Galactic magnetic X fields

Abstract

Aims. Or aim in this note is to compare a recent explanation of the galactic X pattern in Faraday rotation to the pattern produced by the advection part of the classical dynamo. Methods. We find that the characteristic X magnetic field polarization in the plane of the sky, found in edge-on spiral galaxies, can develop magnetohydromagnetically from an initial disk magnetic field combined with wind and rotation. The rotation measure develops a corresponding X distribution in sign, but this distribution is not a universal behavior because it depends primarily on the velocity field. We use Cauchy evolution of an initial magnetic field to find the field at some later time. Results. A battery mechanism that requires the current to always flow out of a galaxy has been recently suggested. This contrasts with the conclusions of this paper. Conclusions. Either explanation has significant consequences for the structure of a spiral galaxy. If the battery mechanism applies, then we have a new method of producing a magnetic field independent of the traditional dynamo. If, however, the flow mechanism applies, then constraints concerning the presence and nature of a galactic wind, together with the signature of the mean radial magnetic field, can be inferred.