S-PASS view of polarized Galactic synchrotron at 2.3 GHz as a contaminant to CMB observations

We have analyzed the southern sky emission in linear polarization at 2.3 GHz as observed by the S -band Polarization All Sky Survey (S-PASS). Our purpose is to study the properties of the diffuse Galactic polarized synchrotron as a contaminant to B-mode observations of the cosmic microwave background (CMB) polarization. We studied the angular distribution of the S-PASS signal at intermediate and high Galactic latitudes by means of the polarization angular power spectra. The power spectra, computed in the multipole interval 20 <= l <= 1000, show a decay of the spectral amplitude as a function of multipole for l less than or similar to 200, typical of the diffuse emission. At smaller angular scales, power spectra are dominated by the radio point source radiation. We find that, at low multipoles, spectra can be approximated by a power law C-l(EE,BB) proportional to l(alpha), with alpha similar or equal to -3, and characterized by a B-to-E ratio of about 0.5. We measured the polarized synchrotron spectral energy distribution (SED) in harmonic space, by combining S-PASS power spectra with low frequency WMAP and Planck ones, and by fitting their frequency dependence in six multipole bins, in the range 20 <= l <= 140. Results show that the recovered SED, in the frequency range 2.3-33 GHz, is compatible with a power law with beta(s) = -3.22 +/- 0.08, which appears to be constant over the considered multipole range and in the different Galactic cuts. Combining the S-PASS total polarized intensity maps with those coming from WMAP and Planck we derived a map of the synchrotron spectral index beta(s) at angular resolution of 2 degrees on about 30% of the sky. The recovered beta(s) distribution peaks at the value around -3.2. It exibits an angular power spectrum which can be approximated with a power law C-l proportional to l(gamma) with gamma similar or equal to -2.6. We also measured a significant spatial correlation between synchrotron and thermal dust signals, as traced by the Planck 353 GHz channel. This correlation reaches about 40% on the larger angular scales, decaying considerably at the degree scales. Finally, we used the S-PASS maps to assess the polarized synchrotron contamination to CMB observations of the B-modes at higher frequencies. We divided the sky in small patches (with f(sky) similar or equal to 1%) and find that, at 90 GHz, the minimal contamination, in the cleanest regions of the sky, is at the level of an equivalent tensor-to-scalar ratio r(synch) similar or equal to 10(-3). Moreover, by combining S-PASS data with Planck 353 GHz observations, we recover a map of the minimum level of total polarized foreground contamination to B-modes, finding that there is no region of the sky, at any frequency, where this contamination lies below equivalent tenor-to-scalar ratio r(FG) similar or equal to 10(-3). This result confirms the importance of observing both high and low frequency foregrounds in CMB B-mode measurements.