THE PROTOGALACTIC ORIGIN FOR COSMIC MAGNETIC-FIELDS

It is demonstrated that strong magnetic fields are produced from a zer o initial magnetic field during the pregalactic era, when the galaxy i s first forming. Their development proceeds in three phases. In the fi rst phase, weak magnetic fields are created by the Biermann battery me chanism. 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 magne tic fields will be amplified to strong magnetic fields by this Kolmogo rov turbulence. During this second phase, the magnetic fields reach sa turation with the turbulent power, but they are coherent only on the s cale of the smallest eddy. During the third phase, which follows this saturation, it is expected that the magnetic field strength will incre ase to equipartition with the turbulent energy and that the coherence length of the magnetic fields will increase to the scale of the larges t turbulent eddy, comparable to the scale of the entire galaxy. The re sulting magnetic field represents a galactic magnetic field of primord ial origin. No further dynamo action after the galaxy forms is necessa ry to explain the origin of magnetic fields. However, the magnetic fie ld will certainly be altered by dynamo action once the galaxy and the galactic disk have formed. It is first shown by direct numerical simul ations that thermoelectric currents associated with the Biermann batte ry build the field up from zero to 10(-21) G in the regions about to c ollapse into galaxies, by z similar to 3. For weak fields, in the abse nce of dissipation, the cyclotron frequency -omega(cyc) = eB/m(H)c and omega/(1 + chi), where omega = del x upsilon is the vorticity and chi is the degree of ionization, satisfy the same equations, and initial conditions omega(cyc) = omega = 0, so that, globally, -omega(cyc)(r, t ) = omega(r, t)/(1 + chi). The vorticity grows rapidly after caustics (extreme nonlinearities) develop in the cosmic fluid. At this time, it is made plausible that turbulence has developed into Kolmogorov turbu lence. Numerical simulations do not yet have the resolution to demonst rate that, during the second phase, the magnetic fields are amplified by the dynamo action of the turbulence. Instead, an analytic theory of the turbulent amplification of magnetic fields is employed to explore this phase of the magnetic field development. From this theory, it is shown that, assuming the turbulence is really Kolmogorov turbulence, the dynamo action of this protogalactic turbulence is able to amplify the magnetic fields by such a large factor during the collapse of the protogalaxy that the power into the magnetic field must reach saturati on with the turbulent power. For the third phase, there is as yet no a nalytic theory capable of describing this phase. However, preliminary turbulence calculations currently in progress seem to confirm that the magnetic fields may proceed to equipartition with the turbulent energ y, and that the coherence length may increase to the largest scales. S imple physical arguments are presented that show that this may be the case. Such an equipartition field is actually too strong to allow imme diate collapse to a disk. Possible ways around this difficulty are dis cussed.