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

Abstract

The configuration of the regular magnetic field in M31 is deduced from radio polarization observations at the wavelengths �� 6, 11 and 20cm. By fitting the observed azimuthal distribution of polarization angles, we find that the regular magnetic field, averaged over scales 1-3kpc, is almost perfectly axisymmetric in the radial range 8 to 14kpc, and follows a spiral pattern with pitch angles of p ≃ −19 ◦ to p ≃ −8 ◦ . In the ring between 6 and 8kpc 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 M31; 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 hth ≃ 1kpc. Faraday effects offer a powerful method to detect thick magneto-ionic disks or halosaround spiral galaxies.