High-resolution radio continuum survey of M 33 III. Magnetic fields

Aims. We study the magnetic field structure, strength, and energy density in the Scd galaxy M 33. Methods. Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across M 33. We fit a 3D model of the regular magnetic field to the observed azimuthal distribution of polarization angles. We also analyze the spatial variation of depolarization across the galaxy. Results. Faraday rotation, measured between 3.6 and 6.2 cm at an angular resolution of 3' (0.7 kpc), shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M 33 at 3.6 cm is polarized. High degrees of polarization of the synchrotron emission (> 20%) and strong regular magnetic fields in the sky plane (similar or equal to 6.6 mu G) exist in-between two northern spiral arms. We estimate the average total and regular magnetic field strengths in M 33 as similar or equal to 6.4 and 2.5 mu G, respectively. Under the assumption that the disk of M 33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric ( m = 0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric ( m = 0 + 1) modes from 3 to 5 kpc radius. Conclusions. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m = 0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M 33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M 33's rotation curve. Generation of interstellar magnetic fields from turbulent gas motion in M 33 is indicated by the equipartition of turbulent and magnetic energy densities.