| A. Ciências Exatas e da Terra - 1. Astronomia - 3. Astrofísica Extragaláctica |
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| FORMATION OF LARGE SCALE STRUCTURES IN A HOMOGENEOUS DARK ENERGY UNIVERSE USING AN X-RAY GALAXY CLUSTER DATA SET |
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Lucio Marassi de Souza Almeida 1
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1. Escola de Ciências e Tecnologia - Universidade Federal do Rio Grande do Norte
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| INTRODUÇÃO: |
| It is now well known that the energy density of the Universe is dominated by a dark energy component, which cause the present accelerated expansion. The large scale structure formation can be used to probe dark energy models, and the mass function is one of the best statistical tools to perform this study. We present here a statistical analysis of mass functions of galaxies under a homogeneous dark energy model, proposed in the work of Percival (2005), using an observational flux-limited X-ray cluster survey, and CMB data. We compare, in our analysis, the standard Press-Schechter (PS) approach (where a Gaussian distribution is used in the primordial density field), and the PL (Power Law) mass function (where we apply instead a nonextensive q-statistical distribution). We conclude that the PS mass function cannot explain at the same time the X-ray and the CMB data, and the PS best fit dark energy equation of state parameter is w=-0.58, which is distant from the cosmological constant case. The PL mass function provides better fits to the HIFLUGCS X-ray galaxy data and the CMB data; we also note that the w parameter is very sensible to modifications in the PL free parameter ('q') suggesting that the PL mass function could be a powerful tool to constrain dark energy models. |
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| METODOLOGIA: |
| We use statistical methods of chi-squared (generating elliptical statistical contours of confidence level) for the cosmological parameters of the theoretical model. We also use intensively the Interactive Data Language program (IDL 7.0) - a powerful data analysis and visualization program - and subroutines of FORTRAN 90 in our analysis. For the data set, we applied the HIFLUGCS X-ray galaxy cluster catalog (Reiprich & Boehringer 2002), which gives galaxy cluster observational data in low redshifts. We study the non-extensive thermodynamics and kinetic theory, to apply these methods to the structure formation, and deriving the PL mass function we use in this work. For the Cosmic Microwave Background (CMB) radiation data, we use the WMAP 3 years data set from Spergel et al. (2007). And we apply the simulations and data analysis based on the Homogeneous Dark Energy Cosmological Model of Percival (2005). |
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| RESULTADOS: |
| We performed a chi-squared statistical procedure using the X-ray HIFLUGCS data sample (Reiprich & Boehringer 2002), and so we determined the statistical confidence contours for the pair of parameters, sigma-8 (the rms density fluctuation at spheres of 8 h-1Mpc) and Omega_m (the Matter Parameter of the Universe). We applied the homogeneous dark energy model of
Percival (2005), and we marginalized over all possible values of the dark energy equation of state parameter (w). We show the contours in the Omega_m-sigma-8 plane obtained by using the PS and the PL approaches. For the PS results: the best-fit values to the HIFLUGCS X-ray galaxy cluster data are, respectively, very low and very high when compared to the nowadays independent CMB Results (from WMAP3 - Spergel 2007). The contours using the Gaussian distribution (the left panel) does not intercept the CMB values. In the other hand, by using the PL mass function with a certain 'q' free parameter, we could overlap the WMAP independent data, for almost the same dark energy w parameter. We see that, for almost the same particular model of dark energy, we obtain better cosmological parameters using the PL mass function, while the standard PS mass function cannot explain the independent WMAP data even at 99% confidence level. |
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| CONCLUSÃO: |
| In this work we perform a statistical analysis, using the X-ray galaxy cluster data, and the independent Cosmic Microwave Background radiation data from WMAP (Spergel 2007), to study the influence of galaxy mass functions under a homogeneous dark energy background. Performing a chi-squared analysis and marginalizing over the 'w' dark energy equation of state parameter, we compare the standard Gaussian Press-Schechter mass function to the PL mass function, which presents a physically motivated free parameter (q). We conclude that the PS mass function cannot explain at the same time the X-ray and the CMB data, even at 3-sigma confidence level, which imposes a problem to the standard Gaussian scenario. Also, the PS approach presents a best fit dark energy equation of state parameter of w=-0.58, which is clearly distant from independent estimates of w=-1 (a cosmological constant). Observing the contours, the PL mass function provides better fits to the HIFLUGCS and the CMB independent data, with almost the same best-fit for the cosmological parameters, but with a huge modification in the best-fit dark energy 'w' parameter, as we change 'q'. This strongly suggests that the PL mass function could be a powerful tool to constrain dark energy models. |
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| Palavras-chave: Mass Function, Galaxy Cluster, Structure Formation. |