Magnetic field transport from disk to halo via the galactic chimney
  process in NGC 6946
  The interstellar medium (ISM) in galaxies is directly affected by the mass
and energy outflows originating in regions of star formation. Magnetic fields
are an essential ingredient of the ISM, but their connection to the gaseous
medium and its evolution remains poorly understood. Here we present the
detection of a gradient in Faraday rotation measure (RM), co-located with a
hole in the neutral hydrogen (HI) distribution in the disk of the nearby spiral
galaxy NGC 6946. The gas kinematics in the same location show evidence for
infall of cold gas. The combined characteristics of this feature point to a
substantial vertical displacement of the initially plane-parallel ordered
magnetic field, driven by a localized star formation event. This reveals how
the large-scale magnetic field pattern in galaxy disks is directly influenced
by internal energetic phenomena. Conversely, magnetic fields are observed to be
an important ingredient in disk-halo interactions, as predicted in MHD
simulations. Turbulent magnetic fields at smaller spatial scales than the
observed RM gradient will also be carried from the disk and provide a mechanism
for the dynamo process to amplify the ordered magnetic field without quenching.
We discuss the observational biases, and suggest that this is a common feature
of star forming galaxies with active disk-halo flows.
