, political and societal landscape, encapsulated within the halo

    On the evolution of star forming galaxies : the metallicity of dwarfs and the effect of environment on local luminous IR galaxies

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    This thesis, entitled "On evolution of star forming galaxies: the metallicity of dwarfs and the effect of environment on local luminous infrared galaxies" is divided into two main areas. First a detailed study of the environment and star formation relationship of local Luminous Infrared Galaxies (LIRGs), with infrared (IR) luminosity (LIR) between 1011L and 1011L, where L is solar luminosity was carried out. Secondly, a chemical abundance analysis was done on three Local Volume dwarf irregular galaxies. In the first part, various redshift surveys were used to quantify the environment around LIRGs. It was found that LIR 1011L is a remarkable luminosity point among IR galaxies

    Patronage, Reputation and Common Agency Contracting in the Scientific Revolution: From Keeping 'Nature's Secrets' to the Institutionalization of 'Open Science'

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    This essay examines the economics of patronage in the production of knowledge and its influence upon the historical formation of key elements in the ethos and organizational structure of publicly funded open science. The emergence during the late sixteenth and early seventeenth centuries of the idea and practice of “open science" was a distinctive and vital organizational aspect of the Scientific Revolution. It represented a break from the previously dominant ethos of secrecy in the pursuit of Nature’s Secrets, to a new set of norms, incentives, and organizational structures that reinforced scientific researchers' commitments to rapid disclosure of new knowledge. The rise of “cooperative rivalries” in the revelation of new knowledge, is seen as a functional response to heightened asymmetric information problems posed for the Renaissance system of court-patronage of the arts and sciences; pre-existing informational asymmetries had been exacerbated by the claims of mathematicians and the increasing practical reliance upon new mathematical techniques in a variety of “contexts of application.” Reputational competition among Europe’s noble patrons motivated much of their efforts to attract to their courts the most prestigious natural philosophers, was no less crucial in the workings of that system than was the concern among their would-be clients to raise their peer-based reputational status. In late Renaissance Europe, the feudal legacy of fragmented political authority had resulted in relations between noble patrons and their savant-clients that resembled the situation modern economists describe as "common agency contracting in substitutes" -- competition among incompletely informed principals for the dedicated services of multiple agents. These conditions tended to result in more favorable contract terms (especially with regard to autonomy and financial support) for the agent-client members of the nascent scientific communities. This left the new scientists better positioned to retain larger information rents on their specialized knowledge, which in turn tended to encourage entry into the emerging disciplines. They also were thereby enabled collectively to develop a stronger degree of professional autonomy for their programs of inquiry within increasingly specialized and formal scientific academies which, during the latter seventeenth century, attracted the patronage of rival absolutist States in Western Europe.open science, new economics of science, economics of institutions, patronage, asymmetric information, principal-agent problems, common agency contracting, social networks, 'invisible colleges', scientific academies

    Geologic, microstructural, and spectroscopic constraints on the origin and history of carbonado diamond

