High-resolution radio continuum survey of M33 III. Magnetic fields

Aims. We study the magnetic field structure, strength, and energy density in the Scd galaxy M33. 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 M33. We fit a 3-D 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 M33 at 3.6 cm is polarized. High degrees of polarization of the synchrotron emission (> 20%) and strong regular magnetic fields in the sky plane (≃ 6.6μG) exist in-between two northern spiral arms. We estimate the average total and regular magnetic field strengths in M33 as ≃ 6.4 and 2.5μG, respectively. Under the assumption that the disk of M33 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 M33. 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 M33’s rotation curve. Generation of interstellar magnetic fields from turbulent gas motions in M33 is indicated by the equipartition of turbulent and magnetic energy densities.