Disentangling Magnetic Fields in NGC 6946 with Wide-band Polarimetry

Abstract

We present λ 13 cm polarization observations of the nearby spiral galaxy NGC 6946 with the Westerbork Synthesis Radio Telescope (WSRT) to examine the nearside halo magnetic fields. Despite λ 13 cm exhibiting similar two-dimensional morphology as observed at longer ( λ 18–22 cm) or shorter ( λ 3 and λ 6 cm) wavelengths, more complete frequency coverage will be required to explain the gap in polarization in the southwest quadrant of the galaxy. We fit models of the turbulent and coherent line-of-sight magnetic fields to the fractional degree of linearly polarized emission at λ 3, λ 6, λ 13, λ 18, and λ 22 cm from observations taken with the WSRT, Karl G. Jansky Very Large Array, and Effelsberg telescopes. The results favor a multilayer turbulent magneto-ionized medium consistent with current observations of edge-on galaxies. We constrain the physical properties of the synchrotron-emitting thin and thick disks (scale heights of 300 pc and 1.4 kpc, respectively) along with the thermal thick disk and halo (scale heights of 1 and 5 kpc, respectively). Our preferred model indicates a clumpy and highly turbulent medium within 1 kpc of the midplane, and a diffuse extraplanar layer with a substantially lower degree of Faraday depolarization. In the halo, we estimate a regular magnetic field strength of 0.4–2.2 μ G and that turbulence and a total magnetic field strength of ∼6 μ G result in a Faraday dispersion of σ _RM = 4–48 rad m ^−2 . This work is an example of how the advanced capabilities of modern radio telescopes are opening a new frontier for the study of cosmic magnetism.

Keywords

• Galaxy magnetic fields
• Radio continuum emission
• Radio astronomy
• Spiral galaxies
• Galaxies
• Magnetic fields