Faraday rotation maps of disk galaxies
  Faraday rotation is one of the most widely used observables to infer the
strength and configuration of the magnetic field in the ionised gas of the
Milky Way and nearby spiral galaxies. Here we compute synthetic Faraday
rotation maps at $z=0$ for a set of disk galaxies from the Auriga
high-resolution cosmological simulations, for different observer positions
within and outside the galaxy. We find that the strength of the Faraday
rotation of our simulated galaxies for a hypothetic observer at the solar
circle is broadly consistent with the Faraday rotation seen for the Milky Way.
The same holds for an observer outside the galaxy and the observed signal of
the nearby spiral galaxy M51. However, we also find that the structure and
angular power spectra of the synthetic all-sky Faraday rotation maps vary
strongly with azimuthal position along the solar circle. We argue that this
variation is a result of the structure of the magnetic field of the galaxy that
is dominated by an azimuthal magnetic field ordered scales of several kpc, but
has radial and vertical magnetic field components that are only ordered on
scales of 1-2 kpc. Because the magnetic field strength decreases exponentially
with height above the disk, the Faraday rotation for an observer at the solar
circle is dominated by the local environment. This represents a severe obstacle
for attempts to reconstruct the global magnetic field of the Milky Way from
Faraday rotation maps alone without including additional observables.
