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

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8.4. Spiral Arms and Magnetic Fields

A standard understanding of the interaction between spiral arms and large-scale magnetic fields is largely based on the idea that the spiral shock compresses the magnetic field and aligns it with the spiral arm (Roberts & Yuan 1970). This leads to a clear prediction that the regular magnetic field must be stronger at the inner edges of the arms and that there p is closer to pSA than in the interarm space. This picture was believed to be supported by the observation that the regular magnetic field in the Milky Way is enhanced within the local arm and that magnetic fields observed in nearby galaxies are well aligned with the spiral arms. It is, however, noteworthy that p neq pSA near the Sun (Section 3.8), whereas the general alignment p approx pSA can arise from dynamo action without any shock compression (Section 8.3).

However, recent observations of most nearby galaxies indicate that the regular magnetic fields are observed to be stronger between the arms, whereas the total field strength is stronger in the arms (Sections 3.3 and 3.4). The implication is straightforward: The action of the spiral pattern on galactic magnetic fields is not as direct and simple as passive compression (at least in these galaxies). (We note also that it is difficult to understand how the primordial theory, which gives only a passive role to the magnetic field, can explain its enhancement between the arms. Possibly, streaming motions induced by spiral arms could help, but this possibility has not been studied.)

The compression of magnetic field in spiral arms becomes much weaker if a large fraction of the interstellar medium is filled with hot gas, which prevents large-scale shocks from occurring. Star formation in spiral arms must then be triggered by, e.g. more frequent collisions of gas clouds (Roberts & Hausman 1984). The nearby spiral galaxies M51 and M81 exhibit strong density waves. In M51 prominent dust lanes, enhanced CO (García-Burillo et al 1993), and radio continuum emission at the inner edges of the optical spiral arms are indicators of narrow compression regions. In M81, however, the compression regions are much broader (Kaufman et al 1989) and can best be explained by the "cloudy" density-wave model of Roberts & Hausman (1984).

Beck (1991) has proposed a qualitative model to explain enhanced field tangling in the arms. He assumes that the field lines are trapped by gas clouds. As the clouds enter a spiral arm, they are decelerated, and their number density, collision rate, and turbulent velocity increase, which gives rise to field tangling and enhanced total field. However, the "magnetic arms" observed between the optical arms of NGC 6946 (Section 3.4) cannot be understood by this model and need a global mechanism such as the dynamo. How to include spiral arms adequately into the theory of galactic magnetic fields remains an important unresolved problem.

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