1,720,990 research outputs found
Galaxy merging, the fundamental plane of elliptical galaxies and the M BHσ0 relation
No full textWe explore the effects of dissipationless merging on the fundamental plane of elliptical galaxies using an N-body code based on a new, high-performance numerical scheme. We investigate the two extreme cases of galaxy growth by equal-mass merging and accretion of small stellar systems; in a subset of simulations we also consider the presence of dark matter haloes around the merging galaxies. Curiously, we found that the fundamental plane is preserved by major merging, while in the accretion scenario its edge-on thickness is only marginally reproduced, with substantial thickening in the case of merging with low angular momentum. We also found that both the Faber-Jackson and Kormendy relations are not reproduced by the simulations, in accordance with the results of a preliminary analysis based on a simple application of the virial theorem. Finally, we discuss the implications of our results for the origin of the M BH-σ0 and Magorrian relations. We found that dissipationless merging is unable to reproduce the MBH-σ 0 relation, if the black hole masses add linearly (while the Magorrian relation is nicely reproduced); in contrast, a black hole merging with substantial emission of gravitational waves reproduces the M BH-σ0 relation but fails to reproduce the Magorrian relation. We argue that our results strongly point towards a major role of dissipation in the formation of early-type galaxies and in the growth of their central supermassive black holes, thus supporting the idea of a link between galaxy formation and quasi-stellar object activity
Radial orbital anisotropy and the Fundamental Plane of elliptical galaxies
No full textThe existence of the Fundamental Plane imposes strong constraints on the structure and dynamics of elliptical galaxies, and thus contains important information on the processes of their formation and evolution. Here we focus on the relations between the Fundamental Plane thinness and tilt and the amount of radial orbital anisotropy: in fact, the problem of the compatibility between the observed thinness of the Fundamental Plane and the wide spread of orbital anisotropy admitted by galaxy models has often been raised. By using N-body simulations of galaxy models characterized by observationally motivated density profiles, and also allowing for the presence of live, massive dark matter haloes, we explore the impact of radial orbital anisotropy and instability on the Fundamental Plane properties. The numerical results confirm a previous semi-analytical finding (based on a different class of one-component galaxy models): the requirement of stability matches almost exactly the thinness of the Fundamental Plane. In other words, galaxy models that are radially anisotropic enough to be found outside the observed Fundamental Plane (with their isotropic parent models lying on the Fundamental Plane) are unstable, and their end-products fall back on the Fundamental Plane itself. We also find that a systematic increase of radial orbit anisotropy with galaxy luminosity cannot explain by itself the whole tilt of the Fundamental Plane, the galaxy models becoming unstable at moderately high luminosities: at variance with the previous case, their end-products are found well outside the Fundamental Plane itself. Some physical implications of these findings are discussed in detail
The mode of gas accretion onto star-forming galaxies
No full textIt is argued that galaxies like ours sustain their star formation by transferring gas from an extensive corona to the star-forming disc. The transfer is effected by the galactic fountain -- cool clouds that are shot up from the plane to kiloparsec heights above the plane. The Kelvin-Helmholtz instability strips gas from these clouds. If the pressure and the the metallicity of the corona are high enough, the stripped gas causes a similar mass of coronal gas to condense in the cloud's wake. Hydrodynamical simulations of cloud-corona interaction are presented. These confirm the existence of a critical ablation rate above which the corona is condensed, and imply that for the likely parameters of the Galactic corona this rate lies near the actual ablation rate of clouds. In external galaxies trails of HI behind individual clouds will not be detectable, although the integrated emission from all such trails should be significant. Parts of the trails of the clouds that make up the Galaxy's fountain should be observable and may account for features in targeted 21-cm observations of individual high-velocity clouds and surveys of Galactic HI emission. Taken in conjunction with the known decline in the availability of cold infall with increasing cosmic time and halo mass, the proposed mechanism offers a promising explanation of the division of galaxies between the blue cloud to the red sequence in the colour-luminosity plane
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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