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 ∼700 known bright sources from the maps, we estimate that residual sources contribute ∼3500 μK2 at 41 GHz and ∼130 μK2 at 94 GHz to the power spectrum [ℓ(ℓ + 1)Cℓ/2π] 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 ℓ ∼ 350. The spectrum clearly exhibits a first acoustic peak at ℓ = 220 and a second acoustic peak at ℓ ∼ 540 (Page and coworkers), and it provides strong support for adiabatic initial conditions (Spergel and coworkers). Kogut and coworkers analyze the C 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 ∼30% on degree angular scales, as a result of secondary scattering.