Whether present day galactic magnetic fields were built up over cosmic time, or very rapidly when the first galaxies were formed, the question remains of whether the first cosmological seed fields were generated during the inflation epoch, the plasma epoch before recombination, or when the.first stars were formed (cf section 5.5). In contrast to tests of the `conventional' mean-field disk dynamo discussed above, there appears to be little immediate hope of observationally discriminating among the various early universe seed field mechanisms described in section 5.5.
We have reviewed some of the alternative possibilities for field generation which can occur in stars, galaxies and supra-galaxy phenomena, such as galaxy collisions, galaxy cluster-scale infall or `Compton drag' on intergalactic plasma. Field-creating battery effects are a natural consequence of the fundamental fact that electrons and ions have the same charge but different masses. Differential Compton drag can potentially induce large scale currents in the interaction between intergalactic plasma and the photon flux of the cosmic background radiation. Large scale dynamics of plasma out-flow and consequent differential ion/electron dece1eration - e.g. in starburst galaxy nuclei (section 3.1) - have also been proposed to generate currents, hence magnetic fields on galactic scales. Battery effects associated with radio jets/accretion disks may also be capable of creating fields through small charge separation effects. Alternatively, possibly in addition, if extragalactic metal lines seen in QSO absorption lines trace magnetized galactic winds, as we speculated in section 5.4.3, we can extend this argument to hypothesize that the seed fields with which most galaxies formed came from stars. Since stars are the origin of these metal lines, and the heavy elements seen in the ICM of galaxy clusters, then the associated magnetic fields may have been expelled stellar fields, created by Biermann's battery, dynamo amplified, and expelled in the vigorous stellar winds from the most massive stars.
The question of what magnetic fields existed at the formation time of the earliest galaxies thus has more than one dimension: when were the first fields created - produced along with the first galaxies - or a cosmological seed field created before recombination? The second, perhaps more important question is; independent of the answer to the first, how strong were the magnetic fields at the first galaxy formation epoch? If they were in equilibrium with the radiation energy density or the turbulent or thermal energy density of protogalactic gas (as is the case in the galactic interstellar medium), it is likely that galaxy formation was significantly influenced by magnetic forces. Magnetic effects were less influential, though probably not negligible if much weaker fields ( 10-9 G) existed at the first galaxy formation time.
The magnetic field in the outflow regions of M82 and other galaxies with pronounced ouflow may be providing us with important clues about the origin of intergalactic fields in general. Since the starburst phenomenon is quite universal, and if most galaxies underwent at least one starburst in their lifetime, they could have `injected' their metal-enhanced magnetoplasma into the IGM. If, in a related scenario, the very first galaxies were low mass (hence low escape velocity) starbursters, the intergalactic magnetic fields could have been largely amplified and injected into the IGM, by starbursters at redshifts greater than 3, i.e. during the first few per cent of the Hubble time, [hereby `filling' the IGM with magnetic field. In this case, most subsequently formed galaxies were born with pre-existing fields not much different from the present, few µG level measured in the interstellar medium of our Milky Way and other galaxies.
Another mechanism for magnetizing the IGM at earlier epochs might have been by jet-lobe radio sources (section 3.2.3), depending on their (as yet unknown) occurrence rate at 3 z 10. None of these possibilities rules out the prior existence of seed fields during and before the recombination epoch.