1,721,072 research outputs found
Simulating the formation and evolution of galaxies. Methods and results
No full textThis Thesis has two closely interconnected goals. The first, and more strictly astrophysical, is the study of a possible theoretical scenario for the formation and the evolution of the galaxies in cosmological context. In order to do so, given the great complexity of the involved physical systems, it has been decided to use the tool of numerical simulations. The second aim is therefore the construction of an instrument suited to deal with this physical problem. In first part of this Thesis the observed properties of galaxies are summarized, with particular attention to their star formation history and their mass assembly. To this aim, a number of results from recent surveys are presented, seemingly suggesting a precise trend: massive galaxies formed before, and in shorter time, than less massive ones (downsizing, Bundy et al. 2006). This seems to be at odds with the current paradigm of cosmological structure formation, that predicts the hierarchical merging of small objects to assemble greater systems. How to make coexist the observational evidences with the theoretical scenario it is still an unsolved problem. Moving to lower masses, in dwarf galaxies regime, the star formation histories become extremely various and complex, apparently requiring the possibility that isolated objects can form stars after the initial burst, even without any external interaction. Then, the bases of theoretical models of formation of the galaxies are exposed, by means of a panoramic on the main physical processes involved: gravitation, hydrodynamics, cooling and heating phenomena, star formation, energetic and chemical feedback. The second part of the Thesis is focused on the numerical simulations made by the candidate. First of all, the basic version of the evolutionary code is presented: Pd-Tsph, a lagrangian numerical code developed in Padova by C. Chiosi and collaborators (Carraro & Chiosi 1998), based on the classic algorithms Tree-code (Barnes & Hut 1986) and SPH (Lucy 1977), developed to study the formation of galactic systems. With this code, two models of formation of massive galaxies in cosmological context have been realized and are here discussed (Merlin & Chiosi 2006). Then, a first study on a fundamental phenomenon, often poorly investigated in numerical simulations - namely, the co-existence of different gaseous phases in the interstellar medium - is presented with another set of simulations, focusing on galaxies of intermediate-high mass (Merlin & Chiosi 2007). Finally EvoL, the new version of code Pd-Tsph, is presented and discussed.
A number of numerical and physical issues have been modified and improved for this new release. After the results of some standard hydrodynamic tests carried out to test its performance, two test-simulations of galaxy formation are presented, this time focused on intermediate-low mass
systems, close to the dwarf galaxies regime, but with different initial density.
The three examples of numerical simulations discussed in this Thesis seem to suggest a possible trail to follow to build a coherent scenario of galaxy formation: while high density systems give life to a single burst of star formation largely independently of their mass, a small mass system can have a more complicated star formation history, if its density is sufficiently low. Future work will be the construction of a number of models in order to explore in detail the mass-density parameter space.Gli obbiettivi di questa tesi sono due, strettamente interconnessi fra loro. Il primo, più prettamente astrofisico, è lo studio di un possibile scenario teorico per la formazione e l’evoluzione delle galassie in contesto cosmologico. Per fare questo, vista la grande complessità dei sistemi fisici in gioco, si è scelta la strada delle simulazioni numeriche. Il secondo scopo è quindi la costruzione di uno strumento adeguato a trattare il problema fisico in questione.
Nella prima parte di questa tesi vengono riassunte dapprima le proprietà osservate delle galassie, soprattutto in termini di storia della formazione stellare e dell’assemblaggio della massa. A tale scopo, vengono brevemente presentati i risultati di numerose campagne di osservazione recenti, le quali sembrano suggerire un preciso andamento: le galassie più massicce si sono formate prima, e in minor tempo, di quelle meno massicce (downsizing, Bundy et al. 2006). Questo sembra contrastare decisamente con l‘attuale paradigma di formazione delle strutture in ambito cosmologico, che prevede la formazione di sistemi più grandi a partire dall’unione gerarchiaca di oggetti più piccoli. Come far convivere l’evidenza osservativa con l’impianto teorico è un problema ancora irrisolto. A masse molto piccole, nel regime delle cosiddette galassie nane, le storie di formazione stellare diventano poi estremamente varie e complesse, e sembrano richiedere la possibilità che singoli oggetti siano in grado di formare stelle in tempi differenti dopo l’intenso episodio iniziale, anche senza interagire con altri corpi.
Vengono quindi esposte le basi dei modelli teorici di formazione delle galassie, mediante una panoramica sui principali processi fisici coinvolti: gravitazione, fluidodinamica, fenomeni di cooling e di heating, formazione stellare, feedback energetico e chimico.
La seconda parte della tesi è dedicata alle simulazioni numeriche. Per prima cosa, viene presentato lo strumento di lavoro nella sua versione “base”: il Pd-Tsph, un codice numerico lagrangiano sviluppato a Padova da C. Chiosi e collaboratori (Carraro & Chiosi 1998), basato sui classici algoritmi Tree-code (Barnes & Hut 1986) e SPH (Lucy 1977), pensato e realizzato per lo studio della formazione di sistemi galattici. Con questo codice, due modelli di formazione di galassie massicce in ambiente cosmologico sono stati realizzati e vengono qui discussi (Merlin & Chiosi 2006). Un primo studio su un fenomeno fondamentale ma spesso poco studiato nell‘ambito delle simulazioni numeriche, ovvero la coesistenza di diverse fasi gassose nel mezzo interstellare, viene affrontato in un‘altro set di simulazioni riguardanti galassie di massa medio-alta (Merlin & Chiosi 2007). Infine, viene presentato EvoL, la nuova versione del codice Pd-Tsph, in cui vengono rivisti e migliorati numerosi aspetti sia tecnico-numerici che di modellizzazione fisica. Vengono presentati i risultati di alcuni test idrodinamici standard effettuati per testare il funzionamento del nuovo codice, seguiti da due nuove simulazioni-test, questa volta focalizzate su sistemi di
massa medio-piccola, vicini al regime delle galassie nane, con diverse densità iniziali. I tre esempi di simulazioni numeriche presentati in questa tesi sembrano indicare una possibile strada da seguire nell’ambito di una possibile teoria generale di formazione delle galassie: mentre sistemi di alta densità iniziale danno vita a un unico burst di formazione stellare in modo essenzialmente indipendentemente dalla loro massa, oggetti di piccola massa possono avere una storia di formazione stellare più complicata, se la loro densità è sufficientemente bassa. Le prospettive di lavoro per il futuro prevedono la costruzione di un sufficientemente elevato numero di modelli per esplorare approfonditamente lo spazio dei parametri massa-densità
Study of ISM tracers in galaxies
No full textWe collected data for two samples of normal and interacting galaxies for a total of 2953 galaxies having fluxes in one or more of the following wavebands: FIR, 21 cm line, CO(1-0) lines and soft X-ray. The large set of data obtained allowed us to revisit some of the already known relations between the different tracers of the interstellar medium (ISM), such as the link between the FIR flux and the CO line emission, the relation between X-ray emission and the blue or FIR luminosity. The relation lacking from observations for early-type galaxies has been discussed and explained in detail in the frame of a suitable theoretical model, obtained by coupling chemo-dynamical N-body simulations with a dusty spectrophotometric code of population synthesis
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
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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