Magnetic fields and cosmic rays in M31. II. Strength and distribution of the magnetic field components

Abstract

The intensities of total, linearly polarized, and unpolarized synchrotron emission are measures for the strengths of total, ordered, and isotropic turbulent fields in the sky plane. Faraday rotation measures (RMs) provide a model of the regular field. The quadratic difference between ordered and regular field strengths yields the strength of the anisotropic turbulent field. - Based on observations of M31 at 3.6 cm, 6.2 cm, and 20.5 cm wavelengths and assuming equipartition between the energy densities of total magnetic fields and total cosmic rays, we measured average equipartition strengths of the magnetic field in the emission torus of M31 of 6.3\pm\0.2 \muG for the total, 5.4\pm0.2 \muG for the isotropic turbulent, and 3.2\pm0.3 \muG for the ordered field in the sky plane. The average strength of the axisymmetric regular field, Breg, is 2.0\pm0.5 \muG and remains almost constant between 7 kpc and 12 kpc radius. Quadratic subtraction of the component Breg,perp in the sky plane from the ordered field Bord,perp yields the strength of the anisotropic turbulent field Ban,perp, which is 2.7\pm0.7 \muG. - The average strength of the regular field is about 40% smaller than the equipartition strength of the ordered field, which is consistent with the assumption of equipartition. The almost constant ratio between regular and isotropic turbulent fields of about 0.39 is consistent with present-day dynamo models. The almost constant ratio between anisotropic and isotropic turbulent fields of about 0.57 indicates that anisotropic turbulent fields are generated by the shearing of isotropic turbulent fields. - The average magnetic energy density is about five times larger than the thermal energy density of the diffuse warm ionized gas, while the magnetic energy density is similar to the kinetic energy density of turbulent motions of the neutral gas.