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

¹ SISSA, Via Bonomea 265, Trieste 34136, Italy
e-mail: nkrach@sissa.it
² INAF Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
³ CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
⁴ INAF – Istituto di Radioastronomia, Via Gobetti 101, Bologna 40129, Italy
⁵ INFN, Via Valerio 2, 34127 Trieste, Italy
⁶ Department of Physics & Electronics, Rhodes University, Grahamstown, South Africa
⁷ Square Kilometre Array South Africa (SKA SA), Park Road, Pinelands 7405, South Africa
⁸ Department of Physics & Astronomy, The University of Iowa, Iowa City, Iowa 52245, USA
⁹ BABL AI Inc., 630 Fairchild St., Iowa City, IA 52245, USA
¹⁰ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George St, Toronto, ON M5S 3H4, Canada
¹¹ ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), Sydney, Australia
¹² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500, GL Nijmegen, The Netherlands
¹³ International Centre for Radio Astronomy Research, University of Western Australia, Crawley, WA 6009, Australia

Received: 4 February 2018
Accepted: 26 July 2018

Abstract

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 ≲ 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 ∝ ℓ^α, with α ≃ −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 β_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 β_s at angular resolution of 2° on about 30% of the sky. The recovered β_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 ∝ ℓ^γ with γ ≃ −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 ≃ 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 ≃ 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 ≃ 10⁻³. This result confirms the importance of observing both high and low frequency foregrounds in CMB B-mode measurements.