1,720,988 research outputs found
Constraining the Star Formation Histories of Gamma-Ray Burst Host Galaxies from their Observed Abundance Patterns
No full textLong-duration Gamma-Ray Bursts (GRBs) are linked to the collapse of massive stars and their hosts are exclusively identified as active, star-forming galaxies. Four long GRBs observed at high spectral resolution at redshift 1.5 <= z <= 4 allowed the determination of the elemental abundances for a set of different chemical elements. In this paper, for the first time, by means of detailed chemical evolution models taking into account also dust production, we attempt to constrain the star formation history of the host galaxies of these GRBs from the study of the measured chemical abundances measured in their interstellar medium. We are also able to provide constraints on the age and on the dust content of GRB hosts. Our results support the hypothesis that long duration GRBs occur preferentially in low metallicity, star-forming galaxies. We compare the specific star formation rate (SSFR), namely the star formation rate (SFR) per unit stellar mass, predicted for the hosts of these GRBs with observational values for GRB hosts distributed across a large redshift range. Our models predict a decrease of the SSFR with redshift, consistent with the observed decrease of the comoving cosmic SFR density between z ~ 2 and z = 0. On the other hand, observed GRB hosts seem to follow an opposite trend in the SSFR versus redshift plot, with an increase of the SSFR with decreasing redshift. Future SSFR determination in larger samples of GRB hosts will be important to understand whether this trend is real or due to some selection effect. Finally, we compare the SSFR of GRB 050730 host with values derived with a sample of Quasar damped Lyman alpha (DLA) systems. Our results indicate that the abundance pattern and the SSFRs of the host galaxies of this GRB are basically compatible with the ones determined for a sample of Quasar DLA systems, suggesting similar chemical evolution paths
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Stochastically Lighting Up Galaxies: Statistical Implications of Stellar Clustering
Full text linkStars form discretely. At the very least, they form in units of individual stars. However, their discretenesslikely extends to much larger spatially and temporally correlated structures known as star clusters. This discretenesshas a profound impact on the light that a population of stars will produce even at fixed star formation rate. Ignoring the effects of this clusteringwhen analyzing observations can lead to significant errors and biases. This work presents an exploration of the effects of this clustering, the foundation of which is the construction of SLUG, a code which Stochastically Lights Up Galaxies. It accounts for the effects of clustering by populating composite stellar populations (``galaxies") one cluster at a time where each cluster is filled by individual stars whose evolution is tracked.This is the first code capable of exploring stochasticity for stellar populations composed of clusters and led to several significant insights in the field. Most notably, the scatter of luminosities due to stochastically placing clusters over the star formation history of a population greatly exceeds the effects of stochastically sampling a population with a stellar initial mass function. This has profound implications for interpretations of star formation rates, deriving initial mass functions, and the star formation rate distribution of the universe. We also explore the statistics of luminosities of clusters themselves, deriving an analytical method (CLOC) for calculating the full distribution of cluster order statistics roughly one billion times faster than a suite of Monte Carlo simulations. This giant leap forward in speed provides the groundwork fora previously impossible robust exploration of the relevant parameter space (e.g. dust opacity distributions, cluster mass function shape and cutoffs, and cluster disruption parameters)
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Fast Radio Bursts: Their Local Environments and Applications to Magnetic Field Measurements
Full text linkTwo primary questions that exist within the field of fast radio bursts (FRBs) are: “What are the sources of Fast Radio Bursts?” and “How can they serve our understanding of magnetism in the universe?”. This has been against the backdrop of a quickly-evolving field with an ever-changing understanding of this mysterious phenomenon. This thesis is a story told in a few parts: (i) an introduction to the world of FRBs; (ii) a look into my work on characterizing the local environments surrounding these bursts, and any connections this may have with their burst properties or any similarities to other transient classes; and (iii) a test of whether or not we can use FRBs as probes of the magnetic fields in their host galaxies. My works find a tentative connection between FRBs and spiral arms which I explore further in subsequent chapters. I develop more rigorous methods of testing this possible relationship between FRBs and spiral structure by measuring the minimum distance between a spiral arm and the FRB position. We find that most FRBs are at distances less than 1 kpc from the nearest arm, tracing the distributions of Type II and Ia relative to spiral arms in their respective hosts. I also explore the connections between burst and local characteristics and find a tentative positive correlation between stellar mass surface density and scattering times. Finally, I explore the power of FRBs to measure magnetic field strengths in their host galaxies. My findings show that we can indeed use FRBs to probe galactic magnetic field
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Food for stars: the role of hydrogen in the formation and evolution of galaxies
Full text linkThe current cosmological model, the Lambda CDM theory, describes with remarkable precision the assembly and growth of the large scale structures and of the dark matter halos in our Universe. A comprehensive theory for the baryon processes that take place within dark matter halos is, instead, still the subject of active research. The three major ingredients of this theory are known: accretion of hydrogen from the intergalactic medium, star formation, and feedback mechanisms in the form of galactic winds. However, the recipe to blend them together has not yet been found. This thesis focuses on the role that two of these ingredients have in the assembly and evolution of galaxies. The underlying questions that this work aims to address are how the accretion of hydrogen onto galaxies occurs and what the conditions needed to convert this raw fuel into stars are. The instruments used for this investigation are diverse, because of the multiplicity of physical processes, spatial scales, and cosmic epochs involved in the problem. Theory, or more specifically the analysis of hydrodynamic simulations to unveil gas accretion onto high-redshift galaxies, is the starting point for this work. In the second part, spectroscopy of bright quasars is used to probe the physical properties of gas and metals around and within distant galaxies. These observations are systematically compared to model predictions. Deep optical imaging is also used to connect the star formation rates of these galaxies to the gas properties that are measured in absorption. Finally, in the third part, the relationship between hydrogen and star formation on smaller scales is investigated by means of multiwavelength observations of local galaxies. This thesis contributes to the aforementioned open questions in four ways. First, itis shown that the accretion of gas onto galaxies as predicted by current simulations imprints characteristic signatures on the distribution of hydrogen and metals of a particular family of absorption line systems, the Lyman limit systems. Second, new spectroscopic observations that led to the discovery of gas clouds with physical properties that match predictions from simulations are presented, paving the way for establishing empirically how galaxies acquire their gas. Third, through a comparison of the hydrogen content and the star formation rates of distant galaxies, this thesis confirms how the presence of significant amounts of hydrogen is not a sufficient condition for the onset of star formation. Finally, after assessing the validity of star formation models in environments that are common to high redshift galaxies, these findings have been interpreted as inefficient star formation in regions with low gas column density and low metallicity
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
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