Multiscale magnetic fields in spiral galaxies: evolution and reversals

Context. Magnetic fields in nearby, star-forming galaxies reveal large-scale patterns, as predicted by dynamo models, but also a variety of small-scale structures. In particular, a large-scale field reversal may exist in the Milky Way while no such reversals have been observed so far in external galaxies. Aims. The effects of star-forming regions in galaxies need to be included when modelling the evolution of their magnetic fields, which can then be compared to future radio polarization observations. The conditions leading to large-scale field reversals also need clarification. Methods. Our simplified model of field evolution in isolated disc galaxies includes a standard mean-field dynamo and continuous injection of turbulent fields (the effect of supernova explosions) in discrete star forming regions by implicit small-scale dynamo action. Synthetic maps of radio synchrotron emission and Faraday rotation measures are computed for galaxies at different evolutionary stages. Results. A large-scale dynamo is essential to obtain regular large-scale spiral magnetic fields, as observed in many galaxies. These appear, on kpc scales in near energy equilibrium with the turbulence, after 1–2 Gyr (corresponding to redshift about 4−3). The injection of turbulent fields generates small-scale field structures. Strong injected small-scale fields and a large dynamo number (e.g. rapid rotation) of a galaxy favour the generation of field reversals. Depending on the model parameters, large-scale field reversals may persist over many Gyrs and can survive until the present epoch. Significant polarized radio synchrotron emission from young galaxies is expected at redshift ≤4. Faraday rotation measures (RM) are crucial to detect field reversals. Large-scale RM patterns of rotation measures can be observed at redshift ≤3. Conclusions. Our model can explain the general form of axisymmetric spiral fields with many local distortions, as observed in nearby galaxies. For a slightly different choice of parameters, large-scale field reversals can persist over the lifetime of a galaxy. Comparing our synthetic radio maps with future observations of distant galaxies with the planned Square Kilometre Array (SKA) and its precursors will allow testing and refinement of models of magnetic field evolution