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Astron. Astrophys. 329, 895-905 (1998)

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1. Introduction

Spiral galaxies appear usually to host magnetic fields on both global and smaller scales, see e.g. the review by Beck et al. (1996). The determination of the detailed morphology of the large-scale fields is difficult (Beck et al. 1996), but it does appear that many possess approximately an axisymmetric structure, perhaps modified by features with azimuthal wave number [FORMULA] (i.e. an axisymmetric structure has [FORMULA]). Others have significant [FORMULA] structure (bisymmetric spiral: BSS), and sometimes the picture may be more complicated. Note that current observational techniques are only adequate to determine the most prominent components of the field structure, and that it is plausible that higher order azimuthal components are quite generally present.

Basic dynamo theory (e.g Ruzmaikin, Sokoloff, Shukurov 1988, Eltsner et al. 1992, Moss & Brandenburg 1992) predicts that [FORMULA] fields are the most readily excited, and they appear also to be stable in the nonlinear regime (see, e.g., Moss et al. 1993a,b). A possible complication is that transients, reflecting the unknown initial conditions, may persist for several Gigayears, before the eventual stable configuration is achieved (Moss & Tuominen 1989, Brandenburg et al. 1992, Moss et al. 1993a, Poezd et al. 1993).

Thus finding an explanation for the maintenance of nonaxisymmetric magnetic features until the present epoch presents an interesting challenge for dynamo theory. The problem of explaining BSS has attracted considerable attention during the last few years. Apart from the possibility that it is a long-lived transient (Moss et al. 1993a), mechanisms investigated rely on nonaxisymmetries in the galactic disc. This can appear directly in the alpha-effect (Moss & Brandenburg 1992), perhaps involving parametric resonance or a related mechanism, or might arise from large-scale streaming velocities driven by the density wave associated with `grand design' spiral arms (Chiba 1991, Mestel & Subramanian 1991, Subramanian & Mestel 1993, Schmidt & Rüdiger 1992, Moss 1995, Moss 1996a,b, Moss 1997, Bikov et al. 1997, and others), or from essentially impulsive galaxy-galaxy encounters (Moss et al. 1993a,b, Moss 1996b, 1997).

Barred spiral galaxies offer another example of a situation where large-scale nonaxisymmetric streaming velocities occur. Any dynamo generated field would be expected to be modified significantly by such motions, and hence to possess substantial nonaxisymmetric components. Chiba & Lesch (1994) presented a study of the effects of non-circular gas motions, of a type that might be associated with barred galaxies, on magnetic field generation and evolution. They used a quasi-local form of the induction equation, and made some quite drastic assumptions about the form of the non-circular velocities. Their very simplified model, together with the absence of any visualization of the resulting global field, make it difficult to assess readily their results, and to compare them with ours. In particular, it is unclear whether their field structures can be maintained against global decay in the absence of an [FORMULA] -effect.

Subsequently, Otmianowska-Mazur & Chiba (1995) also studied the inductive effects of steady large-scale gas streaming on galactic magnetic fields. Their velocities were generated by a SPH simulation of the motion of gas under prescribed nonaxisymmetric disturbances to a given potential. They also did not include any [FORMULA] -effect, and their calculations were strictly linear. The boundary conditions are unclear, but this presumably implies that their magnetic fields eventually decay. In most of the models presented, the disc plane value of the magnetic diffusion is very large ([FORMULA] cm2 s-1). In these, and other, ways their calculations differ from ours.

In this paper we study the behaviour of magnetic fields in the presence of velocity fields derived from numerical simulations of barred galaxies. We use the N-body code developed by Salo (1991; see also Salo and Laurikainen 1993, which describes the 3D version of the code), which follows the evolution of a self-gravitating galactic disc embedded in an analytically modelled spherical halo. The simultaneous evolution of both `gas' and `stars' is included, the former being represented by dissipatively colliding particles. Under suitable conditions a central bar can form, with associated large-scale gas streaming. We have taken the gas velocity fields from two such simulations in which the velocities are restricted to be two dimensional, and included them in both a 2D and a 3D galactic dynamo model, for times of up to nearly 10 Gyr. Most of the calculations presented are with the 2D version of the dynamo code, as shorter trial runs suggest that the global magnetic field morphology calculated with the 3D code is quite similar. The latter simulations are, of course, much more time consuming. However, for inclusion of velocity data from a 3D dynamical simulation, the 3D dynamo code will be essential.

Of course, a dynamical simulation from different initial conditions, and/or with other parameters changed would produce different velocities. We do not claim to give a unique description of magnetic field evolution during the formation and evolution of a galactic bar, nor even to model processes occurring in any particular galaxies. We do think that we can display some of the morphological features of the magnetic fields that arise.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997