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Abstract: We propose a simple way to estimate the parameter beta = Omega m^0.6-b fromthree-dimensional galaxy surveys. Our method consists in measuring the relationbetween the cosmological velocity and gravity fields, and thus requirespeculiar velocity measurements. The relation is measured *directly in redshiftspace*, so there is no need to reconstruct the density field in real space. Inlinear theory, the radial components of the gravity and velocity fields inredshift space are expected to be tightly correlated, with a slope given, inthe distant observer approximation, by g - v = 1 + 6 beta - 5 + 3 beta^2 -7^1-2 - beta. We test extensively this relation using controlled numericalexperiments based on a cosmological N-body simulation. To perform themeasurements, we propose a new and rather simple adaptive interpolation schemeto estimate the velocity and the gravity field on a grid. One of the moststriking results is that nonlinear effects, including `fingers of God-, affectmainly the tails of the joint probability distribution function PDF of thevelocity and gravity field: the 1-1.5 sigma region around the maximum of thePDF is *dominated by the linear theory regime*, both in real and redshiftspace. This is understood explicitly by using the spherical collapse model as aproxy of nonlinear dynamics. Applications of the method to real galaxy catalogsare discussed, including a preliminary investigation on homogeneous volumelimited `galaxy- samples extracted from the simulation with simpleprescriptions based on halo and sub-structure identification, to quantify theeffects of the bias between the galaxy and the total matter distibution, and ofshot noise ABRIDGED.

Autor: Stéphane Colombi IAP Paris, Michał Chodorowski Copernicus, Romain Teyssier CEA Saclay



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