Forecasting Polarized Radio Sources for CMB observations

TL;DR

This paper analyzes the frequency-dependent polarization of extragalactic radio sources across a wide range of frequencies to forecast their impact on CMB polarization measurements, especially B-modes.

Contribution

It combines recent datasets to model polarized ERS emission over a broad frequency range and forecasts their contamination in upcoming CMB experiments.

Findings

Polarization fraction increases with frequency at high radio frequencies.

Up to 2000 polarized ERS sources are expected to be detected in future CMB surveys.

Polarized ERS can significantly contaminate B-mode polarization if not properly controlled.

Abstract

We combine the latest datasets obtained with different surveys to study the frequency dependence of polarized emission coming from Extragalactic Radio Sources (ERS). We consider data over a very wide frequency range starting from $1.4$ GHz up to $217$ GHz. This range is particularly interesting since it overlaps the frequencies of the current and forthcoming Cosmic Microwave Background (\cmb) experiments. Current data suggest that at high radio frequencies, ($ \nu \geq 20$ GHz) the fractional polarization of ERS does not depend on the total flux density. Conversely, recent datasets indicate a moderate increase of polarization fraction as a function of frequency, physically motivated by the fact that Faraday depolarization is expected to be less relevant at high radio-frequencies. We compute ERS number counts using updated models based on recent data, and we forecast the contribution of unresolved ERS in CMB polarization spectra. Given the expected sensitivities and the observational patch sizes of forthcoming \cmb\ experiments about $\sim 200 $ ( up to $\sim 2000 $ ) polarized ERS are expected to be detected. Finally, we assess that polarized ERS can contaminate the cosmological B-mode polarization if the tensor-to-scalar ratio is $r< 0.05$ and they have to be robustly controlled to de-lens \cmb\ B-modes at the arcminute angular scales.