The Westerbork SINGS survey. III. Global magnetic field topology

A sample of large northern Spitzer Infrared Nearby Galaxies Survey (SINGS) galaxies was 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. Large-scale magnetic field structures of two basic types are found: (a) disk fields with a spiral topology in all detected targets ; and (b) circumnuclear, bipolar outflow fields in a subset. He re we explore the systematic patterns of azimuthal modulation of both the Faraday depth and the polarized intensity and their variati on 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 emerge s. 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) relat ive to the front-side zone in its propagation through the turbulent mi d-plane. The field topology which characterizes the thick-d isk emission zone, is in all cases an axisymmetric spiral with a quadrupole dependance on height above the mid-plane. The front-side emission is affected by only mild dispersion (10’s of rad m −2 ) from the thermal plasma in the galaxy halo, while the back-side emission is affected by additional strong dispersion (100’s of rad m −2 ) from an axi-symmetric spiral field in the galaxy mid-plane. The field topology in the upper halo of galaxies is a mixture of two distinct types: a simple extension of the axisymmetric spiral quadrupole fie ld of the thick disk and a radially directed dipole field. The dipole co mponent might be a manifestation of (1) a circumnuclear, bipolar outflow, (2) an in situ generated dipole field, or (3) evidence of a non-stationary g lobal halo.