1,721,131 research outputs found
Stellar systems following the R1/m luminosity law. II. Anisotropy, velocity profiles, and the fundamental plane of elliptical galaxies
No full textFollowing a first paper on this subject (Ciotti 1991, hereafter Paper I), we study the dynamical properties of spherical galaxies with surface luminosity profile described by the R1/m-law, in which a variable degree of orbital anisotropy is allowed. The parameter m for the present models covers the range [1, 10]. For these models we study the self-consistently generated phase-space distribution function (DF), and we derive - as a function of m - the minimum value of the anisotropy radius for the model consistency (i.e., in order to have a nowhere negative DF). Then we study the region in the parameter space where the R1/m models are likely to be stable against radial-orbit instability, and we compare its size with that of the larger region corresponding to the consistency requirement. For stable anisotropic models the spatial and projected velocity dispersion profiles are obtained solving the Jeans equation, and compared to those of the globally isotropic case, already discussed in Paper I. The relevance of the results in connection with the fundamental plane (FP) of elliptical galaxies is pointed out: the effect on the projected velocity dispersion due to the maximum orbital anisotropy allowed by the stability requirement is well within the FP thickness, and so no fine-tuning for anisotropy is required. Finally, the velocity profiles are constructed as a function of the projected radius and for various degrees of anisotropy, and their deviations from a gaussian discussed
Projection effects on the FP thickness
No full textWe study the contribution of projection effects to the
intrinsic thickness of the Fundamental Plane (FP) of elliptical
galaxies. The Monte–Carlo mapping technique between model properties
and observed quantities, introduced by Bertin et al. (2002), is extended to oblate, two–integrals galaxy models, with
non–homologous density profiles, adjustable flattening, variable
amount of ordered rotational support, and for which all the relevant
projected dynamical quantities can be expressed in fully analytical
way. In agreement with previous works, it is found that projection
effects move models not exactly parallel to the edge–on FP, by an
amount that can be as large as the observed FP thickness. The
statistical contribution of projection effects to the FP thickness is
however marginal, and the estimated physical FP rms thickness is
90% of the observed one (when corrected for measurement
errors)
The “dynamical clock”: dating the internal dynamical evolution of star clusters with Blue Straggler Stars
Full text linkWe discuss the observational properties of a special class of objects (the so-called “Blue Straggler Stars”, BSSs) in the framework of using this stellar population as probe of the dynamical processes occurring in high-density stellar systems. Indeed, the shape of the BSS radial distribution and their level of central concentration are powerful tracers of the stage of dynamical evolution reached by the host cluster since formation. Hence, they can be used as empirical chronometers able to measure the dynamical age of stellar systems. In addition, the presence of a double BSS sequence in the color–magnitude diagram is likely the signature of the most extreme dynamical process occurring in globular cluster life: the core collapse event. Such a feature can, therefore, be used to reveal the occurrence of this process and, for the first time, even date it
The tilt of the fundamental plane of elliptical galaxies - I. Exploring dynamical and structural effects
No full textIn this paper we explore several structural and dynamical effects on the projected velocity dispersion as possible causes of the fundamental plane (FP) tilt of elliptical galaxies. Specifically, we determine the size of the systematic trend along the FP in the orbital radial anisotropy, in the dark matter (DM) content and distribution relative to the bright matter, and in the shape of the light profile that would be needed to produce the tilt, under the assumption of a constant stellar mass-to-light ratio. Spherical, non-rotating, two-component models are constructed, where the light profiles resemble the R1/4 law. For the investigated models anisotropy cannot play a major role in causing the tilt, while a systematic increase in the DM content and/or concentration may formally produce it. Also a suitable variation of the shape of the light profile can produce the desired effect, and there may be some observational hints supporting this possibility. However, fine tuning is always required in order to produce the tilt, while preserving the tightness of the distribution of the galaxies about the FP
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
The scaling relations of galaxy clusters and their dark matter halos
No full textLike early-type galaxies, nearby galaxy clusters also define fundamental plane, luminosity-radius, and luminosity-velocity dispersion relations, whose physical origins are still unclear. By means of high-resolution N-body simulations of massive dark matter halos in a ΛCDM (Λ cold dark matter) cosmology, we find that scaling relations similar to those observed for galaxy clusters are already defined by their dark matter hosts. The slopes, however, are not the same, and among the various possibilities in principle able to bring the simulated and the observed scaling relations into mutual agreement, we show that the preferred solution is a luminosity-dependent mass-to-light ratio (M/L~L~0.3) that corresponds well to what is inferred observationally. We then show that at galactic scales there is a conflict between the cosmological predictions of structure formation, the observed trend of the mass-to-light ratio in elliptical galaxies, and the slope of their luminosity-velocity dispersion relation (which significantly differs from the analogous one followed by clusters). The conclusion is that the scaling laws of elliptical galaxies might be the combined result of the cosmological collapse of density fluctuations at the epoch when galactic scales became nonlinear plus important modifications afterward due to early-time dissipative merging. Finally, we briefly discuss the possible evolution of the cluster scaling relations with redshift
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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