Non-Gaussianity due to Possible Residual Foreground Signals in WMAP 1st-year Data Using Spherical Wavelet Approaches

Abstract

We perform multi-scale non-Gaussianity detection and localization to the Wilkinson Microwave Anisotropy Probe (WMAP) first-year data in both wavelet and real spaces. Such an analysis is facilitated by spherical wavelet transform and inverse transform techniques developed by the YAWtb team. Skewness and kurtosis as test statistics are calculated on scales from about $1^{\circ}$ to $30^{\circ}$ on the sky as well as toward different directions using anisotropic spherical Morlet wavelet (SMW). A maximum deviation from Gaussian simulations with a right tail probability of $\sim 99.9%$ is detected at an angular scale of $\sim12^{\circ}$ at an azimuthal orientation of $\sim 0^{\circ}$ on the sky. In addition, some significant non-Gaussian spots have been identified and localized in real space from both the combined Q-V-W map recommended by the WMAP team and the Tegmark foreground-cleaned map. Systematic effects due to beams and noise can be rejected as the source of this non-Gaussianity. Several tests show that residual foreground contamination may significantly contribute to this non-Gaussian feature. It is thus still premature to do more precise tests on the non-Gaussianity of the intrinsic CMB fluctuations before we can identify the origin of these foreground signals, understand their nature, and finally remove them from the CMB maps completely.