Four-year Cobe1 Dmr Cosmic Microwave Background Observations: Maps and Basic Results

In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 yr of COBE Differential Microwave Radiometer (DMR) observations, with additional details in a set of companion Letters. The anisotropy is consistent with a scale-invariant power-law model and Gaussian statistics. With full use of the multifrequency 4 yr DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of n 5 1.2 H 0.3 and a quadrupole normalization Qrms2PS 5 15.322.8 13.8 mK. For n 5 1 the best-fit normalization is Qrms2PSun51 5 18 H 1.6 mK. These values are consistent with both our previous 1 yr and 2 yr results. The results include use of the , 5 2 quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The final DMR 4 yr sky maps, presented in this Letter, portray an accurate overall visual impression of the anisotropy since the signal-to-noise ratio is12 per 108 sky map patch. The improved signal-to-noise ratio of the 4 yr maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics. Subject headings: cosmic microwave background— cosmology: observations