Astronomy & Astrophysics

Issue		A&A Volume 461, Number 3, January III 2007
Page(s)		963 - 976
Section		Galactic structure, stellar clusters, and populations
DOI		10.1051/0004-6361:20065772

A&A 461, 963-976 (2007)
DOI: 10.1051/0004-6361:20065772

The WENSS & Dwingeloo surveys and the Galactic magnetic field

D. H. F. M. Schnitzeler¹, P. Katgert¹, M. Haverkorn^{2, 3}, and A. G. de Bruyn^{4, 5}

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: schnitze@strw.leidenuniv.nl
² Jansky Fellow, National Radio Astronomy Observatory
³ Astronomy Department UC-Berkeley, 601 Campbell Hall, Berkeley CA 94720, USA
⁴ ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁵ Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands

(Received 7 June 2006 / Accepted 5 October 2006)

Abstract
Aims.We investigate the structure of the Galactic magnetic field in the 2nd Galactic quadrant using radio continuum polarization data from the 325 MHz WENSS (WEsterbork Northern Sky Survey) survey in combination with earlier single-dish observations.
Methods.We study gradients in polarization angle along Galactic longitude and latitude in the region $130\degr \la l \la 173\degr$ and $-5\degr \la b \la 31\degr$ . These gradients were determined with a new method that we developed to efficiently and reliably fit linear gradients to periodic data like polarization angles. Since the WENSS data were obtained with a synthesis array they suffer from the "missing short spacing" problem. We have tried to repair this by adding an estimate of the large-scale emission based on the single-dish (Dwingeloo) data obtained by Brouw and Spoelstra. Combining all available data we derive a rotation measure map of the area, from which we estimate all 3 components of the magnetic field vector.
Results.In the part of WENSS where large-scale structure in polarized intensity is relatively unimportant, we find that the magnetic field is predominantly perpendicular to the line-of-sight, and parallel to the Galactic plane. The magnetic field components along the line-of-sight and along Galactic latitude have comparable values, and the strength of these components is much smaller than the strength of the total magnetic field. Our observations also cover part of the so-called "fan" region, an area of strong polarized intensity, where large-scale structure is missing from our WENSS data. We tentatively show that Faraday rotation occurring in front of the Perseus arm is causing both the WENSS RM and the RM towards the fan region observed in previous single-dish surveys, and we suggest that the fan is formed by local emission that originates in front of the emission we see in WENSS.

Key words: magnetic fields -- radio continuum: ISM -- techniques: polarimetric -- ISM: magnetic fields -- ISM: structure -- techniques: miscellaneous

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