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

TL;DR

This study uses S-PASS 2.3 GHz polarization data to characterize Galactic synchrotron emission as a contaminant for CMB B-mode detection, revealing its spectral properties, spatial correlation with dust, and the minimum contamination level across the sky.

Contribution

It provides a detailed analysis of the polarized synchrotron foreground at 2.3 GHz and its impact on CMB B-mode observations, including spectral index mapping and contamination estimates.

Findings

Synchrotron power spectra follow a power law with alpha~-3.

The synchrotron spectral index peaks around -3.2.

Minimum foreground contamination corresponds to a tensor-to-scalar ratio of ~10^-3.

Abstract

We analyze 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 CMB B-mode observations. We study the angular distribution of the S-PASS signal at intermediate and high Galactic latitudes by means of angular power spectra. Power spectra, show a decay of the spectral amplitude as a function of multipole for \ell<200, typical of the diffuse emission. Spectra can be approximated by a power law C_{\ell}\propto\ell^{alpha}, with alpha~-3, and characterized by a B-to-E ratio of ~0.5. We study the synchrotron SED in polarization by computing power spectra of the low frequency WMAP and Planck maps. Results show that the SED, in the frequency range 2.3-33 GHz, is compatible with a power law with beta_s=-3.22\pm0.08. Combining S-PASS polarization maps with those coming from WMAP and Planck we derived a map of the synchrotron spectral index at angular resolution of 2{\deg} on about 30% of the sky. The recovered distribution peaks at the value around -3.2. We also measure 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 use the S-PASS maps to assess the polarized synchrotron contamination to CMB observations of the B-modes. 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 ~10^-3. This result confirms the importance of observing both high and low frequency foregrounds in CMB B-mode measurements.