On the Origin of Radio-loudness in Active Galactic Nuclei Using Far-infrared Polarimetric Observations

Abstract

Abstract The dichotomy between radio-loud (RL) and radio-quiet (RQ) active galactic nuclei (AGNs) is thought to be intrinsically related to radio jet production. This difference may be explained by the presence of a strong magnetic field (B-field) that enhances, or is the cause of, the accretion activity and the jet power. Here we report the first evidence of an intrinsic difference in the polarized dust emission cores of four RL and five RQ obscured AGNs using 89 μm polarization with SOFIA/HAWC+. We find that the thermal polarized emission increases with the nuclear radio-loudness, R 20 = L 5GHz/L 20μm. The dust emission cores of RL AGNs are measured to be polarized, ∼5%–11%, while RQ AGNs are unpolarized, <1%. For RQ AGNs, our results are consistent with the observed region being filled with an unmagnetized or highly turbulent disk and/or expanding outflow at scales of 5–130 pc from the AGNs. For RL AGNs, the measured 89 μm polarization arises primarily from magnetically aligned dust grains associated with a 5–130 pc scale dusty obscuring structure with a toroidal B-field orientation highly offset, 65° ± 22°, with respect to the jet axis. Our results indicate that the size and strength of the B-fields surrounding the AGNs are intrinsically related to the strength of the jet power—the stronger the jet power is, the larger and stronger the toroidal B-field is. The detection of a ≤130 pc scale ordered toroidal B-field suggests that (a) the infalling gas that fuels RL AGNs is magnetized, (b) there is a magnetohydrodynamic wind that collimates the jet, and/or (c) the jet is able to magnetize its surroundings.