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, . 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 to . 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.