The strength of galactic magnetic fields

Abstract

THE magnitudes of galactic magnetic fields are usually estimated from measurements of the radio synchroton emission arising from acceleration of cosmic-ray electrons in the magnetic field. To interpret the emission spectrum, it is usually assumed that the energy density in the magnetic field is equal to that in the cosmic-ray protons (which are assumed to outnumber the electrons by 100:1, as they do in our Galaxy). There is, however, no compelling justification for this assumption of energy equipartition. Here we use measurements by the Compton Gamma Ray Observatory1,2 of the γ-ray flux from the Magellanic clouds together with radio continuum data to estimate the strength of the magnetic fields in these galaxies without having to invoke energy equipartition. We find that the assumption of energy equipartition is not valid in these irregular galaxies, and that the validity cannot be restored by changing the electron/proton ratio. Supporting evidence for our conclusion comes from radio and X-ray observations (M. G. Watson, personal communication) of the starburst galaxy M82. Our results imply that these galactic fields are too large to have been generated by dynamo action alone, and we suggest that recent star formation might instead provide the generating mechanism.