Magnetic fields in galaxies - I. Radio discs in local late-type galaxies

We develop an analytical model to follow the cosmological evolution of magnetic fields in disc galaxies. Our assumption is that fields are amplified from a small seed field via magnetohydrodynamical turbulence. We further assume that this process is fast compared to other relevant time-scales and occurs principally in the cold disc gas. We follow the turbulent energy density using a galaxy formation and evolution model. Three processes are important to the turbulent energy budget as follows: infall of cool gas on to the disc and supernova feedback increase the turbulence, while star formation removes gas and hence turbulent energy from the cold gas. Finally, we assume that field energy is continuously transferred from the incoherent random field into an ordered field by differential galactic rotation.

Model predictions are compared with observations of local late-type galaxies. The model reproduces observed magnetic field strengths and luminosities in low- and intermediate-mass galaxies. These quantities are overpredicted in the most massive hosts, suggesting that inclusion of gas ejection by powerful active galactic nuclei is necessary in order to quench gas cooling and reconcile the predicted and observed magnetic field strengths.