|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
8.5. Dynamo Models for Individual Galaxies
The predictions of 2 -dynamo models are roughly consistent with the large-scale field structures observed in spiral galaxies. In this section we discuss briefly a few individual galaxies for which detailed dynamo models have been developed and/or new problems have arisen.
Kinematic dynamo models for the Milky Way (see Ruzmaikin et al 1988a Sections VII.7 and VII.9) indicate that the axisymmetric mode is dominant, even though the m = 1 mode can also be maintained if the half-thickness of the ionized disk is within a narrow range (500-700 pc near the Sun, but these values are model dependent). In view of the uncertainty concerning the generation of bisymmetric fields in spiral galaxies, an ASS seems more likely but the presence of the BSS cannot be excluded; a superposition of the two modes (MSS) is also possible.
The presence of reversals (Section 3.8) is often considered as an indication of a bisymmetric global structure of magnetic field in the Milky Way. We again stress that this is not true. The possibility of such reversals in an axisymmetric spiral field was demonstrated in a dynamo model for the Milky Way by Poezd et al (1993). Even this simplified model exhibits a reasonable agreement with observations, yielding two or three reversals whose positions along the radius roughly agree with those observed. According to Poezd et al, the reversals represent transient nonlinear magnetic structures (cf Section 6.4).
Both dynamo theory and observations agree that the large-scale magnetic field in M31 is axisymmetric. A notable feature of this galaxy is that both the gas and the large-scale magnetic field are concentrated within a narrow ring of about 10 kpc radius (Section 3.2). The explanation provided by the dynamo models reviewed by Ruzmaikin et al (1988a, Section VII.7) relies on the rotation curve having a pronounced double-peaked shape (Deharveng & Pellet 1975). However, recent interpretations (with better allowance for radial motions) have resulted in a much less pronounced minimum in the rotation curve (Kent 1989, Braun 1991). Even though the new rotation curve has not yet been incorporated into dynamo models, it can be guessed that the kinematic dynamo modes will no longer show any concentration into a ring. Thus, the ring-like structure of magnetic field in M31 probably arises during the nonlinear stage of the dynamo and is associated with a similar distribution of the interstellar gas (Dame et al 1993).
M81 is the only nearby galaxy for which a dominant bisymmetric magnetic field is firmly indicated by observations (Section 3). Apart from kinematic asymptotic dynamo models (Krasheninnikova et al 1989, Starchenko & Shukurov 1989), a three-dimensional, nonlinear dynamo model has been developed for M81 based on the velocity field inferred from simulations of the interaction of this galaxy with its companion NGC 3077 (Moss et al 1993a). The interaction has been shown to result in a persistent bisymmetric structure. To reach a final conclusion about the nature of the magnetic field in M81, these numerical simulations must be extended to include better spatial resolution and a fully time-dependent representation of the velocity field. There is no minimum of polarized intensity observed near the probable location of the magnetic neutral line in M81 (Figure 2). Its absence probably indicates that the reversal in the BSS structure is rather abrupt, reminiscent of a contrast structure (see Section 6.2).