Wide field polarimetry around the Perseus cluster at 350 MHz^⋆

ASTRON, PO Box 2, 7990 AA Dwingeloo The Netherlands
e-mail: brentjens@astron.nl

Received: 1 July 2010
Accepted: 28 October 2010

Abstract

Aims. This paper investigates the fascinating diffuse polarization structures at 350 MHz that have previously been tentatively attributed to the Perseus cluster and, more specifically, tries to find out whether the structures are located at (or near) the Perseus cluster, or in the Milky Way.

Methods. A wide field, eight point Westerbork Synthesis Radio Telescope mosaic of the area around the Perseus cluster was observed in full polarization. The frequency range was 324 to 378 MHz and the resolution of the polarization maps was 2′ × 3′. The maps were processed using Faraday rotation measure synthesis to counter bandwidth depolarization. The RM-cube covers Faraday depths of  −384 to  +381 rad m^-2 in steps of 3 rad m^-2.

Results. There is emission all over the field at Faraday depths between  −50 and  +100 rad m^-2. All previously observed structures were detected. However, no compelling evidence was found supporting association of those structures with either the Perseus cluster or large scale structure formation gas flows in the Perseus-Pisces super cluster. On the contrary, one of the structures is clearly associated with a Galactic depolarization canal at 1.41 GHz. Another large structure in polarized intensity, as well as Faraday depth at a Faraday depth of  +30 rad m^-2, coincides with a dark object in WHAM Hα maps at a kinematic distance of 0.5 ± 0.5 kpc. All diffuse polarized emission at 350 MHz towards the Perseus cluster is most likely located within 1 kpc from the Sun. The layers that emit the polarized radiation are less than 40 pc/∥B_∥∥  thick.