First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations:: The angular power spectrum

We present the angular power spectrum derived from the first-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps. We study a variety of power spectrum estimation methods and data combinations and demonstrate that the results are robust. The data are modestly contaminated by diffuse Galactic foreground emission, but we show that a simple Galactic template model is sufficient to remove the signal. Point sources produce a modest contamination in the low-frequency data. After masking similar to700 known bright sources from the maps, we estimate that residual sources contribute similar to3500 muK(2) at 41 GHz and similar to130 muK(2) at 94 GHz to the power spectrum [l(l+1)C-l/2pi] at l=1000. Systematic errors are negligible compared to the (modest) level of foreground emission. Our best estimate of the power spectrum is derived from 28 cross-power spectra of statistically independent channels. The final spectrum is essentially independent of the noise properties of an individual radiometer. The resulting spectrum provides a definitive measurement of the cosmic microwave background (CMB) power spectrum, with uncertainties limited by cosmic variance, up to lsimilar to350. The spectrum clearly exhibits a first acoustic peak at l=220 and a second acoustic peak at lsimilar to540 (Page and coworkers), and it provides strong support for adiabatic initial conditions (Spergel and coworkers). Kogut and coworkers analyze the C-l(TE) power spectrum and present evidence for a relatively high optical depth and an early period of cosmic reionization. Among other things, this implies that the temperature power spectrum has been suppressed by similar to30% on degree angular scales, as a result of secondary scattering.