THE MAGNETIC FIELD AND POLARIZATION PROPERTIES OF RADIO GALAXIES IN DIFFERENT ACCRETION STATES

We use the integrated polarized radio emission at 1.4 GHz ( &Pgr; 1.4 GHz ?> ) from a large sample of active galactic nuclei (AGN; 796 sources at redshifts z < 0.7 ?> ) to study the large-scale magnetic field properties of radio galaxies in relation to the host galaxy accretion state. We find a fundamental difference in &Pgr; 1.4 GHz ?> between radiative-mode AGN (i.e., high-excitation radio galaxies (HERGs) and radio-loud QSOs) and jet-mode AGN (i.e., low-excitation radio galaxies (LERGs)). While LERGs can achieve a wide range of &Pgr; 1.4 GHz ?> (up to ∼30%), the HERGs and radio-loud QSOs are limited to &Pgr; 1.4 GHz ≲ 15 % ?> . A difference in &Pgr; 1.4 GHz ?> is also seen when the sample is divided at 0.5% of the total Eddington-scaled accretion rate, where the weakly accreting sources can attain higher values of &Pgr; 1.4 GHz ?> . We do not find any clear evidence that this is driven by intrinsic magnetic field differences of the different radio morphological classes. Instead, we attribute the differences in &Pgr; 1.4 GHz ?> to the local environments of the radio sources, in terms of both the ambient gas density and the magnetoionic properties of this gas. Thus, not only are different large-scale gaseous environments potentially responsible for the different accretion states of HERGs and LERGs, we argue that the large-scale magnetized environments may also be important for the formation of powerful AGN jets. Upcoming high angular resolution and broadband radio polarization surveys will provide the high-precision Faraday rotation measure and depolarization data required to robustly test this claim.