Mapping the large-angle deviation from Gaussianity in simulated CMB maps

The detection of the type and level of primordial non-Gaussianity in the CMB data is essential to probe the physics of the early universe. Since one does not expect that a single statistical estimator can be sensitive to all possible forms of non-Gaussianity which may be present in the data, it is important to employ different statistical indicators to study non-Gaussianity of CMB. In recent works we have proposed two new large-angle non-Gaussianity indicators based on skewness and kurtosis of patches of CMB sky sphere, and used them to find significant deviations from Gaussianity in frequency bands and foreground-reduced CMB maps. Simulated CMB maps with assigned type and amplitude of primordial non-Gaussianity are important tools to determine the strength, sensitivity, and limitations of non-Gaussian estimators. Here we investigate whether and to what extent our non-Gaussian indicators have sensitivity to detect non-Gaussianity of local type, particularly with an amplitude within the 7 yr Wilkinson Microwave Anisotropy Probe (WMAP) bounds. We make a systematic study by employing our statistical tools to generate maps of skewness and kurtosis from several thousands of simulated maps equipped with non-Gaussianity of local type of various amplitudes. We show that our indicators can be used to detect large-angle local-type non-Gaussianity only for relatively large values of the nonlinear parameter f(NL)(local). Thus, our indicators do not have enough sensitivity to detect deviation from Gaussianity with the nonlinear parameter within the 7 yr WMAP bounds. This result, along with the outcomes of frequency bands and foreground-reduced analyses, suggests that non-Gaussianity captured in the previous works by our indicators is not of primordial origin, although it might have a primordial component. We have also made a comparative study of non-Gaussianity of simulated maps and of the full-sky WMAP 7 yr foreground-reduced internal linear combination (ILC)-7 yr map. An outcome of this analysis is that the level of non-Gaussianity of the ILC-7 yr map is higher than that of the simulated maps for f(NL)(local) within WMAP bounds. This provides quantitative indications on the suitability of the ILC-7 yr map as Gaussian reconstruction of the full-sky CMB.