1,720,968 research outputs found
Numerical simulations of galaxies in cosmological volumes
No full textModelling galaxy formation in a cosmological context is challenging due to many physical processes and a wide range of scales involved. It is then convenient to address the problem by means of numerical techniques. Cosmological simulations are still orders of magnitude away from capturing directly the spatial scales where stars actually form. The treatment of the interstellar medium, with multiple gas phases co-existing at very different densities and temperatures, needs to be included in simulations in the form of sub-resolution effective models.
In Murante et al. (2015), cosmological simulations of individual disk galaxies, carried out with GADGET-3 (Springel, 2005), are presented including the sub-resolution model MUlti-Phase Particle Integrator (MUPPI). Late-type galaxies are obtained with properties in broad agreement with observations (disk size, mass, surface density, rotation velocity, gas fraction) and also with results shown by other groups. During my PhD project, the properties of the bars in simulated galaxies are quantified and presented in Goz et al. (2015). Morphology, kinematic and radial streaming motions are in agreement with observations of local Universe (Seidel et al., 2015). In the GA galaxy, Fourier analysis of the surface density of the disk reveals that bar length and strength are remarkably independent of resolution. In the GA simulations, the onset of bar instability is found to take place when the disk is Toomre-unstable due to accumulation of mass in the disk. The complex task to calculate the emerging spectral energy distribution of a simulated galaxy, treating the reprocessing of light emitted by stars by means of dusty ISM, has been faced using the post-processing code GRASIL-3D (Dominguez-Tenreiro et al., 2014). During my PhD, GRASIL-3D has been interfaced with the outputs of the simulation and modified to better manage the quantities provided by the MUPPI algorithm. In this Thesis, results of a sample of simulated galaxies at z = 0 are shown, extracted from one large volume simulation presented by Barai et al. (2015), in which feedback and star formation are modelled by MUPPI. In broad agreement with observations of the local Universe, simulated galaxies reproduce the gas mass scaling, the main sequence, the relation between the gas-phase metallicity and stellar mass. Infrared predicted spectra, binned by stellar mass, gas metallicity, SFR and morphology, are remarkably in agreement with the available Herschel Reference Survey (Ciesla et al., 2014) templates calibrated with observations on nearby galaxies. H2 and HD are effective in cooling primordial gas below the temperature of ∼10^4 K, but usually the non-equilibrium radiative rates are only approximate in simulations, typically either assuming collisional ionization equilibrium or ignoring metal-line cooling altogether, rather than computing them explicitly. Maio et al. (2007) have extended a previous non-equilibrium calculations of Yoshida et al. (2003) in order to include in the numerical code GADGET a detailed chemical network and to follow the formation/destruction of a wide range of species, both with primordial and metal-enriched gas.
During my PhD project, the chemical network, presented by Maio et al. (2007), has been extended to take into account the H2 formation onto dust grains and its UV-photodissociation. Then, the modified chemical network has been coupled in two different preliminary forms with the MUPPI algorithm within GADGET-3. The main aim, motivated by the tight observed connection between molecular hydrogen and star formation (e.g. Kennicutt et al., 2007; Bigiel et al., 2008), is to devise a way to track self-consistently non-equilibrium abundance and cooling processes of H2 and H2-based star formation in smoothed particle hydrodynamic simulations
Properties of barred spiral discs in hydrodynamical cosmological simulations
Get PDFWe present a quantification of the properties of bars in two N-body+ smoothed particle hydrodynamics cosmological simulations of spiral galaxies, named GA and AqC. The initial conditions were obtained using the zoom-in technique and represent two dark matter haloes of 2-3 × 1012 M&sun;, available at two different resolutions. The resulting galaxies are presented in the companion paper of Murante et al. We find that the GA galaxy has a bar of length 8.8 kpc, present at the two resolution levels even though with a slightly different strength. Classical bar signatures (e.g. pattern of streaming motions, high m = 2 Fourier mode with roughly constant phase) are consistently found at both resolutions. Though a close encounter with a merging satellite at z ̃ 0.6 (mass ratio 1: 50) causes a strong, transient spiral pattern and some heating of the disc, we find that bar instability is due to secular process, caused by a low Toomre parameter Q ≲ 1 due to accumulation of mass in the disc. The AqC galaxy has a slightly different history: it suffers a similar tidal disturbance due to a merging satellite at z ̃ 0.5 but with a mass ratio of 1: 32, that triggers a bar in the high-resolution simulation, while at low resolution the merging is found to take place at a later time, so that both secular evolution and merging are plausible triggers for bar instability
Simulating realistic disc galaxies with a novel sub-resolution ISM model
Get PDFWe present results of cosmological simulations of disc galaxies carried out with the GADGET-3 TreePM+SPH code, where star formation and stellar feedback are described using our MUlti Phase Particle Integrator model. This description is based on a simple multiphase model of the interstellar medium at unresolved scales, where mass and energy flows among the components are explicitly followed by solving a system of ordinary differential equations. Thermal energy from supernovae is injected into the local hot phase, so as to avoid that it is promptly radiated away. A kinetic feedback prescription generates the massive outflows needed to avoid the overproduction of stars. We use two sets of zoomed-in initial conditions of isolated cosmological haloes with masses (2-3) × 1012 M⊙, both available at several resolution levels. In all cases we obtain spiral galaxies with small bulge-over-total stellar mass ratios (B/T ̃ 0.2), extended stellar and gas discs, flat rotation curves and realistic values of stellar masses. Gas profiles are relatively flat, molecular gas is found to dominate at the centre of galaxies, with star formation rates following the observed Schmidt-Kennicutt relation. Stars kinematically belonging to the bulge form early, while disc stars show a clear inside-out formation pattern and mostly form after redshift z = 2. However, the baryon conversion efficiencies in our simulations differ from the relation given by Moster et al. at a 3σ level, thus indicating that our stellar discs are still too massive for the dark matter halo in which they reside. Results are found to be remarkably stable against resolution. This further demonstrates the feasibility of carrying out simulations producing a realistic population of galaxies within representative cosmological volumes, at a relatively modest resolution
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
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
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
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