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Hemispherical Power Asymmetry in the Third-Year Wilkinson Microwave Anisotropy Probe Sky Maps

Eriksen, H. K. and Banday, A. J. and Górski, K. M. and Hansen, F. K. and Lilje, P. B. (2007) Hemispherical Power Asymmetry in the Third-Year Wilkinson Microwave Anisotropy Probe Sky Maps. Astrophysical Journal Letters, 660 (2). L81-L84. ISSN 2041-8205. doi:10.1086/518091.

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We consider the issue of hemispherical power asymmetry in the third-year WMAP data, adopting a previously introduced modulation framework. Computing both frequentist probabilities and Bayesian evidences, we find that the model consisting of an isotropic CMB sky modulated by a dipole field gives a substantially better fit to the observations than the purely isotropic model, even when accounting for the larger prior volume. For the ILC map, the Bayesian log-evidence difference is ~1.8 in favor of the modulated model, and the raw improvement in maximum log likelihood is 6.1. The best-fit modulation dipole axis points toward (l, b) = (225°, -27°), and the modulation amplitude is 0.114, in excellent agreement with the results from the first-year analyses. The frequentist probability of obtaining such a high modulation amplitude in an isotropic universe is ~1%. These results are not sensitive to data set or sky cut. Thus, the statistical evidence for a power asymmetry anomaly is both substantial and robust, although not decisive, for the currently available data. Increased sky coverage through better foreground handling and full-sky and high-sensitivity polarization maps may shed further light on this issue.

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Additional Information:© 2007 American Astronomical Society. Received 2007 January 4; accepted 2007 March 14; published 2007 April 10. H. K. E. acknowledges financial support from the Research Council of Norway. Some of the results in this Letter have been derived using the HEALPix (Go´rski et al. 2005) software and analysis package. We acknowledge use of the Legacy Archive for Microwave Background Data Analysis (LAMBDA). Support for LAMBDA is provided by the NASA Office of Space Science.
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Research Council of NorwayUNSPECIFIED
Subject Keywords:cosmic microwave background; cosmology: observations; methods: statistical
Issue or Number:2
Record Number:CaltechAUTHORS:20100421-095229602
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Official Citation:H. K. Eriksen et al 2007 ApJ 660 L81 doi: 10.1086/518091
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:18034
Deposited By: Jason Perez
Deposited On:21 Apr 2010 18:41
Last Modified:08 Nov 2021 23:40

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