Ellipticity analysis of the BOOMERanG CMB maps

The properties of the Cosmic Microwave Background (CMB) maps carry valuable cosmological information. Here we report the results of the analysis of ellipticity of the hot and cold CMB anisotropy spots in the BOOMERanG 150 GHz map. We carried out this analysis for the map obtained by summing independent measurement channels (signal plus noise map) and for a comparison map (noise only map) obtained by differencing the same channels. The anisotropy areas (spots) have been identified for both maps for various temperature thresholds. The orientation (obliquity) of the spots is random for both maps. We computed the mean elongation of spots obtained from the maps at a given temperature threshold using a simple estimator. We found that for the sum map there is a region of temperature thresholds where the average elongation is not dependent on the threshold. Its value is ∼ 2.3 for cold areas and ∼ 2.2 for hot areas. The bias of the estimator is < ∼ + 0.4. The presence of noise also biases the ellipticity by < ∼ +0.3. These biases have not been subtracted in the results quoted above. The threshold independent and random obliquity behaviour in the sum map is stable against pointing reconstruction accuracy and noise level of the data, thus confirming that these are actual properties of the dataset. In the sum map the anisotropy areas are elongated more homogeneously than in the difference map. Analogous elongation properties of CMB anisotropies had been detected for COBE-DMR 4 year data.