1,721,029 research outputs found
Intracluster Entropy from Joint X-Ray and Sunyaev-Zel'dovich Observations
No full textThe temperature and density of the hot diffuse medium pervading galaxy groups and clusters combine into one significant quantity, the entropy. Here we express the entropy levels and profiles in model-independent forms by joining two observables: the X-ray luminosity and the change in the cosmic microwave background (CMB) intensity due to the Sunyaev-Zel'dovich (SZ) effect. Thus we present both global scaling relations for the entropy levels from clusters and groups, and a simple expression yielding the entropy profiles in individual clusters from resolved X-ray surface brightness and SZ spatial distributions. We propose that our approach provides two useful tools for comparing large data samples with models, in order to probe the processes that govern the thermal state of the hot intracluster medium. The feasibility of using such a diagnostic for the entropy is quantitatively assessed, based on current X-ray and upcoming SZ measurements
Cosmic microwave background anisotropy induced by gas in clusters of galaxies
No full textThe spectral change induced by Compton scattering of the cosmic microwave background radiation off hot electron gas in clusters of galaxies is an important component of the anisotropy on arcminute scales. The level and spatial characteristics of this anisotropy are explored in detail in the context of flat cold (taking 0.8 and 1 for the index of the density fluctuation power spectrum) and mixed dark matter models. Properties of intracluster gas and its evolution are directly modeled based on X-ray measurements, with an implied decrease in the gas mass fraction with increasing redshift. Our calculations yield levels of rms temperature anisotropy, (DELTAT/T)rms, almost-equal-to a few 10(-6) for a wide range of angular scales and in the context of realistic models for the intracluster gas evolution and spatial distribution. This is the minimum level of anisotropy expected on subdegree angular scales if the universe underwent a phase of late reheating
Structure and History of Dark Matter Halos in Galaxies and Galaxy Systems
No full textWe investigate the structure and history of Dark Matter (DM) halos in galaxies and galaxy systems. Our theoretical framework is provided by the two-stage cosmogonical development of DM halos, and by the related “α-profiles”. The latter solve the Jeans equation for the self-gravitating DM equilibria, and yield the radial runs of the density ρ(r) and the velocity dispersion σr2(r) in terms of the DM “entropy” K≡σr2/ρ2/3∝rα highlighted by recent N-body simulations to have a uniform slope α within the halo “body”. The former constrains the entropy slope α to a value within the narrow range 1.25–1.3; such a value applies in the halo body since the transition time that, both in our semianalytic description and in state-of-the-art numerical simulations, is found to separate two stages in the development of a DM halo: an early fast collapse including a few violent major mergers building up the halo body by dynamical relaxation; and a later, quasi-equilibrium stage during which the body is almost unaffected while the outskirts develop from the inside-out by minor mergers and smooth accretion. These physically based α-profiles meet the overall requirements from gravitational lensing observations, being intrinsically flatter at the center and steeper in the outskirts relative to the empirical NFW formula. In quantitative detail, we test them with the recent extensive dataset from weak and strong lensing observations in and around the cluster A1689. We find an optimal fit at both small and large scales in terms of a halo constituted by an early body with α≈1.25 and by recent extended outskirts making up a concentration parameter c≈10; we consistently interpret the latter value in terms of the variance expected in the two-stage halo development under the standard ΛCDM cosmology
Intracluster Comptonization of the Cosmic Microwave Background: Mean Spectral Distortion and Cluster Number Counts
No full textThe mean sky-averaged Comptonization parameter, ȳ, describing the scattering of the cosmic microwave background (CMB) by hot gas in clusters of galaxies, is calculated in an array of flat and open cosmological and dark matter models. The models are globally normalized to fit cluster X-ray data, and intracluster gas is assumed to have evolved in a manner consistent with current observations. We predict values of ȳ lower than the COBE/FIRAS upper limit. The corresponding values of the overall optical thickness to Compton scattering are <~10-4 for relevant parameter values. Of more practical importance are number counts of clusters across which a net flux (with respect to the CMB) higher than some limiting value can be detected. Such number counts are specifically predicted for the COBRAS/SAMBA and BOOMERANG missions
The supermodel of the intracluster plasma
No full textThe equilibria of the intracluster plasma (ICP) and of the gravitationally dominant dark matter (DM) are governed by the hydrostatic and the Jeans equation, respectively. Jeans, with the DM “entropy” set to K∝rα and α~1.25–1.3 applying from groups to rich clusters, yields our radial α-profiles for DM density and gravitational potential. In the ICP the entropy run k(r) is mainly shaped by shocks, as steadily set by supersonic accretion of gas at the cluster boundary, and intermittently driven from the center by merging events or by AGNs; the resulting equilibrium is described by the exact yet simple formalism constituting the ICP “Supermodel”. With a few parameters, this accurately represents the runs of density n(r) and temperature T(r) as required by recent X-ray data on surface brightness and spectroscopy for both cool core (CC) and non cool core (NCC) clusters; the former are marked by a middle temperature peak, whose location is predicted from rich clusters to groups. The Supermodel inversely links the inner runs of n(r) and T(r), and highlights their central scaling with entropy nc∝kc−1 and Tc∝kc0.35, to yield radiative cooling times tc≈0.3 (kc/15keV cm2)1.2Gy. We discuss the stability of the central values so focused both in CC and NCC clusters. From the Supermodel we derive as limiting cases the classic polytropic β-models, and the “mirror” model with T(r)∝σ2(r) suitable for NCC and CC clusters, respectively; these highlight how the ICP temperature T(r) tends to mirror the DM velocity dispersion σ2(r) away from entropy injections. Finally, we discuss how the Supermodel connects information derived from X-ray and gravitational lensing observations
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
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