73 research outputs found
Pursuing Parameters for Critical Density Dark Matter Models
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 , the Hubble parameter and the spectral
index of density perturbations , 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 must be less than 0.5 before any fit is found and
significantly below unity is preferred. Once a hot dark matter component is
invoked, a wide parameter space is acceptable, including . 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 , and for curious combinations of the other parameters
it is possible to fit the data with 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
This thesis aims to reposition the haloâs 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 âmanifestationâ.
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 âglassâ and âsilkâ haloes, through artistsâ 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 haloâs 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 haloâs 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
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'
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 âopen 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 Natureâs 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 âcooperative rivalriesâ 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 âcontexts of application.â Reputational competition among Europeâs 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
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
This thesis explores the evolution of massive galaxies (M * > 1011M Ê) 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
0:6Ό m to 1:6Ό 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
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
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Appendage development and early Distal-less regulation in arthropods: A study of the chelicerate Tetranychus urticae (Acarida)
A major goal of evolutionary developmental biology is to explore mechanisms and events underlying evolution of the myriad body plan morphologies expressed both genetically and phenotypically within the animal kingdom. Arthropods exhibit an astounding array of morphological diversity both within and between representative sub-phyla, thus providing an ideal phylum through which to address questions of body plan innovation and diversification. Major arthropod groups are recognised and defined by the distinct form and number of articulated appendages present along the antero-posterior axis of their segmented bodies.
A great deal is known about the developmental genetics of limb development in the model insect Drosophila melanogaster, added to which, much comparative gene expression data and a growing body of functional genetic data is emerging for other arthropod species. Arthropod limb primordia are consistently marked by expression of the homeobox gene Distal-less (Dll), and the focus of this thesis is to compare signalling mediated by early Dll regulatory genes activity along antero-posterior and dorso-ventral embryonic axes during limb specification in Drosophila, with the activity of their orthologs in the widely disparate chelicerate, the spider mite Tetranychus urticae â interpreting new data with that available for other arthropods.
Having made a detailed study of spider mite embryonic (and post-embryonic) development, to provide a basis for understanding mRNA transcription and protein activity patterns, I confirmed typical expression of Tetranychus Dll in prosomal limb primordia. I obtained limited results for the candidate antero-posterior positioning genes wingless and engrailed, although one of the two engrailed paralogs I identified is reportedly expressed in posterior segmental compartments, consistent with possible conservation of Engrailed-Wingless interactions in metameric patterning and positive regulation of Dll in arthropod limb specification. In Drosophila, wingless-dependent Dll transcription is restricted along the dorso-ventral axis by dorsal Dpp-mediated and ventral EGFR-mediated signalling gradients. Based on data from Tetranychus and other arthropods, neither dorsal nor ventral signalling regimes appear conserved outside the Drosophila system. Dll suppression in fly abdominal segments occurs due to powerful Hox (Ubx/AbdA) repression of the early Dll cis-regulatory element; this is discussed in relation to the independently evolved limbless chelicerate opisthosoma, informed by hypothetical scenarios of cis (regulatory DNA) and trans (coding sequence) evolution.
Given practical difficulties and limitations encountered while working with spider mites, I offer a final assessment of the place of Tetranychus urticae as a non-model, and yet still valuable chelicerate species to consider carrying into the exciting future of evolutionary developmental biology.This doctoral research work was supported by the Biotechnology and Biological Sciences Research Council, the Newton Trust and Corpus Christi College Cambridge
Modes of engagement with astrology in seventeenth-century England
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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