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    Carbonado is a form of polycrystalline diamond found in placer deposits in South America and Central Africa, and is one of the toughest known materials. The source rock for carbonado is unknown, and it has unusual porosity, textural features, and inclusion mineralogy. These have lead to a wide variety of theories on the genesis of carbonado. The tightly bound, interlocking microtexture of diamond makes it difficult to study, and only one previous study has been done on polished interior sections of a carbonado. This thesis reports the results from studying the polished surfaces of 21 carbonados from Brazil and the Central African Republic. Reflected light and scanning electron microscopy, cathodoluminescence (CL), Photoluminescence, Raman spectrometry, and secondary ion mass spectrometry were performed on these carbonado samples in order to determine their microtexture and evaluate the various theories of carbonado genesis. In addition, carbonado pore minerals and indicator minerals from the Brazilian rivers in which carbonado is found were studied in an attempt to gain some insight into the possible source rock for carbonado. Some of the individual diamond microcrystals in carbonado were found to have morphological and chemical similarities to the monocrystalline microdiamonds found in the Dachine talc schist of French Guiana. Diamonds and chromites from the Dachine talc schist were studied to determine the protolith of the talc schist, and to constrain the residence history of the Dachine microdiamonds in the mantle. Studies of florencite, a common pore mineral in carbonado, show that the Pb that substitutes into the REE site in the florencite crystal lattice is modern, common lead. When combined with previous geochronological studies that show at carbonado has been associated with uranium for at least 2.5 Ga, this modern common lead shows that the pores have been open to exchange with the exterior environment. Raman and CL studies show that the radiation damage previously documented in carbonado is concentrated in the areas around the pores, suggesting that they were filled with a high concentration of uranium. One carbonado was found to host a metallic Fe-Cr inclusions in its pores. This alloy is of a type previously reported only as intracrystalline inclusions. These results have been interpreted as recording a three step history for the pore mineralogy of carbonado. First, carbonado crystallized in the diamond stability field, in equilibrium with reduced metallic phases. After transport to the surface and release from the host rock, U-bearing groundwater dissolved the pore minerals and precipitated uranium in a redox reaction. Finally, recent tropical weathering reoxidized the uranium, leaving recent lateritic minerals in the pores. Because the Pb isotope model ages for carbonado (2.8-3.6 Ga) are older than most other diamonds and much of the craton in which carbonado is found, a detrital zircon study was performed on carbonado-bearing streams to see if any rocks of this age or older were present in the paleo-drainage basin of the conglomerates that contain carbonado. The detrital zircons found in carbonado-bearing streams had ages between 3.7 and 2.1 Ga The clasts that local garimpeiros (prospectors) and sedimentologists believe are related to carbonado had ages between 3.7 and 3.35 Ga. This age distribution is similar to that of detrital zircons found in green Jacobina quartzites, which were found to have the same range of ages, plus a large concentration of 3.30 Ga grains not present in the sediments associated with carbonado. The only possible indicator minerals found were two Cr-rich rutiles, which may originate from metasomatized mantle. One of these Cr-rutiles was tentatively dated using the U-Pb system as having an age of 2933 Ma. This age corresponds with a time of tectonic quiescence in the drainage area of the carbonado source conglomerates. Optical, CL and Raman spectroscopic studies of polished carbonados show that they consist of either a collection of discrete euhedral or anhedral diamond microcrystals, or of a homogenous mix of irregular shaped grains. The ratio of these two textural types varies between carbonado grains, but is generally constant within each individual carbonado. Grain boundaries are generally not straight, and rarely terminate in symmetrical triple junctions. Raman spectroscopy shows that the level of elastic strain and compression or tension of the diamond crystal lattice is much lower than that of diamonds formed through shock synthesis, precipitated in chemical vapor deposition, or recovered from ureilite meteorites. This elastic strain levels in carbonado are similar to those in lithospheric diamonds or synthetic diamonds synthesized at static high pressures. This suggests that carbonados were in the diamond stability field at moderately high temperatures. SIMS measurements of carbonado using the SHRIMP ll ion probe show that the individual crystals in carbonados have slightly different carbon isotopic compositions and nitrogen concentrations. There are two hypotheses that can account for the features observed in carbonado. The first is a two stage process, whereby the euhedral grains grew first, and the matrix diamond rapidly crystallized at a later date. The second is a deformation process, whereby microdiamonds were concentrated and deformed to varying degrees, resulting in the variable ratios of euhedral diamond to matrix diamond in different carbonado stones. Because the undeformed euhedral diamonds were found to be morphologically and chemically similar to the diamonds in the Dachine talc schist in French Guiana, this primary diamondiferous rock was studied in an attempt to determine how such diamond can form. The diamonds in the Dachine talc schist were found to be type IaA-Ib. The lack of total nitrogen aggregation means that they can not have been resident in the mantle for more than 10 million years. The low aggregation state also constrains the temperature of the magma that erupted them to less than about 1500 °C, as the nitrogen in the Dachine diamond would have aggregated during transport if the magmas were any hotter. SHRIMP carbon isotopic measurements show that these diamonds have a range in carbon isotopic composition from typical mantle values down to typical biogenic values. All Dachine diamonds with detectable nitrogen have identical thermal histories, irrespective of carbon isotopic composition. The chromites on the Dachine talc schist were a mix of metasomatized lithospheric mantle chromites typical of kimberlites, and igneous chromites. The igneous chromites had trace elemental compositions less depleted than those found in boninites and komatiites, and were similar to those found in high-Mg shoshonitic intrusive rocks. This, combined with relict volcaniclastic textures, a geologic setting that is interpreted as an early Proterozoic arc, and the low temperatures required by the diamonds, suggest that the Dachine talc schist may have originally been a hydrous, arc-related volcanic rock. Such an interpretation would allow the low 813C of the Dachine diamonds to be caused by the subduction of organic carbon. There is still much research that must be done before carbonado diamond is well understood. However, this theses presents several new and important constraints. The first result is that the radiation damage in carbonado was generated by the deposition of uranium in the pores, and that both this uranium and the original pore minerals have since been replaced by recent lateritic minerals related to tropical weathering. The other important result is that the diamond lattice in carbonado grains is under very little residual stress, so that whatever process that formed the carbonado microstructures must have occurred in the diamond stability field, and not as a result of metastable diamond growth

