Detection of a non-Gaussian spot in WMAP

An extremely cold and big spot in the Wilkinson Microwave Anisotropy Probe (WMAP) 1-yr data is analysed. Our work is a continuation of a previous paper by Vielva et al. that reported the detection of non-Gaussianity, with a method based on the spherical Mexican hat wavelet (SMHW) technique. We study the spots at different thresholds on the SMHW coefficient maps, considering six estimators, namely the number of maxima, the number of minima, the numbers of hot and cold spots, and the number of pixels of those spots. At SMHW scales around 4degrees (10degrees on the sky), the data deviate from Gaussianity. The analysis is performed on all of the sky, the Northern and Southern hemispheres, and on four regions covering all of the sky. A cold spot at (b=-57degrees, l= 209degrees) is found to be the source of this non-Gaussian signature. We compare the spots of our data with 10 000 Gaussian simulations, and conclude that only around 0.2 per cent of them present such a cold spot. Excluding this spot, the remaining map is compatible with Gaussianity, and even the excess of kurtosis in the paper by Vielva et al. is found to be due exclusively to this spot. Finally, we study whether the spot causing the observed deviation from Gaussianity could be generated by systematics or foregrounds. None of them seem to be responsible for the non-Gaussian detection.