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

3.5. ASS, BSS, or What?

The available data on global magnetic structures in spiral galaxies are compiled in Tables 2 and 3. Most of the results were obtained using the RM analysis method (see Section 2.2); the more advanced -analysis method has as yet only been applied to M51 (and in simplified form to M31, IC 342, and M81).

Table 3. Magnetic field structures of (almost) edge-on galaxies

	Instrument a and
Galaxy	wavelength	Field structure	References
M82	V 6, 3.6	Radial	Reuter et al (1994)
NGC253	P 6 cm		Harnett et al (1990),
	V 20, 6 cm		Carilli et al (1992),
	E 2.8 cm	|| plane (ASS?)	Beck et al (1994b)
NGC891	V 20, 6 cm	|| & inclined to plane	Sukumar & Allen (1991),
	E 2.8 cm		Dumke et al (1995)
NGC1808	V 20, 6 cm	Extensions plane	Dahlem et al (1990)
NGC3628	V 20, 6 cm		Reuter et al (1991),
	E 2.8 cm	|| & inclined to plane	Dumke et al (1995)
NGC4565	V 20, 6 cm	|| plane	Sukumar & Allen (1991),
	E 2.8 cm		Dumke et al (1995)
NGC4631	E 6, 2.8 cm	plane (inner region),	Hummel et al (1991a),
	V 20, 6, 3.5 cm	|| & inclined to plane (outer regions)	Golla & Hummel (1994)
NGC4945	P 6 cm	Extensions plane	Harnett et al (1989),
NGC5775	V 6 cm	|| plane	Golla & Beck (1990)
NGC5907	V 6 cm	|| plane	M Dumke & M Krause (in prep.)
NGC7331	E 2.8 cm	Almost || plane	Dumke et al (1995),
	V 20, 6 cm		E Hummel (unpubl.)
Circinus	A 13, 6 cm	northern plume	Elmouttie et al (1995)
a Instruments: E = Effelsberg 100-m, A = Australia Telescope Compact Array, P = Parkes 64-m, W = Westerbork Synthesis Radio Telescope, V = Very Large Array.

Singly-periodic RM variations indicative of ASS have been detected in the disks of M31 (Sofue & Takano 1981, Beck 1982) and IC 342 (Gräve & Beck 1988, Krause et al 1989a, Sokoloff et al 1992). In M31 Ruzmaikin et al (1990) found evidence for the presence of the m = 1 mode at lower amplitude in the outer regions, superimposed on the dominating m = 0 mode. In NGC 6946 the phase of the azimuthal RM variation differs significantly from the value of the mean magnetic pitch angle (Ehle & Beck 1993). Recent high-resolution data for this galaxy (R Beck & P Hoernes, in preparation) indicate some correlation of RM with the optical spiral arms, suggesting local enhancements of RM due to thermal gas, rather than to field geometry. The magnetic spiral arms (where thermal gas density is low) seem to have RMs of opposite sign (R Beck & P Hoernes, in preparation), indicative of the m = 0 mode or, more realistically, a superposition of the m = 0 and the m = 2 mode with about equal amplitudes. In NGC 253, seen almost edge-on, the large-scale magnetic field has opposite directions on the "left" and "right" of the rotation axis of the inner disk. NGC 253 is thus another candidate for an ASS disk field (Beck et al 1994b).

The only clear candidate for a BSS symmetry is M81 (Krause et al 1989b, Sokoloff et al 1992). The analysis of Krause et al (1989b) indicated that the magnetic neutral lines are in the interarm space. In M33 the weak polarized emission leads to large uncertainties in RM, and a bisymmetric field can be claimed only with some caution (Buczilowski & Beck 1991). The same is true for NGC 2276 (Hummel & Beck 1995). The galaxies M33, M81, and NGC 2276 show signs of gravitational interaction, which can be important in producing nonaxisymmetric dynamo fields (see Sections 6.2 and 8.2). Thus these are all candidates for MSS status. Other claims of a dominating bisymmetric field (Sofue et al 1985, 1986) are of much lower significance.

The strongly interacting galaxy M51 is a special case where the pitfalls of data interpretation can be demonstrated. M51 was thought to contain a bisymmetric field (Tosa & Fujimoto 1978, Horellou et al 1990), but this was not confirmed by later Effelsberg and VLA measurements. At 10 cm, Faraday depolarization is strong and the observed polarized emission originates in the upper disk or halo (Horellou et al 1992). By analyzing all available data in terms of the angles, EM Berkhuijsen et al (in preparation) have found that the field in M51 can be described as MSS, with axisymmetric and bisymmetric components having about equal weights in the disk, together with a horizontal axisymmetric halo field with opposite direction.

The RM variation in M83 is doubly-periodic (Neininger et al 1993), but the phase is inconsistent with BSS symmetry. A future analysis of polarization angles including recent observations at 13 cm (Ehle 1995) might clarify the case. The RM pattern in M83 indicates a nonaxisymmetric distribution of gas or velocity field, an MSS field, or both.