Multi-frequency polarimetric study of a complete sample of extragalactic radio sources: radio source populations and cosmological perspectives

The high-frequency (> 20 GHz), bright flux density (> 200 mJy) radio population is dominated by blazars, i.e. compact Doppler-boosted objects, whose emission arises from knot-like synchrotron structures along the relativistic jet. Their polarization properties was so far poorly constrained at high frequency and results in literature are typically biased. Extending the characterization of polarization properties of radio sources to high frequencies provides invaluable information about magnetic fields and plasma in the inner and unresolved regions of relativistic jets. Furthermore, extragalactic radio sources are an important contaminant for the angular power spectrum of CMB (Cosmic Microwave Background) at scales smaller than 30 arcmin, up to 100 GHz: their polarimetry is crucial to search for primordial B-modes, the footprint of inflation. We present the analysis of high sensitivity (rms ~ 0.6 mJy) multi-frequency and multi-epoch polarimetric observations of a complete sample of 104 compact extragalactic radio sources drawn from the faint (> 200 mJy at 20 GHz in total intensity) Planck-ATCA Coeval Observations (PACO) catalogue, performed with the Australia Telescope Compact Array (ATCA) at 7 frequencies, between 2.1 and 38 GHz. We found that polarization spectra of single sources cannot be simply inferred from total intensity ones, as different synchrotron components dominate the different emissions. We distinguish six spectral categories finding different behaviours in polarization fractions and position angles. Multi-epoch variability analysis (from 1.5 up to 10 yr time lags) is also presented. ALMA (Atacama Large Millimetre and sub-millimetre Array) observations extends the analysis up to 100 GHz for a (complete) sub-sample of 32 objects. We still find synchrotron signal at these frequencies with no sign of thermal or dust emissions. We compute number counts in polarization at 20 and 100 GHz to a deeper level than available so far and provide forecasts for forthcoming CMB experiments.