1,721,068 research outputs found
A simple method to construct exact density-potential pairs from a homeoidal expansion
No full textWe start from a study of the density- potential relation for classical homeoids in terms of an asymptotic expansion for small deviations from spherical symmetry. We then show that such expansion is a useful device that allows us to construct a variety of exact density- potential pairs with spheroidal, toroidal, or triaxial shapes for which the deviation from spherical symmetry is finite. As concrete analytical applications, we describe: ( 1) The construction of a family of toroidal axisymmetric density- potential pairs one of which is associated with a perfectly flat rotation curve ( for a member of this family, the supporting two- integral phase- space distribution function is obtained in closed form); ( 2) The determination of the aperture velocity dispersion in a wide class of two- integral axisymmetric models not stratified on homeoids with central black hole, which may be useful for the discussion of the dynamical contributions to the characteristics of the Fundamental Plane of early- type galaxies; and ( 3) For such class of models, the construction of the v/sigma-ellipticity relation, often considered to assess the role of rotation in the structure of elliptical galaxies
Collisionless Evaporation from Cluster Elliptical Galaxies
No full textBy means of simple numerical models we discuss whether collisionless stellar evaporation from cluster elliptical galaxies
could be an effective mechanism for the production of intracluster
stellar populations. The effectiveness of this mechanism is due to
the fact that, for realistic galaxy and cluster models, the galaxy
oscillation periods near equilibrium configurations in the cluster
tidal field are of the same order of stellar orbital times in the
external parts of the galaxies themselves. With the aid of
Monte-Carlo simulations we explore the evolution of stellar orbits in
oscillating galaxies placed near different equilibrium positions. We
found that, over an Hubble time, the main effect is a substantial
expansion of the galactic outskirts, particularly affecting the galaxy
at the cluster center and those orbiting near the cluster core radius:
overall, approximately the 10% of the galaxy mass is affected. Thus,
the proposed mechanism could be of some importance in the shaping of
the halo of cD galaxies and in making easier “galaxy harassment” in
the formation of the intracluster stellar population
One-zone models for spheroidal galaxies with a central supermassive black-hole: self-regulated Bondi accretion
Full text linkBy means of a one-zone evolutionary model, we study the co-evolution of supermassive black holes and their host galaxies, as a function of the accretion radiative efficiency, dark matter content, and cosmological infall of gas. In particular, the radiation feedback is computed by using the self-regulated Bondi accretion. The models are characterized by strong oscillations when the galaxy is in the AGN state with a high accretion luminosity. We found that these one-zone models are able to reproduce two important phases of galaxy evolution, namely an obscured-cold phase when the bulk of star formation and black hole accretion occur, and the following quiescent hot phase in which accretion remains highly sub-Eddington. A Compton-thick phase is also found in almost all models, associated with the cold phase. An exploration of the parameter space reveals that the closest agreement with the present-day Magorrian relation is obtained, independently of the dark matter halo mass, for galaxies with a low-mass seed black hole, and the accretion radiative efficiency ≃0.1
The importance of dry and wet merging on the formation and evolution of elliptical galaxies
Full text linkWith the aid of a simple yet robust approach, we investigate the influence of dissipationless and dissipative merging on galaxy structure and the consequent effects on the scaling laws followed by elliptical galaxies. Our results suggest that elliptical galaxies cannot be originated by parabolic merging of low-mass spheroids only, even in the presence of substantial gas dissipation. However, we also found that scaling laws such as the Faber-Jackson, Kormendy, funda- mental plane, and MBH- relations, when considered over the whole mass range spanned by elliptical galaxies in the local universe, are robust against merging. We conclude that galaxy scaling laws, possibly established at high redshift by the fast collapse in preexisting dark matter halos of gas-rich and clumpy stellar distributions, are compatible with a (small) number of galaxy mergers at lower redshift
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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