Diego Sáez

On the Role of Shock Waves in Galaxy Cluster Evolution

Numerical simulations of galaxy clusters including two species—baryonic gas and dark matter particles—are presented. A cold dark matter spectrum, Gaussian statistics, and flat universe are assumed. The dark-matter component is evolved numerically by means of a standard particle mesh method. The evolution of the baryonic component has been studied numerically by using a multidimensional (three-dimensional) hydrodynamical code based on modern high-resolution shock-capturing techniques. These techniques are specially designed for treating accurately complex flows in which shocks appear and interact. With this picture, the role of shock waves in the formation and evolution of rich galaxy clusters is analyzed. Our results display two well-differentiated morphologies of the shocked baryonic matter: filamentary at early epochs and quasi-spherical at low redshifts.

A relativistic approach to gravitational instability in the expanding Universe: second-order Lagrangian solutions

A Lagrangian relativistic approach to the non–linear dynamics of cosmological perturbations of an irrotational collisionless fluid is considered. Solutions are given at second order in perturbation theory for the relevant fluid and geometric quantities and compared with the corresponding ones in the Newtonian approximation. Specifically, we compute the density, the volume expansion scalar, the shear, the “electric” part, or tide, and the “magnetic” part of the Weyl tensor. The evolution of the shear and the tide beyond the linear regime strongly depends on the ratio of the characteristic size of the perturbation to the cosmological horizon distance. For perturbations on sub– horizon scales the usual Newtonian approximation applies, at least at the considered perturbative order; on super–horizon scales, instead, a new picture emerges, which we call “silent universe”, as each fluid element evolves independently of the environment, being unable to exchange signals with the surrounding matter through either sound waves or gravitational radiation. For perturbations inside the Hubble radius particular attention is paid in singling out non–local effects during the non–linear evolution of fluid elements. These non–local effects are shown to be carried by a traceless and divergenceless tensor, contained in the magnetic part of the Weyl tensor, which is dynamically generated as soon as the system evolves away from the linear regime.

A Multidimensional Hydrodynamic Code for Structure Evolution in Cosmology

A cosmological multidimensional hydrodynamic code is described and tested. This code is based on modern high-resolution shock-capturing techniques. It can make use of a linear or a parabolic cell reconstruction as well as an approximate Riemann solver. The code has been specifically designed for cosmological applications. Two tests including shocks have been considered: the first one is a standard shock tube and the second test involves a spherically symmetric shock. Various additional cosmological tests are also presented. In this way, the performance of the code is proved. The usefulness of the code is discussed; in particular, this powerful tool is expected to be useful in order to study the evolution of the hot gas component located inside nonsymmetric cosmological structures.

On the microwave background anisotropies produced by nonlinear voids

The Tolman-Bondi solution of the Einstein equations is used to study the microwave background anisotropy produced by a pressureless spherical cosmological inhomogeneity. Our method improves on previous ones because it does not involve any approximating condition and it allows us to assume the following: (1) a general Friedmann-Robertson-Walker background, (2) an arbitrary relative location of the observer and the inhomogeneity, (3) either an overdense or an underdense inhomogeneity with arbitrary size, (4) an arbitrary initial energy density profile, and (5) an arbitrary amplitude of the density contrast

On the Rees-Sciama effect: maps and statistics

Small maps of the Rees–Sciama (RS) effect are simulated by using an appropriate N-body code and a certain ray-tracing procedure. A method designed for the statistical analysis of cosmic microwave background (CMB) maps is applied to study the resulting simulations. These techniques, recently proposed – by our team – to consider lens deformations of the CMB, are adapted to deal with the RS effect. This effect and the deviations from Gaussianity associated to it seem to be too small to be detected in the near future. This conclusion follows from our estimation of both the RS angular power spectrum and the RS reduced n-direction correlation functions for n≤ 6.

Beam deconvolution in noisy CMB maps

The subject of this paper is beam deconvolution in small angular scale CMB experiments. The beam effect is reversed using the Jacobi iterative method, which was designed to solved systems of algebraic linear equations. The beam is a non circular one which moves according to the observational strategy. A certain realistic level of Gaussian instrumental noise is assumed. The method applies to small scale CMB experiments in general (cases A and B), but we have put particular attention on PLANCK mission at 100 GHz (cases C and D). In cases B and D, where noise is present, deconvolution allows to correct the main beam distortion effect and recover the initial angular power spectrum up to the end of the fifth acoustic peak. An encouraging result whose importance is analyzed in detail. More work about deconvolution in the presence of other systematics is in progress. This paper is related to the PLANCK LFI activities.

Small Angular Scale Simulations of the Microwave Sky

We describe and compare two types of microwave sky simulations that are good for small angular scales. The first type uses expansions in spherical harmonics, and the second one is based on plane waves and the fast Fourier transform. The angular power spectrum is extracted from maps corresponding to both types of simulations, and the resulting spectra are appropriately compared. In this way, the features and usefulness of Fourier simulations are pointed out. For l ≥ 100, all the simulations lead to similar accuracies; however, the CPU cost of Fourier simulations is ~ 10 times smaller than that for spherical harmonic simulations. For l ≤ 100, the simulations based on spherical harmonics seem to be preferable.

Galaxy clusters and microwave background anisotropy

Previous estimates of the microwave background anisotropies produced by freely falling spherical clusters are discussed. These estimates are based on the Swiss-Cheese and Tolman-Bondi models. It is proved that these models give only upper limits to the anisotropies produced by the observed galaxy clusters. By using spherically symmetric codes including pressureless matter and a hot baryonic gas, new upper limits are obtained. The contributions of the hot gas and the pressureless component to the total anisotropy are compared. The effects produced by the pressure are proved to be negligible; hence, estimations of the cluster anisotropies based on N-body simulations are hereafter justified. After the phenomenon of violent relaxation, any realistic rich cluster can only produce small anisotropies with amplitudes of order

Cosmology in a certain vector-tensor theory of gravitation

10^{-7}

Secondary gravitational anisotropies in open universes

. During the rapid process of violent relaxation, the anisotropies produced by nonlinear clusters are expected to range in the interval