Where has all the polarization gone?

Abstract

We have observed the five radio galaxies NGC1052, NGC4261, Hydra A, Centaurus A and Cygnus A with polarimetric VLBI at 15 GHz to look for possible Faraday rotation in foreground absorbers. Only Cygnus A was found to be polarized at the level of 0.4 %, the other four objects were found to be unpolarized (<0.3 %). We discuss implications for magnetic field structures and physical conditions in the absorbers imposed by the absence of polarized emission at this high frequency.