The Galactic Faraday depth sky revisited
  The Galactic Faraday depth sky is a tracer for both the Galactic magnetic
field and the thermal electron distribution. It has been previously
reconstructed from polarimetric measurements of extra-galactic point sources.
Here, we improve on these works by using an updated inference algorithm as well
as by taking into account the free-free emission measure map from the Planck
survey. In the future, the data situation will improve drastically with the
next generation Faraday rotation measurements from SKA and its pathfinders.
Anticipating this, the aim of this paper is to update the map reconstruction
method with the latest development in imaging based on information field
theory. We demonstrate the validity of the new algorithm by applying it to the
Oppermann et al. (2012) data compilation and compare our results to the
previous map.\\ Despite using exactly the previous data set, a number of novel
findings are made: A non-parametric reconstruction of an overall amplitude
field resembles the free-free emission measure map of the Galaxy. Folding this
free-free map into the analysis allows for more detailed predictions. The joint
inference enables us to identify regions with deviations from the assumed
correlations between the free-free and Faraday data, thereby pointing us to
Galactic structures with distinguishably different physics. We e.g. find
evidence for an alignment of the magnetic field within the line of sights along
both directions of the Orion arm.
