Non-Gaussian signatures in the temperature fluctuation observed by the Wilkinson Microwave Anisotropy Probe

We present results from a test for the Gaussianity of the whole sky subdegree-scale cosmic microwave background (CMB) temperature anisotropy measured by the Wilkinson Microwave Anisotropy Probe (WMAP). We calculate the genus from the foreground-subtracted and Kp0-masked WMAP maps and measure the genus shift parameters defined at negative and positive threshold levels (Deltav- and Deltav+) and the asymmetry parameter (Deltag) to quantify the deviation from the Gaussian relation. At WMAP Q, V and W bands, the genus and genus-related statistics imply that the observed CMB sky is consistent with a Gaussian random phase field. However, from the genus measurement on the Galactic northern and southern hemispheres, we have found two non-Gaussian signatures at the W-band resolution (0.degrees35 scale), i.e. the large difference of genus amplitudes between the north and the south and the positive genus asymmetry in the south, which are statistically significant at 2.6 and 2.4sigma levels, respectively. The large genus amplitude difference also appears in the WMAP Q- and V-band maps, deviating from the Gaussian prediction with a significance level of about 2sigma. The probability that the genus curves show such a large genus amplitude difference exceeding the observed values at all Q, V and W bands in a Gaussian sky is only 1.4 per cent. Such non-Gaussian features are reduced as the higher Galactic cut is applied, but their dependence on the Galactic cut is weak. We discuss possible sources that can induce non-Gaussian features, such as the Galactic foregrounds, the integrated Sachs-Wolfe and the Sunyaev-Zel'dovich effects, and the reionization-induced low l-mode non-Gaussianity that are aligned along the Galactic plane. We conclude that CMB data with higher signal-to-noise ratio and an accurate foreground model are needed to understand the non-Gaussian signatures.