Magnetic fields, cosmic rays and synchrotron emission in spiral galaxies

Abstract

Magnetic fields have been detected in many spiral galaxies. Observations of polarized radio synchrotron emission are the most useful tool in exploring the nature of these fields. We have developed methods to generate synthetic polarization maps of spiral galaxies (with allowance for their position and inclination with respect to the line of sight) from magnetic field models and other data. Polarization maps based on dynamo and gas dynamical models for barred galaxies are compared with maps observed at λλ3.5 and 6.2 cm. We have developed methods and procedures to make the comparisons of theory and observations both direct and meaningful, resulting in the most detailed comparisons to date. It is shown that gas dynamical and dynamo models for the galaxy NGC 1365 are broadly compatible with radio observations, but we have identified deficiencies which are most likely attributable to earlier published gas dynamical models. We also find that the variation of cosmic ray energy density across the galaxy is much weaker than that of the magnetic field, so that energy equipartition between the two can not be maintained locally. To justify and understand this, we have developed a numerical two-fluid MHD model for the evolution of cosmic ray energy density in magnetic field produced by interstellar turbulence. This is the first such model for cosmic ray dynamics where the magnetic field is produced by dynamo action. We have demonstrated that the large diffusivity of cosmic rays can be successfully modelled with a non-Fickian approach. Cosmic ray energy density in our model is seen to exhibit some correlation with gas density, but not with the magnetic field.