1,720,970 research outputs found
Peak Number Density of Non-Gaussian Random Fields
No full textAsymptotic expressions are derived for the mean up-crossing rate, size, and number density of excursions above a high level by a stationary, non-Gaussian, random field in D dimensions. These formulas are relevant in the analysis of the large-scale matter distribution in the Universe
Catelan, Lucchin and Matarrese Reply
No full textReply to P. Coles 'Comment on 'Peak Number Density of Non-Gaussian Random Fields'
On the Spatial Distribution of Dark Matter Halos
No full textWe study the spatial distribution of dark matter halos in the universe in terms of their number density contrast, related to the underlying dark matter fluctuation via a nonlocal and nonlinear bias random field. The description of the matter dynamics is simplified by adopting the "truncated" Zeldovich approximation to obtain both analytical results and simulated maps. The halo number density field in our maps and its probability distribution reproduce with excellent accuracy those of halos in a high-resolution N-body simulation with the same initial conditions. Our nonlinear and nonlocal bias prescription matches the N-body halo distribution better than any Eulerian linear and local bias
Weighted Bias and Galaxy Clustering
No full textWe consider a weighted biasing scheme for galaxy clustering. This differs from previous treatments by the fact that the biased density field coincides with the background mass density whenever the latter exceeds a given threshold value. There is some physical motivation for this scheme, and it is in better accord with intuitive ideas than models based on Kalser's analysis of the clustering of rich clusters. We explain how different classes of object could be biased in different ways with respect to the underlying density distribution but still have b = 1, where b is defined by the square root of the ratio of galaxy to mass autocorrelation functions. We also show that if one applies our scheme consistently, a weak dependence of b upon density can be implied. This could also be the reason why the correlation function of galaxies in groups does not differ substantially from the correlation function of all galaxies
Excursion set approach to the clustering of dark matter haloes in Lagrangian space
No full textWe present a stochastic approach to the spatial clustering of dark matter haloes in Lagrangian space. Our formalism is based on a local formulation of the `excursion set' approach by Bond et al., which automatically accounts for the `cloud-in-cloud' problem in the identification of bound systems. Our method allows us to calculate correlation functions of haloes in Lagrangian space using either a multidimensional Fokker-Planck equation with suitable boundary conditions, or an array of Langevin equations with spatially correlated random forces. We compare the results of our method with theoretical predictions for the halo autocorrelation function considered in the literature, and find good agreement with the results recently obtained within a treatment of halo clustering in terms of `counting fields' by Catelan et al. Finally, the possible effect of spatial correlations on numerical simulations of halo merger trees is discussed
The bias field of dark matter haloes
No full textThis paper presents a stochastic approach to the clustering evolution of dark matter haloes in the Universe. Haloes, identified by a Press-Schechter-type algorithm in Lagrangian space, are described in terms of `counting fields', acting as non-linear operators on the underlying Gaussian density fluctuations. By ensemble-averaging these counting fields, the standard Press-Schechter mass function as well as analytic expressions for the halo correlation function and corresponding bias factors of linear theory are obtained, extending the recent results by Mo & White. The non-linear evolution of our halo population is then followed by solving the continuity equation, under the sole hypothesis that haloes move by the action of gravity. This leads to an exact and general formula for the bias field of dark matter haloes, defined as the local ratio between their number density contrast and the mass density fluctuation. Besides being a function of position and `observation' redshift, this random field depends upon the mass and formation epoch of the objects and is both non-linear and non-local. The latter features are expected to leave a detectable imprint on the spatial clustering of galaxies, as described, for instance, by statistics like the bispectrum and the skewness. Our algorithm may have several interesting applications, among which is the possibility of generating mock halo catalogues from low-resolution N-body simulations
Eulerian Perturbation Theory in Non-Flat Universes: Second Order Approximation
No full textThe problem of solving perturbatively the equations describing the evolution of self-gravitating collisionless matter in an expanding universe considerably simplifies when directly formulated in terms of the gravitational and velocity potentials: the problem can be solved exactly, rather than approximately, even for cosmological models with arbitrary density parameter Omega. The Eulerian approach we present here allows us to calculate the higher-order moments of the initially Gaussian density and velocity fields: in particular, we compute the gravitationally induced skewness of the density and velocity-divergence fields for any value of Omega, confirming the extremely weak Omega-dependence of the skewness previously obtained via Lagrangian perturbation theory. Our results show that the separability assumption of higher-order Eulerian perturbative solutions is restricted to the Einstein-de Sitter case only
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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