Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

6.5. The Influence of Magnetic Fields on the Galactic Disk

Early ideas that magnetic fields might universally give rise directly to spiral structure have now generally been abandoned, because large-scale fields would need to have strengths 10 µG to cause the velocity perturbations of about 20 km s^-1 associated with spiral arms (e.g. Binney & Tremaine 1987, p. 394). This can be compared with typical values of a few µG (Section 3.8). (Note that the above estimate is valid for a gas density appropriate to the Milky Way, and that for gas-rich galaxies, which tend to have larger fields, it would also be increased.) However, Nelson (1988) suggested, from study of a simplified, two-dimensional model, that magnetic fields might have a significant effect on gas dynamics at large galactocentric distances, where the gas density is lower.

Nevertheless, there may be more subtle effects. Magnetic pressure contributes significantly to the overall pressure balance in the ISM (e.g. Bowyer et al 1995), perhaps affecting the vertical distribution of the gas [scale height, etc (see Boulares & Cox 1990)]. This in turn can affect the dynamo efficiency, establishing a feedback loop (Dobler et al 1996). Magnetic fields, of both large and small scale, could affect the formation and motion of clouds, for example, by increasing their effective cross-section. More directly, magnetic fields are believed to mediate the star-formation process, inter alia helping to solve the "angular momentum problem" (see Mestel 1985). A locally stronger magnetic field may bias the initial mass function to more massive stars (e.g. Mestel 1989), which, with their more rapid and violent evolution, could result in a more energetic ISM and perhaps an enhanced -effect, thus providing another feedback loop (Mestel & Subramanian 1991, see also the discussion by Nozakura 1993).

Even the relatively modest azimuthal magnetic torques might affect the centrifugal balance sufficiently to give a significant angular momentum transport. An investigation by Rüdiger et al (1993) suggests that in the case of fields of quadrupolar parity, a substantially subsonic gas inflow will result, with only a small effect on the dynamo field structure.