• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 407
  • 75
  • 63
  • 43
  • 31
  • 15
  • 8
  • 6
  • 5
  • 5
  • 4
  • 4
  • 3
  • 3
  • 3
  • Tagged with
  • 791
  • 98
  • 89
  • 82
  • 82
  • 78
  • 71
  • 65
  • 48
  • 44
  • 43
  • 42
  • 38
  • 37
  • 37
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
151

ALMA observations of atomic carbon in z ∼ 4 dusty star-forming galaxies

Bothwell, M. S., Aguirre, J. E., Aravena, M., Bethermin, M., Bisbas, T. G., Chapman, S. C., De Breuck, C., Gonzalez, A. H., Greve, T. R., Hezaveh, Y., Ma, J., Malkan, M., Marrone, D. P., Murphy, E. J., Spilker, J. S., Strandet, M., Vieira, J. D., Weiß, A. 21 April 2017 (has links)
We present Atacama Large Millimeter Array [C-I](1 - 0) (rest frequency 492 GHz) observations for a sample of 13 strongly lensed dusty star-forming galaxies (DSFGs) originally discovered at 1.4 mm in a blank-field survey by the South Pole Telescope (SPT). We compare these new data with available [C-I] observations from the literature, allowing a study of the interstellar medium (ISM) properties of similar to 30 extreme DSFGs spanning a redshift range 2 < z < 5. Using the [C-I] line as a tracer of the molecular ISM, we find a mean molecular gas mass for SPT-DSFGs of 6.6 x 10(10) M-circle dot. This is in tension with gas masses derived via low-J (CO)-C-12 and dust masses; bringing the estimates into accordance requires either (a) an elevated CO-to-H-2 conversion factor for our sample of alpha(CO) similar to 2.5 and a gas-to-dust ratio similar to 200, or (b) an high carbon abundance X-CI similar to 7 x 10(-5). Using observations of a range of additional atomic and molecular lines (including [C-I], [C-II] and multiple transitions of CO), we use a modern photodissociation region code (3(D)-PDR) to assess the physical conditions (including the density, UV radiation field strength and gas temperature) within the ISM of the DSFGs in our sample. We find that the ISM within our DSFGs is characterized by dense gas permeated by strong UV fields. We note that previous efforts to characterize photodissociation region regions in DSFGs may have significantly under-estimated the density of the ISM. Combined, our analysis suggests that the ISM of extreme dusty starbursts at high redshift consists of dense, carbon- rich gas not directly comparable to the ISM of starbursts in the local Universe.
152

Supernovae seen through gravitational telescopes

Petrushevska, Tanja January 2017 (has links)
Galaxies, and clusters of galaxies, can act as gravitational lenses and magnify the light of objects behind them. The effect enables observations of very distant supernovae, that otherwise would be too faint to be detected by existing telescopes, and allows studies of the frequency and properties of these rare phenomena when the universe was young. Under the right circumstances, multiple images of the lensed supernovae can be observed, and due to the variable nature of the objects, the difference between the arrival times of the images can be measured. Since the images have taken different paths through space before reaching us, the time-differences are sensitive to the expansion rate of the universe. One class of supernovae, Type Ia, are of particular interest to detect. Their well known brightness can be used to determine the magnification, which can be used to understand the lensing systems. In this thesis, galaxy clusters are used as gravitational telescopes to search for lensed supernovae at high redshift. Ground-based, near-infrared and optical search campaigns are described of the massive clusters Abell 1689 and 370, which are among the most powerful gravitational telescopes known. The search resulted in the discovery of five photometrically classified, core-collapse supernovae at redshifts of 0.671&lt;z&lt;1.703 with significant magnification from the cluster. Owing to the power of the lensing cluster, the volumetric core-collapse supernova rates for 0.4 ≤ z &lt; 2.9 were calculated, and found to be in good agreement with previous estimates and predictions from cosmic star formation history. During the survey, two Type Ia supernovae in A1689 cluster members were also discovered, which allowed the Type Ia explosion rate in galaxy clusters to be estimated. Furthermore, the expectations of finding lensed supernovae at high redshift in simulated search campaigns that can be conducted with upcoming ground- and space-based telescopes, are discussed. Magnification from a galaxy lens also allows for detailed studies of the supernova properties at high redshift that otherwise would not be possible. Spectroscopic observations of lensed high-redshift supernovae Type Ia are of special interest since they can be used to test for evolution of the standard candle nature of these objects. If systematic redshift-dependent properties are found, their utility for future surveys could be challenged. In the thesis it is shown that the strongly lensed and very distant supernova Type Ia PS1-10afx at z=1.4, does not deviate from the well-studied nearby and intermediate populations of normal supernovae Type Ia. In a different study, the discovery of the first resolved multiply-imaged gravitationally lensed supernova Type Ia is also reported. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>
153

