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Weekly Seminars for March 2009 Print E-mail

Monday 30th March 2009, 10:00 A.M.

Università "La Sapienza" Roma - Sala Direzione INFN (Physics Dept., Old Building - 2nd floor)

Speaker:  Prof. Jean Einasto (Estonian Academy of Sciences - Tartu Observatory)

Title: The Large-scale structure of the Universe

Abstract: The formation of the large-scale structure of the Universe is studied using Sloan Digital Sky Survey data and numerical simulations of the structure evolution. To understand how density perturbations of various scales influence the formation of the structure we use wavelet technique. We show that the distribution of matter at the present epoch is very far from a random one with random amplitudes and phases of density waves. Superclusters and other rich galaxy systems form in places where density waves of different scale have density maxima in similar locations, i.e. Phases are syncronized. Numerical experiments show that, in accordance with Zeldovich approximation, density waves increase the amplitude during the evolution, but not phases. In other words, seeds of future clusters and superclusters were present already in the very early phase of the evolution of the universe, when dark matter was made free of high radiation background.

Tuesday 31st March 2009, 2:00 P.M.

Università "La Sapienza" Roma - Aula Conversi (Physics Dept., Old Building - 1st floor )

Speaker:  Prof. Jean Einasto (Estonian Academy of Sciences - Tartu Observatory)

Title: Void structure in the Universe

Abstract: The presence of voids in the distribution of galaxies is well known, however it is not clear why even faint dwarf galaxies do not form in voids. Small-scale primordial fluctuations were present in the early stages of the evolution of the universe everywhere, thus the question is: Why in some regions (superclusters) small-scale perturbation evolve to form galaxies, and in other regions (voids) do not. We have used numerical simulations with various size of the simulation box (256, 100, 64 Mpc) to investigate the formation of galaxies (halos) in different environment. To understand the influence of density perturbations of various scales we have used conventional simulations, where perturbations of all scales are present. Additionally we used truncated simulations where density perturbations on scales larger than a certain limit are cutted, i.e. Aplitudes on respective scales were taken equal to zero initially. The cutoff wavelength was varied from 4, 8, 16 to 32 Mpc. Our results show that in truncated models galaxies and galaxy systems form over the whole simulation box. In full models the combined action of large-scale perturbations amplifies galaxy systems in high-density environment (superclusters) and supresses in low-density environmnet (voids). In deep voids the level of all perturbations is so low (due to combined effect of negative parts of large-scale waves), that even maxima of small-scale perturbations do not reach the overall mean density level, thus all small-scale perturbations are damped during the evolution. This explains the absence of even dwarf galaxies in large voids.

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