The Westerbork SINGS survey III. Global magnetic field topology

TL;DR

This study analyzes the global magnetic field topology in nearby galaxies using polarized radio continuum observations, revealing a consistent axisymmetric spiral structure with complex halo components.

Contribution

It introduces a comprehensive model for polarized emission zones and magnetic field topology, highlighting the presence of both quadrupole and dipole components in galaxy halos.

Findings

Polarized emission concentrated above and below galaxy mid-plane.

Field topology is predominantly an axisymmetric spiral with quadrupole dependence.

Halo magnetic fields include both spiral and dipole components.

Abstract

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.