Planck intermediate results. XLII. Large-scale Galactic magnetic fields
  Recent models for the large-scale Galactic magnetic fields in the literature
have been largely constrained by synchrotron emission and Faraday rotation
measures. We use three different but representative models to compare their
predicted polarized synchrotron and dust emission with that measured by the
Planck satellite. We first update these models to match the Planck synchrotron
products using a common model for the cosmic-ray leptons. We discuss the impact
on this analysis of the ongoing problems of component separation in the Planck
microwave bands and of the uncertain cosmic-ray spectrum. In particular, the
inferred degree of ordering in the magnetic fields is sensitive to these
systematic uncertainties, and we further show the importance of considering the
expected variations in the observables in addition to their mean morphology. We
then compare the resulting simulated emission to the observed dust polarization
and find that the dust predictions do not match the morphology in the Planck
data but underpredict the dust polarization away from the plane. We modify one
of the models to roughly match both observables at high latitudes by increasing
the field ordering in the thin disc near the observer. Though this specific
analysis is dependent on the component separation issues, we present the
improved model as a proof of concept for how these studies can be advanced in
future using complementary information from ongoing and planned observational
projects.
