Magnetic fields and cosmic rays in M 31. I. Spectral indices, scale lengths, Faraday rotation, and magnetic field pattern

Abstract

Context. Magnetic fields play an important role in the dynamics and evolution of galaxies; however, the amplification and ordering of the initial seed fields are not fully understood. The nearby spiral galaxy M 31 is an ideal laboratory for extensive studies of magnetic fields. Aims. Our aim was to measure the intrinsic structure of the magnetic fields in M 31 and compare them with dynamo models of field amplification. Methods. The intensity of polarized synchrotron emission and its orientation are used to measure the orientations of the magnetic field components in the plane of the sky. The Faraday rotation measure gives information about the field components along the line of sight. With the Effelsberg 100-m telescope three deep radio continuum surveys of the Andromeda galaxy, M 31, were performed at 2. 645, 4.85, and 8.35 GHz (wavelengths of 11.3, 6.2, and 3.6 cm). The λ3.6 cm survey is the first radio survey of M 31 at such small wavelengths. Maps of the Faraday rotation measures (RMs) are calculated from the distributions of the polarization angle.Results. At all wavelengths the total and polarized emission is concentrated in a ring-like structure of about 7-13 kpc in radius from the centre. Propagation of cosmic rays away from the star-forming regions is evident. The ring of synchrotron emission is wider than the ring of the thermal radio emission, and the radial scale length of synchrotron emission is larger than that of thermal emission. The polarized intensity from the ring in the plane of the sky varies double-periodically with azimuthal angle, indicating that the ordered magnetic field is oriented almost along the ring, with a pitch angle of −14 • ± 2 • at λ6.2 cm. The RM varies systematically along the ring. The analysis shows a large-scale sinusoidal variation with azimuthal angle, signature of an axisymmetric spiral (AS S ) regular magnetic field, plus a superimposed double-periodic variation of a bisymmetric spiral (BS S ) regular field with about six times smaller amplitude. The RM amplitude of (118 ± 3) rad m −2 between λ6.2 cm and λ3.6 cm is about 50% larger than between λ11.3 cm and λ6.2 cm, indicating that Faraday depolarization at λ11.3 cm is stronger (i.e. with a larger Faraday thickness) than at λ6.2 cm and λ3.6 cm. The phase of the sinusoidal RM variation of −7 • ± 1 • is interpreted as the average spiral pitch angle of the regular field. The average pitch angle of the ordered field, as derived from the intrinsic orientation of the polarized emission (corrected for Faraday rotation), is significantly smaller: −26 • ± 3 • . Conclusions. The dominating axisymmetric (ASS) plus the weaker bisymmetric (BSS) field of M 31 is the most compelling case so far of a field generated by the action of a mean-field dynamo. The difference in pitch angle of the regular and the ordered fields indicates that the ordered field contains a significant fraction of an anisotropic turbulent field that has a different pattern than the regular (AS S + BS S ) magnetic field.