Magnetic fields in the disk and halo of M51

Abstract

We discuss the regular magnetic field in the galaxy M51 on the basis of a new interpretation of polarization angles observed at the wavelengths 2.8, 6.2, 18.0 and 20.5 cm. We found a magneto-ionic halo in M51 with a radial extent of about 10 kpc. The regular magnetic fields in the disk and the halo have different structures. The regular magnetic field in the halo is axisymmetric and horizontal. Its field lines are spirals pointing inwards and generally opposite to those in the disk. The azimuthal structure of the magnetic field in the disk is neither axisymme- tric nor bisymmetric but can be represented by a superposition of these two basic harmonics with about equal weights. Magnetic lines of the regular field in the disk are spirals generally directed outwards. They are well aligned with the optical spiral arms as traced by the dust lanes. The regular magnetic field strength averaged in 3 kpc wide rings is about 5-10 muG in the disk and 3muG in the radial range 3--6 kpc in the halo. These values are in good agreement with independent estimates from the synchrotron emission. The general features of the magnetic patterns revealed in the disk and the halo seem to be in agreement with predictions of dynamo theory. Our results are also based on an analysis of the magneto-ionic medium in M51. For a range of radii in the galactic disk, we estimated the scale heights of synchrotron and thermal disks, electron densities and filling factors as well as the amount of depolarization.