Browsing by Subject "statistical"
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Item Bullet Cluster: A Challenge To Lambda CDM Cosmology(2010-07) Lee, Jounghun; Komatsu, Eiichiro; Komatsu, EiichiroTo quantify how rare the bullet-cluster-like high-velocity merging systems are in the standard Lambda cold dark matter (CDM) cosmology, we use a large-volume (27 h(-3) Gpc(3)) cosmological N-body MICE simulation to calculate the distribution of infall velocities of subclusters around massive main clusters. The infall velocity distribution is given at (1-3) R(200) of the main cluster (where R(200) is similar to the virial radius), and thus it gives the distribution of realistic initial velocities of subclusters just before collision. These velocities can be compared with the initial velocities used by the non-cosmological hydrodynamical simulations of 1E0657-56 in the literature. The latest parameter search carried out by Mastropietro & Burkert has shown that an initial velocity of 3000 km s(-1) at about 2R(200) is required to explain the observed shock velocity, X-ray brightness ratio of the main and subcluster, X-ray morphology of the main cluster, and displacement of the X-ray peaks from the mass peaks. We show that such a high infall velocity at 2R(200) is incompatible with the prediction of a Lambda CDM model: the probability of finding 3000 km s(-1) in (2-3) R(200) is between 3.3 x 10(-11) and 3.6 x 10(-9). A lower velocity, 2000 km s(-1) at 2R(200), is also rare, and moreover, Mastropietro & Burkert have shown that such a low initial velocity does not reproduce the X-ray brightness ratio of the main and subcluster or morphology of the main cluster. Therefore, we conclude that the existence of 1E0657-56 is incompatible with the prediction of a Lambda CDM model, unless a lower infall velocity solution for 1E0657-56 with less than or similar to 1800 km s(-1) at 2R(200) is found.Item Five-Year Wilkinson Microwave Anisotropy Probe (WMAP*) Observations: Bayesian Estimation Of Cosmic Microwave Background Polarization Maps(2009-08) Dunkley, J.; Spergel, D. N.; Komatsu, Eiichiro; Hinshaw, G.; Larson, D.; Nolta, M. R.; Odegard, N.; Page, L.; Bennett, C. L.; Gold, B.; Hill, R. S.; Jarosik, N.; Weiland, J. L.; Halpern, M.; Kogut, A.; Limon, M.; Meyer, S. S.; Tucker, G. S.; Wollack, E.; Wright, Edward L.; Komatsu, EiichiroWe describe a sampling method to estimate the polarized cosmic microwave background (CMB) signal from observed maps of the sky. We use a Metropolis-within-Gibbs algorithm to estimate the polarized CMB map, containing Q and U Stokes parameters at each pixel, and its covariance matrix. These can be used as inputs for cosmological analyses. The polarized sky signal is parameterized as the sum of three components: CMB, synchrotron emission, and thermal dust emission. The polarized Galactic components are modeled with spatially varying power-law spectral indices for the synchrotron, and a fixed power law for the dust, and their component maps are estimated as by-products. We apply the method to simulated low-resolution maps with pixels of side 7.2 deg, using diagonal and full noise realizations drawn from the WMAP noise matrices. The CMB maps are recovered with goodness of fit consistent with errors. Computing the likelihood of the E-mode power in the maps as a function of optical depth to reionization, tau, for fixed temperature anisotropy power, we recover tau = 0.091 +/- 0.019 for a simulation with input tau = 0.1, and mean tau = 0.098 averaged over 10 simulations. A "null" simulation with no polarized CMB signal has maximum likelihood consistent with tau = 0. The method is applied to the five-year WMAP data, using the K, Ka, Q, and V channels. We find tau = 0.090 +/- 0.019, compared to tau = 0.086 +/- 0.016 from the template-cleaned maps used in the primary WMAP analysis. The synchrotron spectral index, beta, averaged over high signal-to-noise pixels with standard deviation sigma(beta) < 0.25, but excluding similar to 6% of the sky masked in the Galactic plane, is - 3.03 +/- 0.04. This estimate does not vary significantly with Galactic latitude, although includes an informative prior.