1,721,025 research outputs found
MUSE's privileged look at the sources of cosmic reionization
No full textLa reionisation est le dernier changement d'état de l'Univers qui a fait passer l'ensemble de sa teneur en hydrogène d'un état neutre à un état ionisé. Cette transition rapide ainsi que le réchauffement du gaz ont eu des conséquences majeures sur la formation et l'évolution des structures, d'où l'importance de la reionisation pour comprendre l'Univers actuel. Dans notre compréhension actuelle, la réionisation s'est terminée à z ~ 6 et les sources responsables de cette transition sont probablement les galaxies de faible masse et à formation d'étoiles (SFG). Une des façons d'étudier cette population consiste à déterminer la fonction de luminosité (LF) des galaxies sélectionnées à partir de leur émission Lyman-alpha et à évaluer leur densité de flux ionisant. Toutefois, les études les plus récentes et leurs conclusions sont en général limitées par le flux le plus faible que l'on peut atteindre avec les instruments actuels. L'un des principaux objectifs du travail présenté dans ce manuscrit est l'étude de la LF des émetteurs Lyman-alpha (LAE) à l'aide d'observations profondes d'amas de galaxies avec l'IFU MUSE. L'utilisation combinée des cubes de données IFU et de champs lentillés rend cette analyse difficile sur le plan du temps de calcul. Pour contourner cette difficulté, nous avons mis au point de nouvelles méthodes pour tenir compte de la contribution de chaque LAE, y compris la détermination du volume effectif et de la complétude. Les LFs résultant de cette analyse ont établi un niveau de contrainte sans précédent sur la forme de la partie à très faible luminosité de la LF. En ne faisant aucune hypothèse sur la fraction d'échappement de l'émissions Lyα, nous observons que la population de LAE a un niveau de contribution à l'émissivité ionisante totale similaire à celui des galaxies sélectionnées par UV (LBG) à z ∼ 6. Dans la continuité de ce travail sur la LAE LF, nous étudions l'effet de la méthode de sélection sur cette conclusion. Ces derniers résultats ont montré que la proportion observée de LAEs augmente de manière significative parmi les galaxies à faible continuum UV et à grand redshift.Reionization is the last change of state of the Universe which made its entire hydrogen content transition from a neutral to a completely ionized state. This rapid transition and heating of the gas content had major consequences on the formation and evolution of structures which makes of reionization a key element to understand the present day Universe. In our current understanding, reionization was mostly done by z ~ 6 and the sources responsible for this transition are likely faint, low mass and star-forming galaxies (SFGs). One way to study this population is to determine the Luminosity Function (LF) of galaxies selected from their Lyman-alpha emission and assess their ionizing flux density. However, most recent studies and their conclusions are in general limited by the lowest flux that can be reached with the current observational facilities. One of the major goals of the work presented in this manuscript is the study of the Lyman-alpha emitters (LAE) LF using deep observations of strong lensing clusters with the integral field unit (IFU) MUSE. The combined usage of large IFU data cubes and lensing fields makes this analysis computationally challenging. To get around this difficulty, we have developed new methods to account for the contribution of each individual LAE, including the effective-volume and completeness determinations. The LFs resulting form this analysis set an unprecedented level of constraint on the shape of the faint end. Making no assumption on the escape fraction of Lyα emission, we observe that the LAE population has a similar level of contribution to the total ionising emissivity as the UV-selected galaxies (LBGs) at z ∼ 6. In the continuity of this work on the LAE LF, we investigate the effect of the selection method on this conclusion. The results have shown that the observed proportion of LAEs increases significantly among UV-faint galaxies and at increasing redshift
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
Responsible sources for the reionization seen by MUSE
No full textDurant les deux dernières décennies, de nombreux efforts ont été apportés pour comprendre le processus de formation des structures de l'Univers jeune. Les avancées dans les technologies observationnelles atteintes aujourd'hui permettent d'observer des galaxies de plus en plus loin, y compris celles responsables de la réionisation cosmique qui a eu lors du premier milliard d'années de l'Univers. L'objectif principal de cette thèse a été de poser des contraintes sur la nature et l'abondance des sources responsables de la réionisation cosmique. Plus précisément, l'étude s'est portée sur les galaxies qui forment des étoiles et qui ont une émission Lyman-alpha (LAE) entre z ~ 3 et 6.7. Il est à noter que cette thèse a été réalisée au sein du consortium MUSE, tout nouvel instrument installé au VLT en janvier 2014 dont nous avons exploité les données du GTO. Ce travail de thèse a permis de confirmer la puissance inégalée de MUSE au niveau de la détection et de l'étude de sources extragalactiques faibles sans aucune