Detection of non-Gaussianity in the WMAP 1-year data using spherical wavelets

A non-Gaussian detection in the WMAP 1-year data is reported. The detection has been found in the combined Q-V-W WMAP proposed by the WMAP team (Komatsu et al. 2003) after applying a wavelet technique based on the Spherical Mexican Hat Wavelet (SMHW). The skewness and the kurtosis of the SMHW coefficients are calculated at different scales (ranging from a few arcmins to tens of degrees). A non-Gaussian signal is detected at scales of the SMHW around 4◦ (size in the sky of around 10◦). The significance of this detection is ≥ 99.9%. In addition a study of Gaussianity is performed in each hemisphere. The northern hemisphere is compatible with Gaussianity, whereas the southern one deviates from Gaussianity at a significance level > 99.9%. Systematics and foregrounds are carefully studied in order to identify the possible source of non-Gaussianity. Contribution from systematic effects (related to noise and beams) is rejected: 1) each one of the Q, V and W maps shows the same non-Gaussianity pattern, and 2) several combinations of the different receivers at each band –that highly reduce the CMB and the foreground emissions– do not show this non-Gaussian pattern. Similarly, the Galactic foregrounds do not contribute significantly to the non-Gaussian detection: non-Gaussianity is detected in all all the WMAP maps (from 23 GHz to 94 GHz) and no frequency dependence is observed. Moreover, the expected foreground contribution to the combined WMAP map was added to CMB Gaussian simulations showing a behaviour compatible with the Gaussian model. Hence, possible intrinsic temperature fluctuations (like secondary anisotropies and primordial features) can not be rejected as the source of this non-Gaussian detection. We remark that our result implies not only asymmetries north/south –like other previous WMAP analyses– but also a direct non-Gaussian detection. Subject headings: methods: data analysis – cosmic microwave background e-mail: [email protected]