1,721,118 research outputs found
Spectral imaging of galaxy clusters with Planck
Full text linkThe Sunyaev–Zeldovich (SZ) effect is a promising tool for detecting the presence of hot gas out to the galaxy cluster peripheries. We developed a spectral imaging algorithm dedicated to the SZ observations of nearby galaxy clusters with Planck, with the aim of revealing gas density anisotropies related to the filamentary accretion of materials, or pressure discontinuities induced by the propagation of shock fronts. To optimize an unavoidable trade-off between angular resolution and precision of the SZ flux measurements, the algorithm performs a multi-scale analysis of the SZ maps as well as of other extended components, such as the cosmic microwave background (CMB) anisotropies and the Galactic thermal dust. The demixing of the SZ signal is tackled through kernel-weighted likelihood maximizations. The CMB anisotropies are further analyzed through a wavelet analysis, while the Galactic foregrounds and SZ maps are analyzed via a curvelet analysis that best preserves their anisotropic details. The algorithm performance has been tested against mock observations of galaxy clusters obtained by simulating the Planck High Frequency Instrument and by pointing at a few characteristic positions in the sky. These tests suggest that Planck should easily allow us to detect filaments in the cluster peripheries and detect large-scale shocks in colliding galaxy clusters that feature favorable geometry
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
Comparing the temperatures of galaxy clusters from hydrodynamical N-body simulations to Chandra and XMM-Newton observations
No full textTheoretical studies of the physical processes guiding the formation and
evolution of galaxies and galaxy clusters in the X-ray region are mainly
based on the results of numerical hydrodynamical N-body simulations,
which in turn are often directly compared with X-ray observations.
Although trivial in principle, these comparisons are not always simple.
We demonstrate that the projected spectroscopic temperature of thermally
complex clusters obtained from X-ray observations is always lower than
the emission-weighed temperature, which is widely used in the analysis
of numerical simulations. We show that this temperature bias is mainly
related to the fact that the emission-weighted temperature does not
reflect the actual spectral properties of the observed source. This has
important implications for the study of thermal structures in clusters,
especially when strong temperature gradients, such as shock fronts, are
present. Because of this bias, in real observations shock fronts appear
much weaker than what is predicted by emission-weighted temperature
maps, and may not even be detected. This may explain why, although
numerical simulations predict that shock fronts are a quite common
feature in clusters of galaxies, to date there are very few observations
of objects in which they are clearly seen. To fix this problem we
propose a new formula, the spectroscopic-like temperature function, and
show that, for temperatures higher than 3 keV, it approximates the
spectroscopic temperature to better than a few per cent, making
simulations more directly comparable to observations
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
A dynamical model for the distribution of dark matter and gas in galaxy clusters
No full textUsing the results of an extended set of high-resolution non-radiative
hydrodynamic simulations of galaxy clusters, we obtain simple analytic
formulae for the dark matter and hot gas distribution, in the spherical
approximation. Starting from the dark matter phase-space radial density
distribution, we derive fits for the dark matter density, velocity
dispersion and velocity anisotropy. We use these models to test the
dynamical equilibrium hypothesis through the Jeans equation: we find
that this is satisfied to good accuracy by our simulated clusters inside
their virial radii. This result also shows that our fits constitute a
self-consistent dynamical model for these systems.
We then extend our analysis to the hot gas component, obtaining analytic
fits for the gas density, temperature and velocity structure, with no
further hypothesis on the gas dynamical status or state equation. Gas
and dark matter show similar density profiles down to ~0.06Rv
(with Rv the virial radius), while at smaller radii the gas
flattens, producing a central core. Gas temperatures are almost
isothermal out to roughly 0.2 Rv, then steeply decrease,
reaching at the virial radius a value almost a factor of 2 lower. We
find that the gas is not at rest inside Rv: velocity
dispersions are increasing functions of the radius, motions are
isotropic to slightly tangential, and contribute non-negligibly to the
total pressure support. We test this model using a generalization of the
hydrostatic equilibrium equation, where the gas motion is properly taken
into account. Again we find that the fits provide an accurate
description of the system: the hot gas is in equilibrium and is a good
tracer of the overall cluster potential if all terms (density,
temperature and velocity) are taken into account, while simpler
assumptions cause systematic mass underestimates. In particular, we find
that using the so-called β-model underestimates the true cluster
mass by up to 50 per cent at large radii. We also find that, if gas
velocities are neglected, then a simple isothermal model fares better at
large radii than a non-isothermal one.
The shape of the gas density profile at small radii is at least
partially explained by the gas expansion caused by energy transfer from
dark matter during the collapse. In fact, when gas bulk energy is also
considered, gas and dark matter are in energy equipartition in the final
system at radii r > 0.1Rv, while at smaller radii the gas
is hotter than the dark matter. This energy imbalance is also probably
the reason of the further global halo compression compared with a pure
collisionless collapse, which we point out by comparing the dark matter
and total density profiles of our hydro-simulated clusters with a set of
identical - but pure N-body - ones. The compression has the effect of
raising the mean concentration by an amount of roughly 10 per cent
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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