Botswana &amp; Zimbabwe : En komparativ demokratiseringsstudie

Kungberg, Marcus January 2017 (has links)
Abstract The purpose of this essay is to shed light on why the democratic developments in Botswana and Zimbabwe, despite their many similarities, have gone in very different directions. By the time of their respective independence, both countries proclaimed themselves democratic states. However, only Botswana succeeded in the transition, while Zimbabwe soon fell back into an autocratic regime. The method used is a comparative case study. By comparing the cases Botswana and Zimbabwe in according to five selected democratic variables, this method makes it possible to explain why only Botswana succeeded. The research questions are: How have Botswana’s and Zimbabwe’s respective democratic transitions happened? What factors have influenced and guided the direction of the democratic development in Botswana and Zimbabwe? The following democratic variables will guide the comparison: “Economic development”, “Market economy”, “Strong middleclass”, “External influences” and “Political leaders dedicated to democracy”.                 The results show considerable differences between the democratic transitions of Botswana and Zimbabwe as regards all the selected democratic variables. The comparative method thus proved to be an effective tool to explain the democratic development in the two countries investigated in this case study. However, these factors potentially just scratch the surface and, in a theory-developing spirit, more democratic factors can broaden the explanation even more.
154

OBSERVATION AND CONFIRMATION OF SIX STRONG-LENSING SYSTEMS IN THE DARK ENERGY SURVEY SCIENCE VERIFICATION DATA

Nord, B., Buckley-Geer, E., Lin, H., Diehl, H. T., Helsby, J., Kuropatkin, N., Amara, A., Collett, T., Allam, S., Caminha, G. B., De Bom, C., Desai, S., Dúmet-Montoya, H., da S. Pereira, M. Elidaiana, Finley, D. A., Flaugher, B., Furlanetto, C., Gaitsch, H., Gill, M., Merritt, K. W., More, A., Tucker, D., Saro, A., Rykoff, E. S., Rozo, E., Birrer, S., Abdalla, F. B., Agnello, A., Auger, M., Brunner, R. J., Kind, M. Carrasco, Castander, F. J., Cunha, C. E., da Costa, L. N., Foley, R. J., Gerdes, D. W., Glazebrook, K., Gschwend, J., Hartley, W., Kessler, R., Lagattuta, D., Lewis, G., Maia, M. A. G., Makler, M., Menanteau, F., Niernberg, A., Scolnic, D., Vieira, J. D., Gramillano, R., Abbott, T. M. C., Banerji, M., Benoit-Lévy, A., Brooks, D., Burke, D. L., Capozzi, D., Rosell, A. Carnero, Carretero, J., D’Andrea, C. B., Dietrich, J. P., Doel, P., Evrard, A. E., Frieman, J., Gaztanaga, E., Gruen, D., Honscheid, K., James, D. J., Kuehn, K., Li, T. S., Lima, M., Marshall, J. L., Martini, P., Melchior, P., Miquel, R., Neilsen, E., Nichol, R. C., Ogando, R., Plazas, A. A., Romer, A. K., Sako, M., Sanchez, E., Scarpine, V., Schubnell, M., Sevilla-Noarbe, I., Smith, R. C., Soares-Santos, M., Sobreira, F., Suchyta, E., Swanson, M. E. C., Tarle, G., Thaler, J., Walker, A. R., Wester, W., Zhang, Y. 05 August 2016 (has links)
We report the observation and confirmation of the first group-and cluster-scale strong gravitational lensing systems found in Dark Energy Survey data. Through visual inspection of data from the Science Verification season, we identified 53 candidate systems. We then obtained spectroscopic follow-up of 21 candidates using the Gemini Multi-object Spectrograph at the Gemini South telescope and the Inamori-Magellan Areal Camera and Spectrograph at the Magellan/Baade telescope. With this follow-up, we confirmed six candidates as gravitational lenses: three of the systems are newly discovered, and the remaining three were previously known. Of the 21 observed candidates, the remaining 15 either were not detected in spectroscopic observations, were observed and did not exhibit continuum emission (or spectral features), or were ruled out as lensing systems. The confirmed sample consists of one group-scale and five galaxy-cluster-scale lenses. The lensed sources range in redshift z similar to 0.80-3.2 and in i-band surface brightness i(SB) similar to 23-25 mag arcsec(-2) (2 '' aperture). For each of the six systems, we estimate the Einstein radius theta(E) and the enclosed mass M-enc, which have ranges theta(E) similar to 5 ''-9 '' and M-enc similar to 8 x 10(12) to 6 x 10(13)M(circle dot), respectively.
155

