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Astron. Astrophys. 350, 423-433 (1999)

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2. Observations

NGC 6946 is a typical late-type, gas-rich spiral galaxy. Distance measurements range between 5 and 10 Mpc; we assume 7 Mpc throughout this paper. NGC 6946 reveals a multiple-arm spiral structure in all spectral ranges (optical, infrared, far-infrared and radio continuum). Lacking a pronounced central bar and a companion galaxy, density waves are only of modest strength. The distribution of neutral hydrogen gas is quite smooth out to [FORMULA] kpc from the center (Boulanger & Viallefond 1992), and even molecular gas shows only a moderate arm/interarm contrast (Clausset et al. 1991).

Early radio polarization observations with low spatial resolution showed that the regular magnetic field of NGC 6946 is of spiral shape, similar to that of the optical spiral pattern (Harnett et al. 1989). Faraday rotation data indicated that NGC 6946 has an axisymmetric field (Ehle & Beck 1993). With increased resolution, however, the regular field appeared to be concentrated in two "magnetic arms" between the optical arms (Beck & Hoernes 1996), inconsistent with axisymmetry. Faraday rotation measures indicate a mixture of axisymmetric ([FORMULA]) and [FORMULA] modes, with the field direction pointing inwards everywhere (Krause & Beck 1998).

Fig. 1 shows a new polarization map at [FORMULA]3.5 cm from observations of the 100-m Effelsberg radio telescope of the MPIfR and the VLA synthesis telescope of the NRAO. 1 At this wavelength, Faraday rotation is smaller than a few degrees and Faraday depolarization is negligible (Ehle & Beck 1993) so that Fig. 1 shows directly the orientation of the magnetic field. Details of the [FORMULA]3.5 cm observations will be given elsewhere (Beck, in prep.)

[FIGURE] Fig. 1. Linearly polarized radio intensity of NGC 6946 at [FORMULA]3.5 cm, combined from observations at the VLA and Effelsberg radio telescopes (Beck, in prep.). Contour levels are (1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16)[FORMULA]Jy/beam. The length of the B-vectors is proportional to the polarized intensity. The angular resolution of 15" is shown in the bottom left corner

Note that the total radio continuum emission, signature of the total magnetic field, is at any wavelength distributed similar to the optical and infrared emission, with its strongest intensities on the gaseous spiral arms.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999