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

The configuration of the regular magnetic field in M 31 is deduced from radio polarization observations at the wavelengths and . 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 , and follows a spiral pattern with pitch angles of to . In the ring between 6 and a perturbation of the dominant axisymmetric mode may be present, having the azimuthal wave number . 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 . Faraday effects offer a powerful method to detect thick magneto-ionic disks or halos around spiral galaxies.