Magnetic fields in barred galaxies

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 $\ion{H}{i}$ and $\ion{CO}{}$ 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 $\lambda\lambda$3.5 and 6.2 cm. 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 into the inner 1 kpc of the galaxy at a rate of a few $M_\odot\,{\rm yr}^{-1}$. Conclusions. The dynamo models are successful to some extent in modelling the large scale regular magnetic field in this galaxy. Our results demonstrate that dynamo models and synthetic polarization maps can provide information about both the gas dynamical models and conditions in the interstellar medium. In particular, it seems that large-scale deviations from energy equipartition (or pressure balance) between large-scale magnetic fields and cosmic rays are unavoidable. We demonstrate that the dynamical effects of magnetic fields cannot be everywhere ignored in galaxy modelling