Ordered magnetic fields around radio galaxies: evidence for interaction with the environment

TL;DR

This study uses multi-frequency VLA observations to reveal complex, banded Faraday rotation structures around radio galaxies, indicating interactions between radio lobes and their magnetized, compressed environments.

Contribution

It provides the first detailed imaging and interpretation of RM bands caused by magnetized plasma compression around radio galaxy lobes.

Findings

Detection of banded RM structures perpendicular to radio lobes

Identification of rims of high depolarization coincident with X-ray bright shells

Evidence that Faraday rotation material is in front of radio emission and shaped by lobe interactions

Abstract

We present detailed imaging of Faraday rotation and depolarization for the radio galaxies 0206+35, 3C 270, 3C 353 and M 84, based on Very Large Array observations at multiple frequencies in the range 1365 to 8440 MHz. This work suggests a more complex picture of the magneto-ionic environments of radio galaxies than was apparent from earlier work. All of the sources show spectacular banded rotation measure (RM) structures with contours of constant RM perpendicular to the major axes of their radio lobes. We give a comprehensive description of the banded RM phenomenon and present an initial attempt to interpret it as a consequence of interactions between the sources and their surroundings. We show that the material responsible for the Faraday rotation is in front of the radio emission and that the bands are likely to be caused by magnetized plasma which has been compressed by the expanding radio lobes. A two-dimensional magnetic structure in which the field lines are a family of ellipses draped around the leading edge of the lobe can produce RM bands in the correct orientation for any source orientation. We also report the first detections of rims of high depolarization at the edges of the inner radio lobes of M 84 and 3C 270. These are spatially coincident with shells of enhanced X-ray surface brightness, in which both the field strength and the thermal gas density are likely to be increased by compression.