The extraordinary amount of substructure in the Hubble Frontier Fields cluster Abell 2744

Jauzac, M., Eckert, D., Schwinn, J., Harvey, D., Baugh, C. M., Robertson, A., Bose, S., Massey, R., Owers, M., Ebeling, H., Shan, H. Y., Jullo, E., Kneib, J.-P., Richard, J., Atek, H., Clément, B., Egami, E., Israel, H., Knowles, K., Limousin, M., Natarajan, P., Rexroth, M., Taylor, P., Tchernin, C. 21 December 2016 (has links)
We present a joint optical/X-ray analysis of the massive galaxy cluster Abell 2744 (z = 0.308). Our strong- and weak-lensing analysis within the central region of the cluster, i.e. at R < 1 Mpc from the brightest cluster galaxy, reveals eight substructures, including the main core. All of these dark matter haloes are detected with a significance of at least 5 sigma and feature masses ranging from 0.5 to 1.4 x 10(14) M-circle dot within R < 150 kpc. Merten et al. and Medezinski et al. substructures are also detected by us. We measure a slightly higher mass for the main core component than reported previously and attribute the discrepancy to the inclusion of our tightly constrained strong-lensing mass model built on Hubble Frontier Fields data. X-ray data obtained by XMM-Newton reveal four remnant cores, one of them a new detection, and three shocks. Unlike Merten et al., we find all cores to have both dark and luminous counterparts. A comparison with clusters of similar mass in the Millennium XXL simulations yields no objects with as many massive substructures as observed in Abell 2744, confirming that Abell 2744 is an extreme system. We stress that these properties still do not constitute a challenge to Lambda cold dark matter, as caveats apply to both the simulation and the observations: for instance, the projected mass measurements from gravitational lensing and the limited resolution of the subhaloes finders. We discuss implications of Abell 2744 for the plausibility of different dark matter candidates and, finally, measure a new upper limit on the self-interaction cross-section of dark matter of sigma(DM) < 1.28 cm(2) g(-1) (68 per cent CL), in good agreement with previous results from Harvey et al.
156

Retrospective Perceptions of Early Parent-Child Relations and Occupational Orientation

Smith, Richard E. 12 1900 (has links)
The purpose of the present study was to examine the relationship between early perceptions of parent-child relations, as measured by the Roe and Siegelman Parent-Child Relations Questionnaire (PCR), and the occupational orientation of the child, as measured by the Strong Vocational Interest Blank (SVIB).
157

High Lyman Continuum Escape Fraction in a Lensed Young Compact Dwarf Galaxy at z=2.5

Bian, Fuyan, Fan, Xiaohui, McGreer, Ian, Cai, Zheng, Jiang, Linhua 02 March 2017 (has links)
We present the HST WFC3/F275W UV imaging observations of A2218-Flanking, a lensed compact dwarf galaxy at redshift z approximate to 2.5. The stellar mass of A2218-Flanking is log(M-*/M-circle dot) = 9.14(-0.04)(+0.07) and SFR is 12.5(-7.4)(+3.8) M-circle dot yr(-1) after correcting the magnification. This galaxy has a young galaxy age of 127. Myr and a compact galaxy size of r(1/2) = 2.4 kpc. The HST UV imaging observations cover the rest-frame Lyman continuum (LyC) emission (similar to 800 angstrom) from A2218-Flanking. We firmly detect (14s) the LyC emission in A2218-Flanking in the F275W image. Together with the HST F606W images, we find that the absolute escape fraction of LyC is f(abs,esc) > 28%-57% based on the flux density ratio between 1700 and 800 angstrom (f(1700)/f(800)). The morphology of the LyC emission in the F275W images is extended and follows the morphology of the UV continuum morphology in the F606W images, suggesting that the f(800) is not from foreground contaminants. We find that the region with a high star formation rate surface density has a lower f(1700)/f(800) (higher f(800)/f(1700)) ratio than the diffused regions, suggesting that LyC photons are more likely to escape from the region with the intensive star-forming process. We compare the properties of galaxies with and without LyC detections and find that LyC photons are easier to escape in low-mass galaxies.
158

