SPATIAL-FREQUENCY ANALYSIS OF THE COBE DATA AND THE CMB ANISOTROPY SPECTRUM

The problem of obtaining estimates for the cosmic microwave background (CMB) anisotropy spectrum when applying the Galaxy-removal procedure is discussed. A method of two-step spatial-frequency filtering that re tains the orthogonality of spherical harmonics is proposed. This metho d is used to analyze the two-year COBE data. After the analysis of the difference maps containing only noise, the l = 7, 9, 13, 23, and 25 m ultipoles were excluded from the data under consideration because the noise power in these multipoles in the difference maps turned out to b e statistically anomalous. This procedure is shown to introduce no sig nificant systematic errors for the model noise-spectrum employed. Once the multipoles that are anomalous in terms of noise have been removed , the analysis yielded a power-law index of the CMB anisotropy power s pectrum of n = 1.84 +/- 0.29 and a quadrupole mean of Q(2) = 15.22 +/- 3.0. It has been established that the COBE data are not described by the Harrison-Zel'dovich spectrum at 99% confidence. The need to increa se the instrument sensitivity in order to more reliably separate the c osmological component from the microwave background is demonstrated.