The Faraday rotation of Cygnus A - Magnetic fields in cluster gas

Abstract

Multifrequency, multiconfiguration observations with the VLA have allowed, for the first time, detailed mapping of the rotation measure (RM) of the prominent extragalactic radio source Cygnus A. It is found that this source lies behind a deep Faraday screen, with the rotation measure varying from -4000 to +3000 rad/sq m for the eastern lobe, and from -2000 to +1500 rad/sq m for the western lobe. Gradients in both lobes commonly exceed 300 rad/sq m/arcsec. The overall pattern in RM is the same in each lobe, increasing from east to west, leading to the conclusion that the magnetic field is ordered on scales of about 20 to 30 kpc. There is no evidence for internal depolarization. The authors examine, and reject, the hypothesis that this Faraday screen has a Galactic origin and propose that its origin is in the intracluster gas in which Cygnus A is embedded.