Astron. Astrophys. 318, 700-720 (1997)

7. Summary and conclusions

We developed a method to determine a three-dimensional structure of the regular magnetic field from multi-frequency polarization observations of spiral galaxies. Using this method we analyzed polarization observations of the galaxy M 51 at 2.8, 6.2, 18.0 and 20.5 cm which have a resolution of about 3.5 kpc in the galactic plane. At each wavelength the observed polarization angles were averaged over sectors of width in the rings at radial distances 3-6, 6-9, 9-12 and 12-15 kpc. For each ring the azimuthal distributions of polarization angles were fitted using a three-dimensional model of the magnetic field.

The galaxy is not completely transparent for polarized emision at 18 and 20 cm, which allowed us to analyze the line-of-sight structure of the magneto-ionic medium by considering simultaneously the data at the above four wavelengths.

We obtained the strengths of the total, regular and random magnetic fields in the disk for each ring from the total and polarized intensities of the nonthermal emission using equipartition arguments and synchrotron scale heights scaled from the Milky Way (see Sect. 3.1 and Table 1).

We also derived estimates of the volume density of thermal electrons, their scale height and filling factor for the disk based on the thermal radio emission from M 51 and an analogy with the Milky Way (see Sect. 3.2 and Table 2). Using these, we estimated the strength of the regular magnetic field in the disk and the halo of M 51 from our fits.

Our general conclusions are as follows:

1. The global magnetic pattern in M 51 at kpc can be represented as a superposition of the two lowest azimuthal Fourier modes.

2. Our analysis indicates the existence of a magneto-ionic halo and shows that the magnetic fields in the disk and the halo have different configurations.

3. The radial extent of the halo is about 10 kpc, in agreement with X-ray data. The halo field is horizontal and axisymmetric. The regular magnetic fields in the halo and in the disk are spirals. The field directions along the spirals are generally opposite running inwards and outwards in the halo and the disk, respectively.

4. In the disk a superposition of axisymmetric () and bisymmetric () magnetic modes provides a satisfactory fit to the observations. The mode slightly dominates at kpc and the two modes have about equal amplitudes at kpc. The field is predominantly horizontal between 3 and 12 kpc and has a weak vertical component at kpc.

In the rings between 3 and 9 kpc the field structure in the disk is strongly non-axisymmetric with a field maximum in the eastern part of the galaxy and a weak field in the western part. Details are given in Table 4 and shown in Fig. 8.

5. The magnetic field pattern in the disk of M 51 shows a discontinuity at kpc, the position at which the inner and the outer spiral structure join (Elmegreen et al. 1989). The relatively strong, coherent magnetic field in the inner rings occurs in the system of spiral arms excited by density waves, whereas the weaker and partly distorted field in the outer rings exists in the area of the material spiral arms produced by the encounter with the companion.

6. The azimuthally averaged strength of the regular magnetic field obtained for the disk decreases from about 7µG at radius 3- to about 4µG at 12-15 kpc (see Fig. 7). The strength of the regular field in the halo decreases from about 3µG at 3-6 kpc to zero beyond 9 kpc. Details are given in Table 5.

7. The azimuthal averages of the regular magnetic field strength in the disk obtained from our fits are in good agreement with independent estimates from the total synchrotron emission and the degree of polarization. The radial scale length of the regular magnetic field is  kpc.

8. We compared the pitch angles of the regular magnetic field obtained from the fits with those of the dust lanes delineating spiral arms for the sectors they have in common. Their mean values agree to within the errors.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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