The Protogalactic Origin for Cosmic Magnetic Fields

Abstract

It is demonstrated that strong magnetic elds are produced from a zero initial magnetic eld during the pregalactic era, when the galaxy is rst forming. Their development proceeds in three phases. In the rst phase, weak magnetic elds are created by the Biermann battery mechanism. During the second phase, results from a numerical simulation make it appear likely that homogenous isotropic Kolmogorov turbulence develops that is associated with gravitational structure formation of galaxies. Assuming that this turbulence is real, then these weak magnetic elds will be amplied to strong magnetic elds by this Kolmogorov turbulence. During this second phase, the magnetic elds reach saturation with the turbulent power, but they are coherent only on the scale of the smallest eddy. During the third phase, which follows this saturation, it is expected that the magnetic eld strength will increase to equipartition with the turbulent energy and that the coherence length of the magnetic elds will increase to the scale of the largest turbulent eddy, comparable to the scale of the entire galaxy. The resulting magnetic eld represents a galactic magnetic eld of primordial origin. No further dynamo action after the galaxy forms is necessary to explain the origin of magnetic elds. However, the magnetic eld will certainly be altered by dynamo action once the galaxy and the galactic disk have formed.