# Fractional Polarisation of Extragalactic Sources in the 500-square-degree SPTpol Survey

## TL;DR

This study measures the polarisation properties of extragalactic sources at 95 and 150 GHz using the SPTpol survey, finding low polarisation fractions that suggest minimal foreground contamination for CMB polarisation studies.

## Contribution

The paper introduces a stacking method to estimate polarisation fractions of extragalactic sources in the SPTpol survey, providing new constraints on their polarisation properties at high frequencies.

## Key findings

- Mean-squared polarisation fraction at 95 GHz is approximately 8.9e-4.
- Mean-squared polarisation fraction at 150 GHz is approximately 6.9e-4.
- Extragalactic foregrounds are subdominant to CMB polarisation signals up to high multipoles.

## Abstract

We study the polarisation properties of extragalactic sources at 95 and 150 GHz in the SPTpol 500 deg$^2$ survey. We estimate the polarised power by stacking maps at known source positions, and correct for noise bias by subtracting the mean polarised 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 polarisation fraction $\langle p^2 \rangle= [8.9\pm1.1] \times 10^{-4}$ at 95 GHz and $[6.9\pm1.1] \times 10^{-4}$ at 150~GHz for the full sample. This is consistent with the values obtained for a sub-sample of active galactic nuclei. For dusty sources, we find 95 per cent upper limits of $\langle p^2 \rangle_{\rm 95}<16.9 \times 10^{-3}$ and $\langle p^2 \rangle_{\rm 150}<2.6 \times 10^{-3}$. We find no evidence that the polarisation fraction depends on the source flux or observing frequency. The 1-$\sigma$ upper limit on measured mean squared polarisation fraction at 150 GHz implies that extragalactic foregrounds will be subdominant to the CMB E and B mode polarisation power spectra out to at least $\ell\lesssim5700$ ($\ell\lesssim4700$) and $\ell\lesssim5300$ ($\ell\lesssim3600$), respectively at 95 (150) GHz.