INCREASING EVIDENCE FOR HEMISPHERICAL POWER ASYMMETRY IN THE FIVE-YEAR WMAP DATA

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, (p) over cap. Previous analyses of this model have been restricted to very low resolution (i.e., 3 degrees.6 pixels, a smoothing scale of 9 degrees FWHM, and l less than or similar to 40) due to computational cost. In this paper, we double the angular resolution (i.e., 1 degrees.8 pixels and 4 degrees.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 3.3 sigma, and the preferred direction points toward Galactic coordinates (l, b) = (224 degrees, -22 degrees) +/- 24 degrees. The corresponding results for l less than or similar to 40 from earlier analyses were A = 0.11 +/- 0.04 and (l, b) = (225 degrees,-22 degrees). The statistical significance of a nonzero amplitude thus increases from 2.8 sigma to 3.3 sigma when increasing l(max) from 40 to 64, and all results are consistent to within 1 sigma. Similarly, the Bayesian log-evidence difference with respect to the isotropic model increases from Delta ln E = 1.8 to Delta ln E = 2.6, ranking as strong evidence on the Jeffreys' scale. The raw best-fit log-likelihood difference increases from Delta ln L = 6.1 to Delta ln 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.