FIGURE 1. Boomerang sky maps (equatorial coordinates).

The sky maps at 90, 150 and 240 GHz (left panels) are shown with a common colour scale, using a thermodynamic temperature scale chosen such that CMB anisotropies will have the same amplitude in the three maps. Only the colour scale of the 400 GHz map (bottom right) is 14 times larger than the others: this has been done to facilitate comparison of emission from interstellar dust (ISD), which dominates this map, with ISD emission present in the lower-frequency maps. The maps at 90 and 400 GHz are each from a single detector, while maps at 150 and 240 GHz have each been obtained by co-adding data from three detectors. For purposes of presentation, the maps have been smoothed with gaussian filters to obtain FWHM effective resolution of 22.5' (small circle in the bottom right side of each panel). Structures along the scan direction larger than 10° are not present in the maps. Several features are immediately evident. Most strikingly, the maps at 90, 150 and 240 GHz are dominated by degree-scale structures that fill the map, have well-correlated morphology and are identical in amplitude in all three maps. These structures are not visible at 400 GHz. The 400 GHz map is dominated by diffuse emission which is correlated with the ISD emission mapped by IRAS/DIRBE²². This emission is strongly concentrated towards the right-hand edge of the maps, near the plane of the Galaxy. The same structures are evident in the 90, 150 and 240 GHz maps at Galactic latitude b >-15°, albeit with an amplitude that decreases steeply with decreasing frequency. The large-scale gradient evident especially near the right edge of the 240 GHz map is a result of high-pass-filtering the very large signals near the Galactic plane (not shown). This effect is negligible in the rest of the map. The two top right panels show maps constructed by differencing the 150 and 90 GHz maps and the 240 and 150 GHz maps. The difference maps contain none of the structures that dominate the maps at 90, 150 and 240 GHz, indicating that these structures do indeed have the ratios of brightness that are unique to the CMB. The morphology of the residual structures in the 240–150 GHz map is well-correlated with the 400 GHz map, as is expected if the residuals are due to the ISD emission. Three compact sources of emission are visible in the lower-frequency maps, as indicated by the circles. These are known radio-bright quasars from the SEST pointing catalogue at 230 GHz. The boxed area has been used for computing the angular power spectrum shown in Fig. 2.