Volume 618, October 2018
|Number of page(s)||18|
|Section||Cosmology (including clusters of galaxies)|
|Published online||30 October 2018|
S–PASS view of polarized Galactic synchrotron at 2.3 GHz as a contaminant to CMB observations
SISSA, Via Bonomea 265, Trieste 34136, Italy
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
Accepted: 26 July 2018
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 CℓEE,BB ∝ ℓα, 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 βs = −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 βs at angular resolution of 2° on about 30% of the sky. The recovered βs distribution peaks at the value around −3.2. It exibits an angular power spectrum which can be approximated with a power law Cℓ ∝ ℓγ 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 fsky ≃ 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 rsynch ≃ 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 rFG ≃ 10−3. This result confirms the importance of observing both high and low frequency foregrounds in CMB B-mode measurements.
Key words: polarization / methods: data analysis / Galaxy: general / cosmic background radiation / diffuse radiation / radio continuum: ISM
© ESO 2018
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