Structure Determination and Prediction of Zeolites : A Combined Study by Electron Diffraction, Powder X-Ray Diffraction and Database Mining

Guo, Peng January 2016 (has links)
Zeolites are crystalline microporous aluminosilicates with well-defined cavities or channels of molecular dimensions. They are widely used for applications such as gas adsorption, gas storage, ion exchange and catalysis. The size of the pore opening allows zeolites to be categorized into small, medium, large and extra-large pore zeolites. A typical zeolite is the small pore silicoaluminophosphate SAPO-34, which is an important catalyst in the MTO (methanol-to-olefin) process. The properties of zeolite catalysts are determined mainly by their structures, and it is therefore important to know the structures of these materials in order to understand their properties and explore new applications. Single crystal X-ray diffraction has been the main technique used to determine the structures of unknown crystalline materials such as zeolites. This technique, however, can be used only if crystals larger than several micrometres are available. Powder X-ray diffraction (PXRD) is an alternative technique to determine the structures if only small crystals are available. However, peak overlap, poor crystallinity and the presence of impurities hinder the solution of structures from PXRD data. Electron crystallography can overcome these problems. We have developed a new method, which we have called “rotation electron diffraction” (RED), for the automated collection and processing of three-dimensional electron diffraction data. This thesis describes how the RED method has been applied to determine the structures of several zeolites and zeolite-related materials. These include two interlayer expanded silicates (COE-3 and COE-4), a new layered zeolitic fluoroaluminophosphate (EMM-9), a new borosilicate (EMM-26), and an aluminosilicate (ZSM-25). We have developed a new approach based on strong reflections, and used it to determine the structure of ZSM-25, and to predict the structures of a series of complex zeolites in the RHO family. We propose a new structural principle that describes a series of structurally related zeolites known as “embedded isoreticular zeolite structures”, which have expanding unit cells. The thesis also summarizes several common structural features of zeolites in the Database of Zeolite Structures. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript.</p>
159

Does a pint a day affect your child’s pay? : Prenatal alcohol exposure and child outcomes, Evidence from a policy experiment

Olsson, Thomas January 2007 (has links)
In this thesis I evaluate the impact of an experiment with free sales of strong beer in two Swedish counties that took place in the 1960s. I do this by studying adult earnings of persons in utero during the experiment. My data includes date and place of birth and allows me to evaluate the impact of the experiment using a difference-in differences methodology, comparing earnings across cohorts and counties. Since the availability of alcohol increased most heavily for persons under the age of 21, and male fetuses are less physiologically robust than female fetuses, I choose to study persons born by mothers younger than 21 separately and also estimate the impact of the experiment separately for men and women. I find that persons born by mothers under the age of 21 during the experiment have lower average earnings than persons born before the experiment, and that the impact is larger on men. My results indicate that the experiment has led to adverse effects on adult earnings, probably caused by the prenatal alcohol exposure’s negative impact on fetal development. This means that alcohol consumption have long-term consequences that represent large costs to society. Since these costs are generally disregarded when evaluating the cost of alcohol consumption, society’s cost of alcohol is probably higher than usually estimated.
160

Modélisation des pales d'éoliennes ou d'hydroliennes en environnement naturel à l'aide d'un code fluide-structure / Fluid-structure interaction on wind turbine blades

Lothodé, Corentin 24 September 2018 (has links)
Ce travail porte sur la mise en œuvre de simulations sur des pales de machines tournantes. Une première partie de la thèse porte sur l’amélioration des performances du couplage fluide-structure. Des nouveaux algorithmes sont présentés. Une nouvelle méthode de déformation de maillage est évaluée. Les développements sont validés à partir de plusieurs cas tests. La deuxième partie porte sur l’application des avancées à des machines tournantes. Une première validation est faite sur une hydrolienne. La vibration d’une pale au passage du mat est étudiée. Enfin, des résultats sur une hydrolienne industrielle sont exposés. / A methodology to simulate blades of turbines is developed. A first part is dedicated to improving the performance of the fluid-structure coupling. New algorithms are presented. A new mesh morphing solution is shown. Developments are validated on many test cases. A second part is dedicated to applying the developments on turbines. A first validation is made on a water turbine. The vibration of a blade interacting with a mast is studied. Finally, some results of an industrial water turbine are shown.

Page generated in 0.0449 seconds