The magnetic field of M 31 from multi-wavelength radio polarization observations

Free access article

Issue		A&A Volume 414, Number 1, January IV 2004


Page(s)		53 - 67
Section		Galactic structure and dynamics
DOI		http://dx.doi.org/10.1051/0004-6361:20034133

A&A 414, 53-67 (2004)
DOI: 10.1051/0004-6361:20034133

The magnetic field of M 31 from multi-wavelength radio polarization observations

A. Fletcher¹, E. M. Berkhuijsen¹, R. Beck¹ and A. Shukurov²

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² School of Mathematics and Statistics, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK

(Received 29 July 2003 / Accepted 6 October 2003 )

Abstract
The configuration of the regular magnetic field in M 31 is deduced from radio polarization observations at the wavelengths $\lambda\lambda6,\ 11$ and $20~{\rm cm}$ . By fitting the observed azimuthal distribution of polarization angles, we find that the regular magnetic field, averaged over scales 1-3 kpc, is almost perfectly axisymmetric in the radial range 8 to $14~{\rm kpc}$ , and follows a spiral pattern with pitch angles of $p\simeq -19\degr$ to $p\simeq -8\degr$ . In the ring between 6 and $8~{\rm kpc}$ a perturbation of the dominant axisymmetric mode may be present, having the azimuthal wave number m=2. A systematic analysis of the observed depolarization allows us to identify the main mechanism for wavelength dependent depolarization - Faraday rotation measure gradients arising in a magneto-ionic screen above the synchrotron disk. Modelling of the depolarization leads to constraints on the relative scale heights of the thermal and synchrotron emitting layers in M 31; the thermal layer is found to be up to three times thicker than the synchrotron disk. The regular magnetic field must be coherent over a vertical scale at least similar to the scale height of the thermal layer, estimated to be $h_{{\rm th}}\simeq 1~{\rm kpc}$ . Faraday effects offer a powerful method to detect thick magneto-ionic disks or halos around spiral galaxies.

Key words: galaxies: magnetic fields -- galaxies: individual: M 31 -- galaxies: spiral -- ISM: magnetic fields -- radio continuum: galaxies -- polarization

Offprint request: A. Fletcher, fletcher@mpifr-bonn.mpg.de

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