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The Westerbork SINGS survey III. Global magnetic field topology

A sample of large northern Spitzer Infrared Nearby Galaxies Survey (SINGS) galaxies was recently observed with the Westerbork Synthesis Radio Telescope (WSRT) at 1300-1760 MHz. In Paper II of this series, we described sensitive observations of the linearly polarized radio continuum emission in this WSRT-SINGS galaxy sample. Here we explore the systematic patterns of azimuthal modulation of both the Faraday depth and the polarized intensity and their variation with galaxy inclination. A self-consistent and fully general model for both the locations of net polarized emissivity at 1-2 GHz frequencies and the global magnetic field topology of nearby galaxies emerges. Net polarized emissivity is concentrated into two zones located above and below the galaxy mid-plane, with the back-side zone suffering substantial depolarization (by a factor of 4-5) relative to the front-side zone in its propagation through the turbulent mid-plane. The field topology, which characterizes the thick-disk emission zone is in all cases an axisymmetric spiral with a quadrupole dependence on height above the mid-plane. The front-side emission is affected by only mild dispersion (10's of rad/m2) from the thermal plasma in the galaxy halo, while the back-side emission is affected by additional strong dispersion (100's of rad/m2) from an axisymmetric spiral field in the galaxy mid-plane. The field topology in the upper halo of galaxies is a mix of two distinct types: a simple extension of the axisymmetric spiral quadrupole field of the thick disk and a radially directed dipole field. The dipole component might be a manifestation of (1) a circumnuclear, bipolar outflow, (2) an in situ generated dipole field, or (3) evidence of a non-stationary global halo.