Cosmic-ray history and its implications for galactic magnetic fields

There is evidence that cosmic rays were present in galaxies at moderately high redshift. This suggests that magnetic fields were also present. If cosmic rays and magnetic fields must always be close to equipartition, as they are to an order of magnitude within the local universe, this would provide a powerful constraint on theories of the origin and evolution of magnetic fields in galaxies. We evaluate the role of magnetic field strength in cosmic-ray acceleration and confinement. We find that the properties of small-scale hydromagnetic turbulence are fundamentally changed in the presence of cosmic rays. As a result, magnetic fields several orders of magnitude weaker than present galactic fields can accelerate and retain a population of relativistic cosmic rays, provided that the fields are coherent over length scales greater than a cosmic-ray gyroradius.