GALACTIC DYNAMICS AND MAGNETIC-FIELDS .2. MAGNETIC-FIELDS IN BARRED GALAXIES

We present a magnetic-field amplification process in galaxies in conju nction with bar dynamics. Our model considers especially the observed non-circular gas velocities in barred spiral galaxies. The bar drives the spirally-falling gas flow toward the center, which consists of a n et radial flow (referred to as flow b) and an elliptically elongated f low rotating in the azimuth (flow a). Such two-dimensional velocity fi elds (upsilon(r), upsilon(phi)) are subject to the so-called ''anti-dy namo theorem for toroidal motion'' for which planar two-dimensional fl ows are a special case, i.e. no dynamo action by a two-dimensional vel ocity field. This is due to the rapid dissipation of the field by turb ulent diffusion. In a two-dimensional model the B(r) and B(phi) compon ents are decoupled from the B(z) component and B(z) cannot act as a so urce for B(r) and B(phi) However, as is well known from observations, galaxies exhibit considerable gas flows perpendicular to the disk in t he z-direction, which are driven either by large-scale galactic winds (as in NGC 4631), supernova explosions (as in M82) or by locally enhan ced star formation (as in NGC 891). Thus, we can expect that mechanism s exist which couple B(z) with B(r) and B(phi) via these gas flows, so that our model would not violate the anti-dynamo theorem and an inter esting possibility is inferred that a bar driven gas flow can act as a source for magnetic field amplification. The induced radial flow by a bar (flow b) produces a magnetic field, whose exponential growth is c losely related with the angular-momentum transport by the non-axisymme tric bar perturbation. The field grows on the same time scale (partial derivative upsilon(r)/partial derivative r)-1 is-proportional-to 2pi/ OMEGA at which the angular momentum is transferred outwards. Furthermo re, the non-axisymmetric gas flow (flow a) also leads to the exponenti al and oscillatory growth of magnetic fields by driving a growing magn etic wave. The interplay of both flows in a bar hence induces an oscil latory amplification of magnetic fields, and the resulting magnetic fi eld pattern rotates with a bar and holds the azimuthal wavenumber m = 1 or 2, depending on the strength of velocity disturbances. This model naturally explains the characteristic radio features observed in M83, where the m = 1 magnetic field is aligned with the bar, and the bar e nds are dominated by the vertical component B(z), giving the holes in polarized intensity map. It is emphasized that the evolution of galact ic magnetic fields is closely related with galactic dynamics and evolu tion.