IONIZED-GAS AND INTRINSIC MAGNETIC-FIELDS IN THE SPIRAL GALAXY NGC-6946

The spiral galaxy NGC 6946 was observed in radio continuum at lambda2.8 cm with the Effelsberg 100-m telescope both in total intensity and linear polarization at 68'' (congruent-to 2.5 kpc ) angular resolution. The distribution of polarized intensity at lambda2.8 cm is rather symmetric with respect to the nucleus, in contrast to observations at decimeter wavelengths which are strongly affected by asymmetric Faraday depolarization effects. A consistent model for Faraday rotation, depolarization and thermal radio emission is presented in which most of the ionized gas resides in small clouds within a thin disk of congruent-to 100 pc full thickness. Each cloud has a mean electron density of 5.4 +/- 2.7 cm-3, decreasing with galactocentric radius proportional to the star-formation rate, and a mean size of congruent-to 1 pc. Their averages of < n(e) >= 0.27 +/- 0.05 cm-3 and < n(e)2 > = 1.5 +/- 0.7 cm-6 are higher than the values for classical H II regions and for the diffuse ionized gas in our Galaxy. The average filling factor in NGC 6946 of f = 0.05 +/- 0.03 is congruent-to 3 x higher than for the suspected similar population of small H II regions in our Galaxy. The uniform component of the intrinsic magnetic field almost precisely follows the spiral arms. The strongest polarized intensities and degrees of polarization (up to 19%) occur in interarm regions. The pitch angles in interarm regions are smaller than those in the spiral arms which indicates an influence of the density-wave gas flow on the field lines. A large fraction of the magnetic field is ordered on scalelengths below 2.5 kpc, not yet resolved by the present observations. The observed large-scale structure of the uniform magnetic field and the general distribution of polarized intensities are fairly well simulated by a dynamo model (axisymmetric spiral mode SO). The model parameters indicate that the dynamo does not only operate in the disk, but also in the halo. Faraday rotation between lambda2.8 cm and lambda6.3 cm shows a singly-periodical variation with azimuthal angle in the plane of NGC 6946 giving further support to an axisymmetric SO field. The Faraday rotation measures (RM ) are explained by the thin disk of small H II regions containing a uniform field of 12 +/- 2 muG. At lambda20.5 cm the thickness effective for RM is reduced to congruent-to 15 pc because the disk is no longer transparent to polarized radio waves due to Faraday depolarization. A close correspondence between RM and H I velocity dispersion is interpreted in terms of vertical field lines due to Parker instabilities or outflow of gas.