Average fractional polarization of extragalactic sources at Planck frequencies

¹ INAF, Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy
e-mail: trombetti@ira.inaf.it
² Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Giuseppe Saragat 1, 44122 Ferrara, Italy
e-mail: burigana@ira.inaf.it
³ INFN, Sezione di Ferrara, Via Giuseppe Saragat 1, 44122 Ferrara, Italy
⁴ INFN, Sezione di Bologna, Via Irnerio 46, 40127 Bologna, Italy
⁵ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: gianfranco.dezotti@oapd.inaf.it
⁶ INAF, Osservatorio Astronomico di Trieste, Via Giambattista Tiepolo 11, 34143 Trieste, Italy

Received: 22 November 2017
Accepted: 22 May 2018

Abstract

Recent detailed simulations have shown that an insufficiently accurate characterization of the contamination of unresolved polarized extragalactic sources can seriously bias measurements of the primordial cosmic microwave background (CMB) power spectrum if the tensor-to-scalar ratio r ∼ 0.001, as predicted by models currently of special interest (e.g., Starobinsky’s R² and Higgs inflation). This has motivated a reanalysis of the median polarization fraction of extragalactic sources (radio-loud AGNs and dusty galaxies) using data from the Planck polarization maps. Our approach, exploiting the intensity distribution analysis, mitigates or overcomes the most delicate aspects of earlier analyses based on stacking techniques. By means of simulations, we have shown that the residual noise bias on the median polarization fraction, Π_median, of extragalactic sources is generally ≲0.1%. For radio sources, we have found Π_median ≃ 2.83%, with no significant dependence on either frequency or flux density, in good agreement with the earlier estimate and with high-sensitivity measurements in the frequency range 5–40 GHz. No polarization signal is detected in the case of dusty galaxies, implying 90% confidence upper limits of Π_dusty ≲ 2.2% at 353 GHz and of ≲3.9% at 217 GHz. The contamination of CMB polarization maps by unresolved point sources is discussed.