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

TL;DR

This study uses deep radio surveys of M 31 to analyze its magnetic field structure, cosmic ray propagation, and polarization properties, revealing an axisymmetric spiral magnetic field generated by a mean-field dynamo.

Contribution

It provides detailed measurements of spectral indices, scale lengths, Faraday rotation, and magnetic field patterns in M 31, supporting the presence of a dominant axisymmetric spiral magnetic field.

Findings

The magnetic field is almost aligned along the ring with a pitch angle of -14°.

Faraday rotation measures indicate an axisymmetric spiral magnetic field with superimposed bisymmetric component.

The ordered magnetic field's pitch angle is about -26°, smaller than the regular field's pitch angle.

Abstract

Three deep radio continuum surveys of the Andromeda galaxy, M 31, were performed at 11.3, 6.2, and 3.6 cm wavelength with the Effelsberg 100-m telescope. At all wavelengths, the total and polarized emission is concentrated in a ring-like structure between about 7 kpc and 13 kpc radius from the center. Propagation of cosmic rays away from 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 varies double-periodically with azimuthal angle, indicating that the ordered magnetic field is almost oriented along the ring, with a pitch angle of -14{\deg} $\pm$ 2{\deg}. Faraday rotation measures (RM) show a large-scale sinusoidal variation with azimuthal angle, signature of an axisymmetric spiral (ASS) regular magnetic field, plus a superimposed double-periodic variation of a bisymmetric spiral (BSS) field with about 6x smaller amplitude. The dominating ASS field of M 31 is the most compelling case so far of a field generated by the action of a mean-field dynamo. The RM amplitude between 6.2 cm and 3.6 cm is about 50% larger than between 11.3 cm and 6.2 cm, indicating that Faraday depolarization at 11.3 cm is stronger than at 6.2 cm and 3.6 cm. The phase of the sinusoidal RM variation of -7{\deg} $\pm$ 1{\deg} 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{\deg} $\pm$ 3{\deg}. 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 (ASS + BSS) field.