| Summary: | We study the polarization properties of extragalactic sources at 95 and 150 GHz in the SPTpol 500 deg² survey. We estimate the polarized power by stacking maps at known source positions, and correct for noise bias by subtracting the mean polarized power at random positions in the maps. We show that the method is unbiased using a set of simulated maps with similar noise properties to the real SPTpol maps. We find a flux-weighted mean-squared polarization fraction 〈p²〉 = [8.9 ± 1.1] × 10â»â´ at 95 GHz and [6.9 ± 1.1] × 10â»â´ at 150 GHz for the full sample. This is consistent with the values obtained for a subsample of active galactic nuclei. For dusty sources, we find 95 per cent upper limits of 〈p²〉₉₅ < 16.9 × 10â»Â³and 〈p²〉â‚â‚…â‚€ < 2.6 × 10â»Â³. We find no evidence that the polarization fraction depends on the source flux or observing frequency. The 1σ upper limit on measured mean-squared polarization fraction at 150 GHz implies that extragalactic foregrounds will be subdominant to the CMB E and B mode polarization power spectra out to at least â„“ ≲ 5700 (â„“ ≲ 4700) and â„“ ≲ 5300 (â„“ ≲ 3600), respectively, at 95 (150) GHz. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2019 October 7. Received 2019 October 1; in original form 2019 July 11. Published: 21 October 2019. South Pole Telescope (SPT) is supported by the National Science Foundation through grant PLR-1248097. Partial support is also provided by the NSF Physics Frontier Center grant PHY-1125897 to the Kavli Institute of Cosmological Physics at the University of Chicago, the Kavli Foundation, and the Gordon and Betty Moore Foundation grant GBMF 947. This research used resources of the National Energy ...
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