1,721,042 research outputs found
The cosmic-ray positron excess from a local Dark Matter over-density
Get PDFWe show that the cosmic-ray positron excess measured by PAMELA and AMS could be induced by Dark Matter annihilations in a local over-density. In such a context leptophilic DM is not needed and good fits to positron data, in agreement with antiproton and gamma-ray measurements, are obtained for DM annihilations to WW, hh, ZZ, tt̄, bb̄, qq̄ channels. The classic Dark Matter candidates, such as the pure supersymmetric Wino with standard thermal annihilation cross-section, can fit the positron excess, without invoking any additional assumption on Dark Matter properties. © 2013 The Authors. Published by Elsevier B.V
Galaktikate evolutsiooni mõistmine nende heledusfunktsiooni abil
Get PDFVäitekirja elektrooniline versioon ei sisalda publikatsioone.Galaktikad, mis koosnevad kuni sadadest miljarditest tähtedest, gaasist ja tolmust, on ühed tähelepanuväärsemad süsteemid Universumis. Juba visuaalsete vaatluste põhjal on näha, et neid on väga mitmesuguseid: spiraalseid, elliptilisi ning korrapäratu kujuga. Selline mitmekesisus tekitab küsimuse, kuidas galaktikad on tekkinud? Galaktikate tekkimine ja evolutsioon hõlmab mitmeid erinevaid füüsikalisi protsesse ja sõltub paljudest faktoritest. Tänapäeva kosmoloogias on selle mõistmine üks aktuaalsemaid probleeme.
Käesoleva töö eesmärgiks on uurida, kuidas galaktikate evolutsioon sõltub Universumi suuremastaabilisest struktuurist. Selleks kasutame galaktikate heledusfunktsiooni - vaadeldud galaktikate heleduste jaotust, Uurides galaktikate heledusfunktsiooni sõltuvana suuremastaabilisest struktuurist, järeldasime, et elliptiliste galaktikate evolutsioon sõltub tugevalt ümbritsevast suuremastaabilisest keskkonnast, seevastu spiraalsete galaktikate heledusfunktsioon jääb erineva tihedusega piirkondades muutumatuks. Elliptiliste galaktikate heleduste üldine sõltuvus globaalsest ümbrusest oli oodatav, kuna elliptilised galaktikad tekivad vastavalt praegusele galaktikate tekke paradigmale peamiselt galaktikate ühinemise tulemusel. Spiraalgalaktikate heledusfunktsiooni sarnasus erinevates piirkondades näitab aga, et spiraalgalaktikate tekkimine erinevates tingimustes on sarnane. Kuna hierarhilise kuhjumise teooria järgi peaksid ka spiraalgalaktikad sõltuma ümbritsevast globaalsest keskkonnast, piirab see tulemus spiraalgalaktikate tekke võimalikke viise.
Käesoleva töö tulemused näitasid selgelt, et lisaks lokaalsele (gruppide) ümbrusele on galaktikate evolutsioonis oluline ka globaalne, suuremastaabiline ümbrus. Loodetavasti aitab suuremastaabilise ümbruse mõju arvestamine lahendada mitmeid probleeme galaktikate tekke stsenaariumides.Galaxies, which are complex objects containing up to several tens of billions stars, as well as gas and dust, are remarkable objects. The Universe contains a very diverse “zoo” of galaxies: there are galaxies with a discy shape and spiral structure, elliptical galaxies, and even galaxies, which show no sign of structure. This variety of galaxies leads to the basic question: how the galaxies form and evolve and which processes shape the structure of galaxies? Due to the complexity of galaxy formation and evolution, this question is still an unresolved puzzle and it is one of the biggest challenges in modern cosmology.
The present thesis is based on large galaxy surveys and concentrates on the large-scale structure: how galaxy evolution is related to the surrounding large-scale environment of superclusters and voids. To study the evolution of galaxies, we use the luminosity function, which is in this respect one of the most fundamental of all cosmological observables.
One of the principal results of the present study was the conclusion that the evolution of spiral galaxies is almost independent of the global environment, especially for blue and red spirals separately, showing that the formation of spiral galaxies has to be similar in all environments. Meanwhile, the luminosity function of elliptical galaxies depends strongly on the environment. This shows that the global environmental density is an important factor (via merging history) in the formation of elliptical galaxies.
The results of the present study show clearly, that besides the local/group environment, the global (supercluster-void) environment plays also an important role in the formation and evolution of galaxies. Accounting for the role of global environment can help to solve several problems in the present picture of galaxy formation and evolution
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
The Cosmic Ballet: spinning in the web
Get PDFAll the celestial objects, from small asteroids and planets to large galaxies and even the long filaments of the cosmic web are rotating. The origin of rotation is yet to be fully understood, especially in the context of galaxies and their dark matter haloes. In the young Universe, as matter began to clump together, the resulting anisotropic distribution of matter torqued up proto-galaxies. Simultaneously, the matter overdensities collapsed to form the large-scale filaments, clusters, walls and voids of the cosmic web that we see today. As a result, we expect a correlation between galaxy spin and the cosmic web. In this thesis, we uncover the role of the cosmic web environment in establishing the rotation of haloes and galaxies using large cosmological simulations. We show that haloes in filaments are in general spinning faster than in other web components. We have studied the correlations between the spin-axis of haloes/galaxies with the orientation of the cosmic filaments that they are growing in. The thesis explores in detail the spin transition from parallel to perpendicular as a function of the halo or galaxy mass with respect to the spine of the host filament. We show how these trends evolve with cosmic time and filament properties using the MMF/Nexus and Bisous formalisms for cosmic web detection. The results from this thesis are important for various reasons. They not only hold information on how structure in the Universe emerged but also the notion of spin-transition mass promises to be a potential tool to probe the Universe
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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