Examining the Effect of the Map‐making Algorithm on Observed Power Asymmetry inWMAPData

We analyze first-year data of WMAP to determine the significance of asymmetryin summed power between arbitrarily defined opposite hemispheres, using mapsthat we create ourselves with software developed independently of the WMAPteam. We find that over the multipole range l=[2,64], the significance ofasymmetry is ~ 10^-4, a value insensitive to both frequency and power spectrum.We determine the smallest multipole ranges exhibiting significant asymmetry,and find twelve, including l=[2,3] and [6,7], for which the significance -> 0.In these ranges there is an improbable association between the direction ofmaximum significance and the ecliptic plane (p ~ 0.01). Also, contours of leastsignificance follow great circles inclined relative to the ecliptic at thelargest scales. The great circle for l=[2,3] passes over previously reportedpreferred axes and is insensitive to frequency, while the great circle forl=[6,7] is aligned with the ecliptic poles. We examine how changing map-makingparameters affects asymmetry, and find that at large scales, it is renderedinsignificant if the magnitude of the WMAP dipole vector is increased byapproximately 1-3 sigma (or 2-6 km/s). While confirmation of this result wouldrequire data recalibration, such a systematic change would be consistent withobservations of frequency-independent asymmetry. We conclude that the use of anincorrect dipole vector, in combination with a systematic or foreground processassociated with the ecliptic, may help to explain the observed asymmetry.