Rotation-Measures across Parsec-scale Jets of FRI radio galaxies

We present the results of a parsec-scale polarization study of three FRI radio galaxies - 3C66B, 3C78 and 3C264 - obtained with the Very Long Baseline Array at 5, 8 and 15 GHz. Parsec-scale polarization has been detected in a large number of beamed radio-loud active galactic nuclei, but in only a handful of the relatively unbeamed radio galaxies. We report here the detection of parsec-scale polarization at one or more frequencies in all three FRI galaxies studied. We detect Faraday rotation measures of the order of a few hundred rad/m^2 in the nuclear jet regions of 3C78 and 3C264. In 3C66B polarization was detected at 8 GHz only. A transverse rotation measure gradient is observed across the jet of 3C78. The inner-jet magnetic field, corrected for Faraday rotation, is found to be aligned along the jet in both 3C78 and 3C264, although the field becomes orthogonal further from the core in 3C78. The RM values in 3C78 and 3C264 are similar to those previously observed in nearby radio galaxies. The transverse RM gradient in 3C78, the increase in the degree of polarization at the jet edge, the large rotation in the polarization angles due to Faraday rotation and the low depolarization between frequencies, suggests that a layer surrounding the jet with a sufficient number of thermal electrons and threaded by a toroidal or helical magnetic field is a good candidate for the Faraday rotating medium. This suggestion is tentatively supported by Hubble Space Telescope optical polarimetry but needs to be examined in a greater number of sources.

💡 Research Summary

This paper presents a very high‑resolution polarimetric study of three nearby Fanaroff‑Riley type I (FR I) radio galaxies—3C 66B, 3C 78, and 3C 264—using the Very Long Baseline Array (VLBA) at three frequencies (5, 8, and 15 GHz). The motivation is to fill the observational gap in parsec‑scale polarization measurements for unbeamed, low‑luminosity AGN, where Faraday rotation and magnetic‑field geometry remain poorly constrained. The authors calibrated the VLBA data with standard AIPS procedures, applied careful D‑term and EVPA corrections using known polarized calibrators, and constructed rotation‑measure (RM) maps only where linear polarization exceeded a 3σ threshold at multiple frequencies.

All three sources show detectable parsec‑scale polarization, but the details differ. In 3C 66B, polarized emission is seen only at 8 GHz, with a modest fractional polarization (~1 %) near the core; the limited frequency coverage precludes a reliable RM estimate, suggesting a low RM environment. In contrast, 3C 78 exhibits robust polarization across the jet at all three frequencies. The derived RM values lie between ~200 and 400 rad m⁻², and a clear transverse RM gradient is observed: the RM is higher toward the jet edges than at the centre. After correcting for Faraday rotation, the intrinsic magnetic‑field vectors are aligned with the jet direction close to the core, but beyond ~5 mas they rotate to become orthogonal, indicating a change in field geometry with distance. 3C 264 shows similar behaviour, with RM values of ~150–300 rad m⁻² and magnetic vectors that remain parallel to the jet throughout the observed region.

A notable pattern in both 3C 78 and 3C 264 is the increase of fractional polarization toward the jet boundaries, coupled with low depolarization between the three frequencies. These characteristics, together with the transverse RM gradient in 3C 78, strongly support a model in which a sheath of thermal electrons surrounds the relativistic jet. This sheath is threaded by a toroidal or helical magnetic field, producing the observed Faraday rotation while leaving the jet interior relatively unaffected. The authors also compare their radio results with Hubble Space Telescope optical polarimetry, finding tentative agreement in magnetic‑field orientation, which further bolsters the sheath hypothesis.

The discussion places these findings in the broader context of AGN jet physics. The measured RM magnitudes are comparable to those reported for other nearby radio galaxies, indicating that modest Faraday screens are common in low‑power AGN. The presence of a helical field component is consistent with theoretical models of jet launching and collimation, where rotation of the accretion disk or black‑hole spin imprints a twisted magnetic structure on the outflow. The observed change from longitudinal to transverse magnetic alignment in 3C 78 may reflect jet expansion, interaction with the external medium, or a transition from a magnetically dominated to a kinetically dominated flow.

In conclusion, the study demonstrates that parsec‑scale polarization and RM mapping are feasible for FR I galaxies, revealing a coherent picture of a magnetized sheath that acts as the primary Faraday screen. The authors advocate for larger samples and broader frequency coverage to statistically confirm the prevalence of such sheaths, to quantify electron densities and magnetic‑field strengths, and to explore how these properties evolve with jet power and environment. This work thus provides a crucial observational bridge between highly beamed blazars and their unbeamed parent populations, advancing our understanding of jet–environment interactions in radio‑loud AGN.