    Massive galaxies at 1 < z < 3

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    This thesis explores the evolution of massive galaxies (M * > 1011M ʘ) by conducting the largest multiple-component Sersic light-profile fitting study to date of the rest-frame optical and ultra-violet morphologies of galaxies at redshifts 1 < z < 3. Despite many of the recent advances in galaxy formation and evolution models, the physical processes which are responsible for driving morphological transformations and star-formation quenching remain unclear. By undertaking a detailed study of the individual bulge and disk components of these massive systems, the work presented in this thesis addresses these outstanding issues by exploring not only how the sizes of the individual components evolve with redshift, but also how the overall bulge and disk fractions evolve, and how these trends are connected to star-formation quenching of the separate components. In order to perform this analysis, I have combined the latest high-resolution near-infrared HST WFC3/IR and ACS imaging provided by the CANDELS survey in the UDS and COSMOS fields and have presented a robust procedure for morphological multiple-component Sersic light-pro le model fitting across the 0:6ÎŒ m to 1:6ÎŒ m wavelength range sampled by CANDELS. This procedure is discussed in depth along with the tests I have undertaken to assess its reliability and accuracy. This approach has enabled me to generate separate bulge and disk component model photometry, allowing me to conduct individual component SED fitting in order to determine decomposed stellar-mass and star-formation rate estimates for the separate bulge and disk components. The results presented in this work reveal that the sizes of the bulge and disk components lie both on and below the local size-mass relations, confirming that the size evolution required by the previously reported compact sizes of high-redshift galaxies extends to both galaxy components. However, I find evidence that the bulge components display a stronger size evolution with redshift than the disks as, at 1 < z < 3, the bulges are a median factor of 3:09 ± 0:2 times smaller than similarly massive local early-type galaxies, whereas the disks are a median factor of 1:77 ± 0:1 times smaller than similarly massive local late-type galaxies. By including decomposed star-formation rates for the individual bulge and disk components, this work also reveals that while the growth of individual components through, for example, inside-out processes such as minor merging, are consistent with the size evolution of these systems, the addition of larger newly quenched systems to the galaxy population, for the disk components at least, may also play an important role in the observed size evolution of massive galaxies. By exploring the evolution of the bulge and disk-dominated fractions with redshift, I find that 1 < z < 3 marks a key transition era in cosmic time where these most massive galaxies appear to be undergoing dramatic structural transformations. Within this redshift range there is a decline in the population of disk-dominated galaxies and a gradual emergence of increasingly bulge-dominated systems. However, despite the rise of S0-type galaxies, even by z = 1 I do not yet find a significant fraction of "pure" bulges comparable to the giant ellipticals which comprise the majority of the local massive galaxy population. In addition to studying how the overall bulge and disk dominated fractions evolve with redshift, by incorporating the star-formation rate and stellar-mass estimates for the separate components and imposing new, highly conservative criteria, I con firm that a significant fraction of passive galaxies are disk-dominated (18± 5%) and a significant fraction of star-forming galaxies are bulge-dominated (11 ±4%). The presence of passive disks and star-forming bulges has interesting implications for the models of galaxy evolution as they suggest that the processes which quench star-formation may be distinct from the mechanisms which cause morphological transformations. Finally, the detailed morphological analysis presented in this work has also allowed me to explore the axial ratio distributions of these most massive high-redshift galaxies, which provides additional insight into the structure of the passive and star-forming bulge and disk-dominated sub-populations. Whilst the overall axial ratio distributions for star-forming disks are peaked, I find tentative evidence that the largest and most active star-forming disks are flatter. I have also been able to further demonstrate that by selecting the most active star-forming disks and comparing to extreme star-forming (sub-)mm selected galaxies, the axial ratio distributions of the two samples appear to be comparably flat, thus reconciling the observed structures of these populations

