Inhomogeneity In Intracluster Medium And Its Cosmological Implications
Creators: name: Kawahara, Hajime
Publisher: name: Zenodo
Year: 2009

description: <em>Dissertation, 2009, The University of Tokyo</em>

Recent progress both in numerical simulations and observations has improved physical modeling of galaxy clusters beyond a simple isothermal and spherical approximation for a variety of astrophysical and cosmological applications such as departure from isothermal distribution and non-spherical effect. Despite the previous studies, no physical model has been proposed for the statistical nature of inhomogeneities in the intracluster medium (ICM). We investigate the nature of inhomogeneity in ICM. We construct a statistical model of the density and temperature fluctuations on the basis of cosmological hydrodynamic simulations. We find that the local inhomogeneities of the temperature and density, after corrected for the global radial profiles, have nearly a universal distribution that resembles the lognormal probability density function.

With the lognormal model of the inhomogeneity, we investigate its impact on cosmological applications of galaxy clusters. We explore the origin of the recently reported systematic bias in the spectroscopic temperature of galaxy clusters. We develop an analytical model that explains the bias in the spectroscopic temperature. On the basis of the analytical model, we find that not only the radial profiles but also the local inhomogeneities are largely responsible for the above mentioned bias of cluster temperatures.

We also apply the lognormal model to the Hubble constant measurement from the combined analysis of the Sunyaev-Zel’dovich effect and X-ray observations of galaxy clusters. The Hubble constant estimated from this method has been known to be systematically lower than those from other methods by 10-15 percent. We examine the origin of the systematic underestimate and compare the prediction with idealistic triaxial models and with clusters extracted from cosmological hydrodynamic simulations. We identify three important sources for the systematic errors; density and temperature inhomogeneities in the ICM, departures from isothermality, and asphericity. We find that these systematics well reproduce both the observed bias and the intrinsic dispersions of the Hubble constant estimated from the Sunyaev-Zel’dovich effect.

In order to compare the lognormal model directly with X-ray observations, we develop a method of extracting statistical information about the density fluctuations from the X-ray surface brightness. Performing mock observations, we find that the resulting X-ray surface brightness fluctuations also follow the lognormal distribution fairly well. We analyze Chandra observations of the galaxy cluster Abell 3667, and find that its X-ray surface brightness fluctuations follow the lognormal distribution. While the lognormal model was originally motivated by hydrodynamic simulations, this is the observational confirmation of the lognormal signature in a real cluster.

We conclude that the inhomogeneity in the ICM significantly affects the interpretations of cosmological implications of galaxy clusters. The statistical model we develop will be important to understand the current and future results of observations.
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