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Increasing Evidence for Hemispherical Power Asymmetry in the Five-Year WMAP Data

Hoftuft, J. and Eriksen, H. K. and Banday, A. J. and Gόrski, K. M. and Hansen, F. K. and Lilje, P. B. (2009) Increasing Evidence for Hemispherical Power Asymmetry in the Five-Year WMAP Data. Astrophysical Journal, 699 (2). pp. 985-989. ISSN 0004-637X.

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Motivated by the recent results of Hansen et al. concerning a noticeable hemispherical power asymmetry in the Wilkinson Microwave Anisotropy Probe (WMAP) data on small angular scales, we revisit the dipole-modulated signal model introduced by Gordon et al.. This model assumes that the true cosmic microwave background signal consists of a Gaussian isotropic random field modulated by a dipole, and is characterized by an overall modulation amplitude, A, and a preferred direction, ṕ. Previous analyses of this model have been restricted to very low resolution (i.e., 3°.6 pixels, a smoothing scale of 9° FWHM, and ℓ ≾40) due to computational cost. In this paper, we double the angular resolution (i.e., 1°.8 pixels and 4°.5 FWHM smoothing scale), and compute the full corresponding posterior distribution for the five-year WMAP data. The results from our analysis are the following: the best-fit modulation amplitude for ℓ ≤ 64 and the ILC data with the WMAP KQ85 sky cut is A = 0.072 ± 0.022, nonzero at 3.3σ, and the preferred direction points toward Galactic coordinates (l, b) = (224°, – 22°) ± 24°. The corresponding results for ℓ ≾ 40 from earlier analyses were A = 0.11 ± 0.04 and (l, b) = (225°, – 27°). The statistical significance of a nonzero amplitude thus increases from 2.8σ to 3.3σ when increasing ℓ_(max) from 40 to 64, and all results are consistent to within 1σ. Similarly, the Bayesian log-evidence difference with respect to the isotropic model increases from Δln E = 1.8 to Δln E = 2.6, ranking as "strong evidence" on the Jeffreys' scale. The raw best-fit log-likelihood difference increases from Δln L=6.1 to Δln L=7.3. Similar, and often slightly stronger, results are found for other data combinations. Thus, we find that the evidence for a dipole power distribution in the WMAP data increases with ℓ in the five-year WMAP data set, in agreement with the reports of Hansen et al.

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Additional Information:© 2009. The American Astronomical Society. Received 2009 March 6; accepted 2009 April 30; published 2009 June 17. H.K.E. acknowledges financial support from the Research Council of Norway. The computations presented in this paper were carried out on Titan, a cluster owned and maintained by the University of Oslo and NOTUR. Some of the results in this paper have been derived using the HEALPix (Gόrski 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:20090902-083209737
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Official Citation:Increasing Evidence for Hemispherical Power Asymmetry in the Five-Year WMAP Data J. Hoftuft, H. K. Eriksen, A. J. Banday, K. M. Górski, F. K. Hansen, and P. B. Lilje 2009 ApJ 699 985-989 doi: 10.1088/0004-637X/699/2/985
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:15548
Deposited By: Ruth Sustaita
Deposited On:14 Sep 2009 19:22
Last Modified:03 Oct 2019 00:59

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