    Morphologie de sprites et conditions de productions de sprites et de jets dans les systÚmes orageux de méso-échelle

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    Ce document décrit l'analyse des conditions de production de phénomÚnes lumineux transitoires dans la mésosphÚre, produits en réponse à des décharges électriques énergétiques orageuses localisées au-dessous. Pendant les campagnes d'observation EuroSprite, quelques centaines d'images de sprites ont été obtenues, fournissant des informations sur la morphologie, la localisation et le moment de leur production. Des données issues de radars météorologiques, de satellite météosat, de deux types de systÚme de détection d'éclairs, et de récepteur radio large bande ont été analysées. Des études de cas et une étude statistique sur un grand nombre de cas de sprites produits par 7 orages distincts sont réalisées. L'analyse porte sur le rÎle de la composante intranuage des éclairs nuage-sol positifs à l'origine des sprites et notamment le lien avec leur morphologie, sur la relation avec le stade d'évolution des orages, et enfin sur les conditions associées à la production d'un jet géant aux Etats-Unis. Les sprites observés ont été produits par des systÚmes convectifs de moyenne échelle (MCS) lorsque la partie stratiforme était en phase d'expansion. Les séquences des éclairs nuage-sol et l'activité intranuage observées au moment des sprites confirment une propagation horizontale importante (convective-vers-stratiforme). Les sprites de type colonne sont produits avec des délais plus courts que les sprites de type carotte. Plus le délai est court plus le nombre d'éléments est grand et plus leur luminosité est concentrée à une altitude élevée. Le jet géant semble avoir été favorisé par la configuration de charge et l'activité d'éclairs plutÎt que l'altitude du sommet du nuage.This dissertation is devoted to the description of the conditions of production of transient luminous phenomena (sprites, jets, elves) in the mesosphere, which occur in response to energetic lightning discharges in thunderstorms underneath. During EuroSprite observation campaigns, a few hundred images of sprites have been obtained, providing information about event morphology, location and timing. Precipitation data from weather radar and cloud top altitude from Meteosat, as well as two lightning detection networks and a wide-band radio receiver have been analyzed. The methodology includes case studies and a statistical study over a large number of sprites produced by 7 different storms. The work focuses on the aspect of the intracloud lightning component associated with positive cloud-to-ground flashes, the link with the morphology of sprites, and the life cycle of thunderstorm systems. Additionally, a storm which produced a rare gigantic jet observed in the United States is analyzed in detail. The observed sprites were produced by mesoscale convective systems (MCS) during the expanding phase of the stratiform region. The cloud-to-ground flash sequences and the intracloud lightning component observed at the time of sprites confirm a large horizontal convective-to-stratiform propagation, as mechanism of charge collection, explaining displaced sprites. Sprites of column-type are produced with shorter delays than carrot sprites, and the shorter the delay, the more elements, their luminosity concentrating at greater altitudes. The gigantic jet appears to have been promoted by a certain charge configuration and lightning activity pattern, rather than a high cloud top altitude

    Modes of engagement with astrology in seventeenth-century England

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    Astrology played an important part in the propaganda wars which accompanied the mid-seventeenth-century English Civil Wars, and it remained both influential and controversial in the decades following. At present, the dissemination of astrological ideas in seventeenth-century English publications is better understood than their audience's reception of those ideas. The discrepancy is due partly to the fact that the former is better documented than the latter. It is exacerbated by the controversy surrounding seventeenth-century English astrology, as a result of which much of the commentary on its public reception has likely been skewed to fit the commentator's argument. This thesis will investigate some of the letters which well-known seventeenth-century astrologers received from correspondents with varying levels of interest and expertise in astrology. In context, the letters present an opportunity to examine the flow of communication into the core astrological community from those outside or on the periphery
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