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

Understand

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.