présélection. Nous avons observé quatre amas-lentilles dont l'amplification de la lumière permet la détection de sources à faible luminosité, au prix d'une diminution du volume d'Univers observé. Nous nous sommes d'abord focalisés sur l'étude de l'amas de galaxies Abell 1689 afin de structurer une méthodologie applicable aux autres amas. En comparant la densité volumique des LAEs détectés aux différentes fonctions de luminosité (FdL) de la littérature, nous sommes arrivés à la conclusion suivante : la pente de la loi de puissance que décrit la fonction de Schechter pour la partie la plus faible est plus petite que alpha = 7. Cette thèse a apporté des nouvelles contraintes sur la partie faible de la FdL des LAEs pour un redshift allant jusqu'à z ~ 6, un début donc de ce que l'on va pouvoir faire dans les années à venir pour des redshifts de l'ordre de z ~ 7-8.Significant efforts have been put for the past two decades to understand the formation process of structure in the early Universe. The recent technological advances in the observational field allow for observing galaxies further and further, even the ones responsible for the cosmic reionization which occurred during the first billion years of the Universe. The main goal of this thesis was to impose constraints on the nature and the abundance of the sources responsible of the cosmic reionization. More specifically, the study was focused on the star-forming galaxies that have a Lyman-alpha emission (LAE) between z ~ 3 and 6.7. This thesis has been conducted within the framework of the MUSE consortium, a brand new instrument installed on the VLT in January 2014, as part of the exploitation of the Guaranteed Time (GTO). This thesis work has enabled us to confirm the unrivalled power of MUSE concerning the detection and the study of weak extragalactic sources without any preselection. We have observed four lensing clusters which magnify the incident light and make it possible to detect faint sources, at the expense of a decrease of the volume of the observed Universe. At first we started with the study of the galaxy cluster Abell 1689 in order to build up a methodology we intend to apply on other galaxy clusters. By comparing the volume density of the detected LAEs to the luminosity functions (LF) coming from the literature, we have reached the following conclusion : the slope of the power law from the Schechter function is smaller than alpha = 7 candidates. This thesis brought new constraints on the faint-end part of the LF of LAEs for a redshift up to z ~ 6, which represents a beginning with respect to all we can do in the coming years for redshifts up to z ~ 7-8
MOSAIC for the ELT
No full textInternational audienceMOSAIC is the future multi-object fibre spectrograph for the ESO 39 m Extremely Large Telescope (ELT), working in both the Visible and NIR domains. It is a first generation instrument, to be located on one of the Nasmyth platforms of the ELT, with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution. While ELT first light instruments will be using only the central part of the ELT focal plane, MOSAIC is designed to cover the largest possible area (~40 arcmin2) thanks to an original stepped and tiled focal plane design which allows parallel observations in different modes. MOSAIC is optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from our Galaxy and the local Universe to the epoch of the reionization. In this respect, it will be a privileged survey machine for the follow-up of sources being discovered by current and future major facilities, such as James Webb Space Telescope, Euclid, ROMAN or SKA. During this presentation, the status of the project will be updated, and a short review will be provided on the main expected contributions of MOSAIC to extragalactic science, focusing on several hot topics related to the study of the first galaxies formed in the Universe
MOSAIC for the ELT
No full textInternational audienceMOSAIC is the future multi-object fibre spectrograph for the ESO 39 m Extremely Large Telescope (ELT), working in both the Visible and NIR domains. It is a first generation instrument, to be located on one of the Nasmyth platforms of the ELT, with unique capabilities in terms of multiplex, wavelength coverage and spectral resolution. While ELT first light instruments will be using only the central part of the ELT focal plane, MOSAIC is designed to cover the largest possible area (~40 arcmin2) thanks to an original stepped and tiled focal plane design which allows parallel observations in different modes. MOSAIC is optimized to achieve the best possible signal-to-noise ratio on the faintest sources, from our Galaxy and the local Universe to the epoch of the reionization. In this respect, it will be a privileged survey machine for the follow-up of sources being discovered by current and future major facilities, such as James Webb Space Telescope, Euclid, ROMAN or SKA. During this presentation, the status of the project will be updated, and a short review will be provided on the main expected contributions of MOSAIC to extragalactic science, focusing on several hot topics related to the study of the first galaxies formed in the Universe
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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