Letter to the Editor

WSRT Faraday tomography of the Galactic ISM at  m

First results for a field at (l, b)

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 Leiden, The Netherlands e-mail: schnitze@strw.leidenuniv.nl
² ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
³ Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 12 April 2007
Accepted: 6 June 2007

Abstract

Aims.We investigate the distribution and properties of Faraday rotating and synchrotron emitting regions in the Galactic ISM in the direction of the Galactic anti-centre.

Methods.We apply Faraday tomography to a radio polarization dataset that we obtained with the WSRT. We developed a new method to calculate a linear fit to periodic data, which we use to determine rotation measures from our polarization angle data. From simulations of a Faraday screen + noise we could determine how compatible the data are with Faraday screens.

Results.An unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an unresolved main component in the Faraday depth spectrum. For lines-of-sight with a single unresolved component we demonstrate that a Faraday screen in front of a synchrotron emitting region that contains a turbulent magnetic field component can explain the data.