|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
3.4. IC 342 and NGC 6946: Magnetic Spiral Arms
These two galaxies exhibit high star-formation rates, but their spiral structure is less regular than that of M51. Long arms of polarized emission are present in IC 342 (Krause et al 1989a, Krause 1993).
Recent high-resolution observations of the similar galaxy NGC 6946 (Beck & Hoernes 1996; see Figure 3) revealed a surprisingly symmetric distribution of polarized emission with two major spiral features, in the north and in the south, located between optical spiral arms and running perfectly parallel to the adjacent optical arms over at least 12 kpc. This regular two-armed structure is much more symmetric than the distribution of total field, gas, and stars, which all show a quite irregular, multiarmed pattern. Two further, weaker, magnetic spiral arms are visible between the two main ones (Figure 3).
Figure 3. Polarized synchrotron intensity (contours) and magnetic field orientation of NGC 6946 (obtained by rotating E-vectors by 90°) observed at 6.2 cm with the VLA (12.5 arcsec synthesized beam) and combined with extended emission observed with the Effelsberg 100-m telescope (2.5 arcmin resolution). The lengths of the vectors are proportional to the degree of polarization. (From Beck & Hoernes 1996.)
The main magnetic spiral arms in NGC 6946 do not fill all of the interarm regions, unlike the polarized emission in M81, but are only about 0.5-1 kpc wide. As they are also visible in total emission, both the regular and total magnetic fields are enhanced there. The strength of the (resolved) regular field varies between 3 and 13 µG along the arm. The peak values of polarized intensity and degree of polarization occur in the northern magnetic arm of NGC 6946, in a region between the optical arms, where the density of warm gas is exceptionally low. Subtraction of the diffuse, unpolarized background gives a degree of polarization of 30-65%, with the implication that the fields in the magnetic spiral arms must be almost totally aligned with the optical arms.