1,721,196 research outputs found
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 CDM ( 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 / L0:3) 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
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 Chemical Composition of NGC 5824, a Globular Cluster without Iron Spread but with an Extreme Mg-Al Anticorrelation
Full text linkNGC 5824 is a massive Galactic globular cluster suspected to have an intrinsic spread in its iron content, according to the strength of the calcium triplet lines. We present chemical abundances of 117 cluster giant stars using high-resolution spectra acquired with the multi-object spectrograph FLAMES. The metallicity distribution of 87 red giant branch stars is peaked at [Fe/H] =-2.11 ±0.01 dex, while that derived from 30 asymptotic giant branch stars is peaked at [Fe/H] =-2.20 ±0.01 dex. Both the distributions are compatible with a null spread, indicating that this cluster did not retain the ejecta of supernovae. The small iron abundance offset between the two groups of stars is similar to the abundances already observed among red and asymptotic giant branch stars in other clusters. The lack of intrinsic iron spread rules out the possibility that NGC 5824 is the remnant of a disrupted dwarf galaxy, as previously suggested. We also find evidence of the chemical anomalies usually observed in globular clusters, namely the Na-O and the Mg-Al anticorrelations. In particular, NGC 5824 exhibits a huge range of [Mg/Fe] abundance, observed in only a few metal-poor and/or massive clusters. We conclude that NGC 5824 is a normal globular cluster, without spread in [Fe/H] but with an unusually large spread in [Mg/Fe], possibly due to an efficient self-enrichment driven by massive asymptotic giant branch stars
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
THE WFPC2 ULTRAVIOLET SURVEY: THE BLUE STRAGGLER POPULATION IN NGC 5824
Full text linkWe have used a combination of high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and wide-field ground-based observations, in ultraviolet and optical bands, to study the blue straggler star population of the massive outer halo globular cluster NGC 5824 over its entire radial extent. We have computed the center of the cluster and constructed the radial density profile from detailed star counts. The profile is well reproduced by a Wilson model with a small core (rc ~= 4.''4) and a concentration parameter c ~= 2.74. We also present the first age determination for this cluster. From a comparison with isochrones, we find t = 13 ± 0.5 Gyr. We discuss this result in the context of the observed age-metallicity relation of Galactic globular clusters. A total of 60 bright blue stragglers has been identified. Their radial distribution is found to be bimodal, with a central peak, a well-defined minimum at r ~ 20'', and an upturn at large radii. In the framework of the dynamical clock recently defined by Ferraro et al., this feature suggests that NGC 5824 is a cluster of intermediate dynamical age
ANOTHER BRICK IN UNDERSTANDING CHEMICAL AND KINEMATICAL PROPERTIES OF BSSs: NGC 6752
Full text linkWe used high-resolution spectra acquired with the multifiber facility FLAMES at the Very Large Telescope of the European Southern Observatory to investigate the chemical and kinematical properties of a sample of 22 blue straggler stars (BSSs) and 26 red giant branch stars in the nearby globular cluster NGC 6752. We measured radial and rotational velocities and Fe, O, and C abundances. According to radial velocities, metallicity, and proper motions, we identified 18 BSSs as likely cluster members. We found that all the BSSs rotate slowly (less than 40 km s-1), similar to the findings in 47 Tucanae, NGC 6397, and M30. The Fe abundance analysis reveals the presence of three BSSs affected by radiative levitation (showing [Fe/H] significantly higher than that measured in "normal" cluster stars), confirming that element transport mechanisms occur in the photosphere of BSSs hotter than ≃8000 K. Finally, BSS C and O abundances are consistent with those measured in dwarf stars. No C and O depletion ascribable to mass transfer processes has been found on the atmospheres of the studied BSSs (at odds with previous results for 47 Tucanae and M30), suggesting the collisional origin for BSSs in NGC 6752 or that the CO depletion is a transient phenomenon
INVESTIGATING the MASS SEGREGATION PROCESS in GLOBULAR CLUSTERS with BLUE STRAGGLER STARS: The IMPACT of DARK REMNANTS
Full text linkWe present the results of a set of N-body simulations aimed at exploring how the process of mass segregation (as traced by the spatial distribution of blue straggler stars, BSSs) is affected by the presence of a population of heavy dark remnants (as neutron stars and black holes (BHs)). To this end, clusters characterized by different initial concentrations and different fractions of dark remnants have been modeled. We find that an increasing fraction of stellar-mass BHs significantly delay the mass segregation of BSSs and the visible stellar component. In order to trace the evolution of BSS segregation, we introduce a new parameter (A+), which can be easily measured when the cumulative radial distribution of these stars and a reference population are available. Our simulations show that A+ might also be used as an approximate indicator of the time remaining to the core collapse of the visible component
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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