Origin of coherent magnetic fields in high redshift objects

Abstract

Large scale strong magnetic fields in galaxies are generally thought to have been generated by a mean field dynamo. In order to have generated the fields observed, the dynamo would have had to have operated for a sufficiently long period of time. However, magnetic fields of similar intensities and scales to the one in our galaxy, are observed in high redshift galaxies, where a mean field dynamo would not have had enough time to produce the observed fields. Instead of a mean field dynamo, we study the emergence of strong large scale magnetic fields in the first objects formed in the universe due to the action or a turbulent, helical stochastic dynamo, for redshifts $5 \leq z \leq 10$. Ambipolar drift plays an important role in this process due to the low level of ionization of the gas, allowing a large scale stochastic dynamo to operate. We take into account the uncertainties in the physics of high redshift objects by examining a range of values for the parameters that characterize the turbulent plasma. By numerically integrating the nonlinear evolution equations for the magnetic field correlations, we show that for reasonable values of the parameters in the time interval considered, fields can grow to high intensities ($\sim 10^{-6}$ G), with large coherence lengths ($\sim 2-6$ kpc), essentially independent of the initial values of the magnetic field.