1,720,992 research outputs found
Halo model for cosmic shear predictions
Full text linkThe thesis we present is a numerical study of the lensing properties of the large scale structures.
The current theory of structure formation assumes that it forms via gravitational instability from initial perturbations in the density field. As the Universe cooled, clumps of dark matter began to condense. Gas and dark matter were gravitationally attracted to the higher density areas and formed the seeds for the primordial celestial objects. This scenario is called top-bottom or hierarchical scenario, because the smaller objects are the first to be assembled, then they merge to give origin to the large scale structure, (Coles and Lucchin, 2002).
At present time the LSS shows up in a cosmic web, in which galaxy clusters occupy the nodes and are connected by filaments of matter, and in between there are the voids, (Einasto, 2012; Coil, 2013). Being the most bound and latest forming structures, clusters can be used as an ideal laboratory to test the prediction of the LambdaCDM model, (Davis et al., 1985; Hung et al., 2016). In particular, since clusters are dominated by dark matter, we can use them to investigate the properties of dark matter too (Kunz et al., 2016).
Because the dark matter interacts only with gravity, it is not possible to trace it directly, as we can do when we observe the hot gas bremsstrahlung emission in X-ray band (Bahcall and Sarazin, 1977; Sarazin, 1986) and the light coming from galaxies in the optical band (Zwicky, 1937; Munari et al., 2013). To detect dark matter we need to recognize its gravitational effects on the images of background galaxies, (Bacon et al., 2000; Refregier, 2003).
In fact the gravitational field produced by massive objects extends far into the space-time and, deforming it, it is able to bend light rays passing close to the objects and to refocus them somewhere else. This produces a pattern of distortions in size and shape of the images. The effect of deflection of light by gravity is called gravitational lensing and the objects responsible of it are called gravitational lenses, (Narayan and Bartelmann, 1996).
In the near future several new large scale surveys, like the Large Synoptic Survey Telescope (LSST, LSST Science Collaboration et al., 2009) and Euclid (Refregier et al., 2010, Laureijs et al., 2011) are including among their goals the measurements of weak lensing.
The main purpose of this work is to determine the effects of cosmic shear due to the dark matter distribution in the LSS produced on the background galaxies, by understanding how and how much its convergence field changes, changing the model used to describe the density field.
In order to represent the large scale structure, to estimate its lensing properties and be able to compare the observations with the numerical results, we have two possible alternatives. The first one is to simulate the evolution of a piece of universe by reproducing mock light cones with cosmological simulations, but, for making a statistical analysis, a large number of realizations is needed while the N-body simulations are very expensive in terms of computational time and space, or we can treat the LSS as an ensemble of dark matter clumps, with a given density profile and a cosmologically consistent distribution, compatibly with the halo-model idea, (Cooray and Sheth, 2002). We proceeded by choosing the second approach.
Our work is mainly based on the developing of Weak Lensing-Matter density distributiOn Kode for grAvitational lenses (WL-MOKA), a semi-analytical tool based on MOKA (Giocoli et al., 2012), which is able, given a three-dimensional distribution of dark matter halos, to create the surface density distribution of each spherical halo and then to calculate the convergence fields. This code respect to N-body simulations is very fast, so to test its limits and fine tune it, we took the available dark matter halo catalogues extracted from the COupled Dark Energy Cosmological Simulations (CoDECS) project, in particular the LambdaCDM simulation, implemented with the parallel TreePMSPH N-body code GADGET (Springel, 2005) by Baldi (2012), in which, they added the physical effect due to the interaction between the cold dark matter fluid and the dark energy scalar field. Starting from the outputs of simulation, we created mock light cones with a field of view of 25 deg2 and filled with halos with a given density profile and we calculated the convergence maps. At this step we considered the extension and the density profile outside the virial radius of the halos as free parameters. Then, we proceeded into a one-point and a two-point statistical analysis by deriving the probability distribution function and the power spectrum of the convergence, and we compared our results with the same quantities extracted from the simulation. Once the best match has been determined, we continued our study by analysing the entire a set of 25 light cones of the LambdaCDM simulation and we calculated the covariance maps and the cross-correlation coefficient matrices of the power spectrum. At least we compared them from those derived directly from the simulation.
We also considered another cosmological simulation, LambdaCDM-HS8, with the same cosmological parameters of the LambdaCDM simulation and a different value for sigma8, and we applied the previous recipe.
All these comparisons allow us both to fix the parameters which better reproduce the simulations results and to point out to the pros and cons of our method.
The thesis is divided into four chapters. The first one gives an essential cosmological scenario. Its aim is to provide the theoretical concepts necessary to understand the cosmological framework in which the research takes place. For this reason, after a brief introduction to the cosmological principle and the equations that lead the Universe’s evolution, there is a focus on cosmological models and a dealing with the growth of perturbations in linear and non-linear regime. In the second chapter we depict the large scale structure, under an observational and a numerical point of view. We point out the clusters and their physical properties, in particular their connection with dark matter. The third chapter is dedicated to the gravitational lensing theory. Starting from simple assumptions, we can derive all the quantities we need to understand and give a global description of lensing, with the aim to introduce the concepts used in the research. In the last chapter we described in detail our work, the analysis and the
results coming from the comparison with the simulations. In the conclusions we summarize all the results we achieved, the emerged critical points and the future perspectives
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
The Three-Dimensional Shapes of Galaxy Clusters
No full textWhile clusters of galaxies are considered one of the most important cosmological probes, the standard spherical modelling of the dark matter and the intracluster medium is only a rough approximation. Indeed, it is well established both theoretically and observationally that galaxy clusters are much better approximated as triaxial objects. However, investigating the asphericity of galaxy clusters is still in its infancy. We review here this topic which is currently gathering a growing interest from the cluster community. We begin by introducing the triaxial geometry. Then we discuss the topic of deprojection and demonstrate the need for combining different probes of the cluster's potential. We discuss the different works that have been addressing these issues. We present a general parametric framework intended to simultaneously fit complementary data sets (X-ray, Sunyaev Zel'dovich and lensing data). We discuss in details the case of Abell 1689 to show how different models/data sets lead to different haloe parameters. We present the results obtained from fitting a 3D NFW model to X-ray, SZ, and lensing data for 4 strong lensing clusters. We argue that a triaxial model generally allows to lower the inferred value of the concentration parameter compared to a spherical analysis. This may alleviate tensions regarding, e.g. the over-concentration problem. However, we stress that predictions from numerical simulations rely on a spherical analysis of triaxial halos. Given that triaxial analysis will have a growing importance in the observational side, we advocate the need for simulations to be analysed in the very same way, allowing reliable and meaningful comparisons. Besides, methods intended to derive the three dimensional shape of galaxy clusters should be extensively tested on simulated multi-wavelength observations
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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