The impact of Galactic synchrotron emission on CMB anisotropy measurements. I. Angular power spectrum analysis of total intensity all-sky surveys

Context: Galactic foreground emission fluctuations are a limiting factor for precise cosmic microwave background (CMB) anisotropy measurements.
Aims: We intend to improve current estimates of the influence of Galactic synchrotron emission on the analysis of CMB anisotropies.
Methods: We perform an angular power spectrum analysis (APS) of all-sky total intensity maps at 408 MHz and 1420 MHz, which are dominated by synchrotron emission out of the Galactic plane. We subtract the brighter sources from the maps, which turns out to be essential for the results obtained. We study the APS as a function of Galactic latitude by considering various cuts and as a function of sky position by dividing the sky into patches of 15°×15° in size.
Results: The APS of the Galactic radio diffuse synchrotron emission is best fitted by a power law, C_ell ∼ k ell^α, with α in [-3.0,-2.6], where the lower values of α typically correspond to the higher latitudes. Nevertheless, the analysis of the patches reveals that strong local variations exist. A good correlation is found between the APS normalized amplitude, k₁₀₀ = k×100^α, at 408 MHz and 1420 MHz. The mean APS for ell in [20,40] is used to determine the mean spectral index between 408 MHz and 1420 MHz, β_{(0.408-1.4) GHz} in [-3.2,-2.9] (C_ell(ν) propto ν^-2β), which is then adopted to extrapolate the synchrotron APS results to the microwave range.
Conclusions: We use the 408 MHz and 1420 MHz APS results to predict the Galactic synchrotron emission fluctuations at frequencies above 20 GHz. A simple extrapolation to 23 GHz of the synchrotron emission APS found at these radio frequencies does not explain all the power in the WMAP synchrotron component even at middle/high Galactic latitudes. This suggests a significant microwave contribution (of about 50% of the signal) by other components such as free-free or spinning dust emission. The comparison between the extrapolated synchrotron APS and the CMB APS shows that a mask excluding the regions with |b_gal| ⪉ 5° would reduce the foreground fluctuations to about half of the cosmological ones at 70 GHz even at the lowest multipoles. The main implications of our analysis for the cosmological exploitation of microwave temperature anisotropy maps are discussed.