Magnetic fields in barred galaxies

Abstract

Aims. We present a model of the global magnetic field in the barred galaxy NGC 1365 based jointly on the large-scale velocity field of interstellar gas fitted to H i and CO observations of this galaxy and on mean-field dynamo theory. The aim of the paper is to present a detailed quantitative comparison of a galactic dynamo model with independent radio observations. Methods. We consider several gas dynamical models, based on two rotation curves. We test a range of nonlinear dynamo models that include plausible variations of those parameters that are poorly known from observations. Models for the cosmic ray distribution in the galaxy are introduced in order to produce synthetic radio polarization maps that allow direct comparison with those observed at λλ3. 5a nd 6. 2c m. Results. We show that the dynamo model is robust in that the most important magnetic features are controlled by the relatively well established properties of the density distribution and gas velocity field. The optimal agreement between the synthetic polarization maps and observations is obtained when a uniform cosmic ray distribution is adopted. These maps are sensitive to the number density of thermal ionized gas because of Faraday depolarization effects. Our results are compatible with the observed polarized radio intensity and Faraday rotation measure if the degree of ionization is between 0.01 and 0.2 (with respect to the total gas density, rather than to the diffuse gas alone). We find some indirect evidence for enhanced turbulence in the regions of strong velocity shear (spiral arms and large-scale shocks in the bar) and within 1–2 kpc of the galactic centre. We confirm that magnetic stresses can drive an inflow of gas � ,